JP7589080B2 - Aqueous electrolyte bipolar secondary battery, battery pack, vehicle, and stationary power source - Google Patents

Description

Embodiments of the present invention relate to secondary batteries, battery packs, vehicles, and stationary power sources.

Non-aqueous electrolyte batteries such as lithium-ion batteries are used as power sources in a wide range of fields. Non-aqueous electrolyte batteries come in a wide variety of forms, from small ones for use in various electronic devices to large ones for use in electric vehicles. Non-aqueous electrolyte batteries use non-aqueous electrolytes that contain flammable substances such as ethylene carbonate, so safety measures are necessary.

Instead of a non-aqueous electrolyte, development is underway to develop aqueous electrolyte batteries that use an aqueous electrolyte containing a non-flammable aqueous solvent.

Bipolar electrodes can be connected in series within a single cell, making it possible to achieve high voltages without external connections. However, in aqueous electrolyte batteries that use aqueous electrolytes, different aqueous electrolytes may be used on each electrode side to achieve high performance. When bipolar electrodes are used in these aqueous electrolyte batteries, there are issues with the durability of the bipolar electrode current collectors.

JP 2020-53385 A

The objective is to provide an aqueous electrolyte bipolar secondary battery and battery pack with excellent life characteristics by being equipped with a bipolar electrode having a highly durable current collector, as well as a vehicle and stationary power source equipped with the battery pack.

According to an embodiment, an aqueous electrolyte bipolar secondary battery is provided. The aqueous electrolyte bipolar secondary battery includes a bipolar electrode having a bipolar electrode collector, a bipolar electrode having a first negative electrode active material-containing layer on at least one surface of a negative electrode collector, a positive electrode having a first positive electrode active material-containing layer on at least one surface of a positive electrode collector, a bipolar electrode having a bipolar electrode collector on one surface of which a second positive electrode active material-containing layer is provided, and a water-based electrolyte. The bipolar electrode collector has a metal layer a and a metal layer b, and the metal layer a contains at least one element selected from the group consisting of Cr, Ni, Fe, Ti, and Al, and the metal layer b contains 10 wt. % or more and 100 wt. % or less of any one of Zn and Sn with respect to the weight per layer of the metal layer b. Metal layer a and metal layer b are electrically connected, and a second positive electrode active material-containing layer is provided on the surface of metal layer a, and a second negative electrode active material-containing layer is provided on the surface of metal layer b.

1 is a schematic diagram of an aqueous electrolyte bipolar secondary battery according to an embodiment. FIG. 2 is a schematic diagram of a bipolar electrode current collector included in the aqueous electrolyte bipolar secondary battery according to the embodiment. FIG. 1 is a cross-sectional view illustrating an example of a secondary battery according to an embodiment. FIG. 4 is a cross-sectional view taken along line IV-IV of the secondary battery shown in FIG. FIG. 4 is a partially cutaway perspective view illustrating a secondary battery according to another embodiment of the present invention. FIG. 6 is an enlarged cross-sectional view of a portion B of the secondary battery shown in FIG. 5 . FIG. 1 is a perspective view illustrating an example of a battery pack according to an embodiment. FIG. 1 is a perspective view illustrating an example of a battery pack according to an embodiment. FIG. 13 is an exploded perspective view illustrating a battery pack according to another embodiment of the present invention. FIG. 10 is a block diagram showing an example of an electrical circuit of the battery pack shown in FIG. 9 . 1 is a partially see-through view illustrating an example of a vehicle according to an embodiment. FIG. 1 is a block diagram showing an example of a system including a stationary power supply according to an embodiment.

The following describes the embodiments. Unless otherwise specified, the values obtained by pH and other measurements are values measured at atmospheric pressure and 25°C.

(First embodiment)
The aqueous electrolyte bipolar secondary battery according to the first embodiment is an aqueous electrolyte bipolar secondary battery comprising: a negative electrode having a first negative electrode active material-containing layer provided on at least one surface of a negative electrode current collector; a positive electrode having a first positive electrode active material-containing layer provided on at least one surface of a positive electrode current collector; a bipolar electrode having a bipolar electrode current collector having a second positive electrode active material-containing layer provided on one surface and a second negative electrode active material-containing layer provided on the other opposing surface; and an aqueous electrolyte, wherein the bipolar electrode current collector has a metal layer a and a metal layer b, the metal layer a contains at least one element selected from the group consisting of Cr, Ni, Fe, Ti, and Al, and the metal layer b contains 10 wt. % or more and 100 wt. % or less of the weight of each metal layer b. % or less of any one of elements Zn and Sn, the metal layer a and the metal layer b are electrically connected to each other, a second positive electrode active material-containing layer is provided on a surface of the metal layer a, and a second negative electrode active material-containing layer is provided on a surface of the metal layer b.

FIG. 1 is a schematic diagram of an aqueous electrolyte bipolar secondary battery according to a first embodiment. The aqueous electrolyte bipolar secondary battery 500 includes an anode 3, a cathode 5, and a bipolar electrode 501. The anode 3 includes a first anode active material-containing layer 3b1 on at least one surface of the anode current collector 3a. The cathode 5 includes a first cathode active material-containing layer 5b1 on at least one surface of the cathode current collector 5a. The bipolar electrode 501 includes a second cathode active material-containing layer 5b2 that faces the first anode active material-containing layer 3b1 across the first separator 4a, and a bipolar electrode current collector 502 that is in contact with the second anode active material-containing layer 3b2 that faces the first cathode active material-containing layer 5a1 across the second separator 4b. The first separator 4a and the second separator 4b may be porous or non-porous. The first separator 4a and the second separator 4b may be the same type or different types. The bipolar electrode current collector 502 has a metal layer a 502a and a metal layer b 502b, the metal layer a 502a being in contact with the positive electrode active material containing layer 5b2, and the metal layer b 502b being in contact with the negative electrode active material containing layer 3b2. The aqueous electrolyte of the aqueous electrolyte bipolar secondary battery 500 is held in the positive electrode active material containing layer and the negative electrode active material containing layer. For convenience, FIG. 1 shows the negative electrode side aqueous electrolyte 504 and the positive electrode side aqueous electrolyte 505. There is a positive electrode side aqueous electrolyte 505 in contact with the metal layer a 502a and a negative electrode side aqueous electrolyte 504 in contact with the metal layer b 502b, and the positive electrode side aqueous electrolyte and the negative electrode side aqueous electrolyte are different. The aqueous electrolytes contained in the first positive electrode active material containing layer and the second positive electrode active material containing layer may be the same type or different. Similarly, the aqueous electrolytes contained in the first negative electrode active material-containing layer and the second negative electrode active material-containing layer may be the same or different.

If the first and second separators are porous, the aqueous electrolyte is also retained in the separators.

The first negative electrode active material and the second negative electrode active material may be of different types or may be the same. The first positive electrode active material and the second positive electrode active material may be of different types or may be the same.

Unless otherwise specified, the positive electrode aqueous electrolyte and the negative electrode aqueous electrolyte are collectively referred to simply as "aqueous electrolyte."

By using bipolar in an aqueous electrolyte battery, it is possible to connect in series within a single cell, making it possible to achieve high voltage without external connections. In particular, by using an aqueous electrolyte that matches the active materials used in the positive and negative electrodes, it is possible to further improve the battery characteristics. However, although appropriate current collectors should be used for the positive and negative electrodes, these current collectors are prone to corrosion depending on the type of aqueous electrolyte. Therefore, by using a bipolar electrode current collector that matches the aqueous electrolyte and electrode, it is possible to suppress corrosion of the current collector, and improve battery characteristics such as the capacity retention rate of the secondary battery, improvement of initial discharge capacity, and improvement of the self-discharge rate.

Here, we will explain the components of the aqueous electrolyte bipolar secondary battery according to this embodiment.

(Positive electrode)
The positive electrode may include a positive electrode current collector and a positive electrode active material-containing layer provided on at least one surface of the positive electrode current collector. The first positive electrode active material-containing layer and the second positive electrode active material-containing layer are collectively referred to as positive electrode active material-containing layers. The positive electrode active material-containing layer contains a positive electrode active material and, optionally, a conductive agent and a binder.

The positive electrode current collector includes metals such as stainless steel, aluminum (Al) and titanium (Ti). The positive electrode current collector has, for example, a foil, a porous body or a mesh shape. In order to prevent corrosion due to a reaction between the positive electrode current collector and the aqueous electrolyte, the surface of the positive electrode current collector may be coated with a different element. The positive electrode current collector is preferably one having excellent corrosion resistance and oxidation resistance, such as Ti foil. In addition, when Li ₂ SO ₄ is used as the aqueous electrolyte, Al may be used as the positive electrode current collector because corrosion does not progress.

The positive electrode current collector may also include a portion on its surface that does not have a positive electrode active material-containing layer. This portion can function as a positive electrode current collecting tab.

The positive electrode active material-containing layer contains a positive electrode active material. The positive electrode active material-containing layer may be supported on both the front and back surfaces of the positive electrode current collector.

As the positive electrode active material, a compound having a lithium ion insertion-extraction potential of 2.5 V to 5.5 V (vs. Li/Li ⁺ ) relative to the oxidation-reduction potential of lithium can be used. The positive electrode may contain one type of compound alone as the positive electrode active material, or may contain two or more compounds as the positive electrode active material.

Examples of compounds that can be used as the positive electrode active material include lithium manganese composite oxide, lithium nickel composite oxide, lithium cobalt aluminum composite oxide, lithium nickel cobalt manganese composite oxide, spinel-type lithium manganese nickel composite oxide, lithium manganese cobalt composite oxide, lithium iron oxide, lithium fluorinated iron sulfate, and phosphate compounds having an olivine crystal structure (for example, a compound represented by Li _k FePO ₄ where 0<k≦1, a compound represented by Li _k MnPO ₄ where 0<k≦1), etc. Phosphate compounds having an olivine crystal structure have excellent thermal stability.

Examples of compounds that can obtain a high positive electrode potential include lithium manganese composite oxides such as a compound represented by Li _k Mn ₂ O ₄ having a spinel structure and having 0 < k ≦ 1, and a compound represented by Li _k MnO ₂ and having 0 < k ≦ 1; lithium nickel aluminum composite oxides such as a compound represented by Li _k Ni _1-i Al _i O ₂ and having 0 < k ≦ 1, 0 < i <1; lithium cobalt composite oxides such as a compound represented by Li _k CoO ₂ and having 0 < k ≦ ₁ ; lithium _{nickel cobalt} composite oxides such as a compound represented by Li _k Ni _1-i-t Co _i Mn _t O ₂ and having 0 < k ≦ 1, 0 < i < 1, 0 ≦ t <1; lithium manganese cobalt composite oxides such as a compound represented by Li _k Mn _1-i Ni _i O ₄ where 0 < k ≦ 1, 0 < i <1; spinel-type lithium manganese nickel composite oxides such as a compound represented by Li _k FePO ₄ where 0 < k ≦ 1, a compound represented by _{Li k Fe 1-y} Mn _y PO ₄ where 0 < k ≦ 1, and a compound represented by Li _k CoPO ₄ where 0 < k ≦ 1; and fluorinated iron sulfates (for example, a compound represented by Li _k FeSO ₄ F where 0 < k ≦ 1).

The positive electrode active material preferably contains one or more selected from the group consisting of lithium cobalt composite oxide, lithium manganese composite oxide, and lithium phosphate having an olivine structure. The operating potential of these compounds is 3.5 V (vs. Li/Li ⁺ ) to 4.2 V (vs. Li/Li ⁺ ). That is, the operating potential of these compounds as active materials is relatively high. By using these compounds in combination with the above-mentioned negative electrode active materials such as spinel-type lithium titanate and anatase-type titanium oxide, a high battery voltage can be obtained.

The positive electrode active material is contained in the positive electrode in the form of particles, for example. The positive electrode active material particles can be single primary particles, secondary particles which are aggregates of primary particles, or a mixture of primary particles and secondary particles. The shape of the particles is not particularly limited, and can be, for example, spherical, elliptical, flat, or fibrous.

The average particle size (diameter) of the primary particles of the positive electrode active material is preferably 10 μm or less, more preferably 0.1 μm or more and 5 μm or less. The average particle size (diameter) of the secondary particles of the positive electrode active material is preferably 100 μm or less, more preferably 10 μm or more and 50 μm or less. The primary particle size and secondary particle size of the positive electrode active material can be measured in the same manner as for the negative electrode active material particles.

The positive electrode active material-containing layer may contain a conductive agent and a binder in addition to the positive electrode active material. The conductive agent is mixed as necessary to improve current collection performance and reduce contact resistance between the active material and the positive electrode current collector. Examples of conductive agents include carbonaceous materials such as acetylene black, ketjen black, graphite, and coke. The conductive agent may be one type, or two or more types may be mixed together.

Examples of binders include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), fluorine-based rubber, ethylene-butadiene rubber, polypropylene (PP), polyethylene (PE), carboxymethyl cellulose (CMC), polyimide (PI), polyacrylimide (PAI), etc. One type of binder may be used, or two or more types may be mixed together.

The compounding ratios of the positive electrode active material, conductive agent, and binder in the positive electrode active material-containing layer are preferably 70% by mass or more and 95% by mass or less, 3% by mass or more and 20% by mass or less, and 2% by mass or more and 10% by mass or less, respectively. When the compounding ratio of the conductive agent is 3% by mass or more, the conductivity of the positive electrode can be improved, and when it is 20% by mass or less, the decomposition of the aqueous electrolyte on the conductive agent surface can be reduced. When the compounding ratio of the binder is 2% by mass or more, sufficient electrode strength can be obtained, and when it is 10% by mass or less, the insulating part of the electrode can be reduced.

The positive electrode current collector is in contact with the second surface of the positive electrode active material-containing layer. The positive electrode current collector may be in contact with the second surface of one positive electrode active material-containing layer, or may be in contact with each of the second surfaces of two positive electrode active material-containing layers. For example, the positive electrode current collector may be in contact with the second surface of one positive electrode active material-containing layer on one surface and in contact with the second surface of another positive electrode active material-containing layer on the other surface on the back side. In other words, it may include two positive electrode active material-containing layers sandwiched between one positive electrode current collector.

The positive electrode can be obtained, for example, by the following method. First, a positive electrode active material, a conductive agent, and a binder are suspended in an appropriate solvent to prepare a slurry. This slurry is applied to one or both sides of a positive electrode current collector. The coating on the positive electrode current collector is dried to form a positive electrode active material-containing layer. Thereafter, the positive electrode current collector and the positive electrode active material-containing layer formed thereon are pressed. As the positive electrode active material-containing layer, a mixture of the positive electrode active material, the conductive agent, and the binder formed into a pellet shape may be used.

(Negative electrode)
The negative electrode may include a negative electrode current collector and a negative electrode active material-containing layer provided on at least one surface of the negative electrode current collector. The first negative electrode active material-containing layer and the second negative electrode active material-containing layer are collectively referred to as negative electrode active material-containing layers. The negative electrode active material-containing layer contains a negative electrode active material, and optionally a conductive agent and a binder. When a compound having a lower limit of lithium ion insertion-extraction potential of 1 V or more (vs. Li/Li ⁺ ) is used as the negative electrode active material contained in the negative electrode active material-containing layer, a secondary battery having high capacity and excellent stability can be realized.

Examples of compounds having a lithium ion insertion-extraction potential of 1 V or more and 3 V or less (vs. Li/Li ⁺ ) based on the oxidation-reduction potential of lithium include titanium oxides and titanium-containing oxides. Examples of titanium-containing oxides include lithium titanium composite oxides, niobium titanium composite oxides, and sodium niobium titanium composite oxides. The negative electrode active material may contain one or more of titanium oxides and titanium-containing oxides.

Titanium oxide includes, for example, titanium oxide with a monoclinic structure, titanium oxide with a rutile structure, and titanium oxide with an anatase structure. The composition of titanium oxide with each crystal structure before charging can be expressed as _TiO2 , _and the composition after charging can be expressed as _LiyTiO2 (subscript y is 0≦y≦1). The structure of titanium oxide with a monoclinic structure before charging can be expressed as _TiO2 (B).

Examples of the lithium titanium oxide include lithium titanium oxides having a spinel structure (e.g., a compound represented by the general formula _{Li4 + jTi5O12} , where -1≦j≦3), lithium titanium oxides having a ramsdellite structure (e.g., a compound represented by Li2 _{+ jTi3O7 ,} where -1≦j≦ ₃ ), a compound represented by Li1 _{+ yTi2O4} , where 0≦y≦1, a compound represented by Li1.1 _{+ yTi1.8O4} , where 0≦y≦1, a compound represented by Li1.07 _{+ yTi1.86O4} , where 0≦y≦ ₁ , a compound represented by _{LikTiO2 ,} where 0<k≦1, etc. The _lithium titanium oxide may also be a lithium titanium composite oxide having a different element introduced therein.

The niobium titanium composite oxide includes, for example, a compound represented by Li _χ TiMe _α Nb _2±β O _7±σ , where 0≦χ≦5, 0≦α≦0.3, 0≦β≦0.3, and 0≦σ≦0.3, and Me is one or more selected from the group consisting of Fe, V, Mo, and Ta.

The sodium niobium titanium composite oxide is, for example, represented by the general formula Li _2+d Na _2-e Me1 _f Ti _6-g-h Nb _g Me2 _h O _14+δ , where 0≦d≦4, 0≦e<2, 0≦f<2, 0<g<6, 0≦h<3, -0.5≦δ≦0.5, Me1 includes one or more selected from Cs, K, Sr, Ba, and Ca, and Me2 includes one or more selected from Zr, Sn, V, Ta, Mo, W, Fe, Co, Mn, and Al. It includes an orthorhombic type Na-containing niobium titanium composite oxide.

As the negative electrode active material, it is preferable to use titanium oxide with anatase structure, titanium oxide with monoclinic structure, lithium titanium oxide with spinel structure, niobium titanium composite oxide, or a mixture of these. On the one hand, when titanium oxide with anatase structure, titanium oxide with monoclinic structure, or lithium titanium oxide with spinel structure is used as the negative electrode active material, a high electromotive force can be obtained by combining it with a positive electrode using lithium manganese composite oxide as the positive electrode active material, for example. On the other hand, by using niobium titanium composite oxide, a high capacity can be exhibited.

The negative electrode active material may be contained in the negative electrode active material-containing layer in the form of particles, for example. The negative electrode active material particles may be primary particles, secondary particles which are aggregates of primary particles, or a mixture of a single primary particle and a secondary particle. The shape of the particles is not particularly limited, and may be, for example, spherical, elliptical, flat, fibrous, etc.

The secondary particles of the negative electrode active material can be obtained, for example, by the following method. First, the raw active material is reacted and synthesized to produce an active material precursor with an average particle size of 1 μm or less. The active material precursor is then calcined and pulverized using a pulverizer such as a ball mill or jet mill. Next, in the calcination process, the active material precursor is aggregated and grown into secondary particles with a large particle size.

The average particle size (diameter) of the secondary particles of the negative electrode active material is preferably 3 μm or more, and more preferably 5 μm or more and 20 μm or less. In this range, the surface area of the active material is small, so water decomposition can be further suppressed.

It is desirable that the average particle diameter of the primary particles of the negative electrode active material is 1 μm or less. This shortens the diffusion distance of Li ions inside the active material and increases the specific surface area. Therefore, excellent high input performance (fast charging) can be obtained. On the other hand, if the average particle diameter of the primary particles of the negative electrode active material is small, the particles are more likely to aggregate. If the particles of the negative electrode active material aggregate, the aqueous electrolyte in the secondary battery is more likely to be unevenly distributed in the negative electrode, which may lead to a depletion of ion species in the positive electrode. Therefore, it is preferable that the average particle diameter of the primary particles of the negative electrode active material is 0.001 μm or more. It is more preferable that the average particle diameter of the primary particles of the negative electrode active material is 0.1 μm or more and 0.8 μm or less.

The primary particle size and secondary particle size refer to the particle size at which the volume cumulative value is 50% in the particle size distribution determined by a laser diffraction particle size distribution measuring device. For example, a Shimadzu SALD-300 is used as a laser diffraction particle size distribution measuring device. The light intensity distribution is measured 64 times at 2 second intervals. The sample used for this particle size distribution measurement is a dispersion diluted with N-methyl-2-pyrrolidone so that the concentration of active material particles is 0.1% to 1% by mass. Alternatively, the measurement sample is one in which 0.1 g of active material is dispersed in 1 ml to 2 ml of distilled water containing a surfactant.

The negative electrode active material has a specific surface area, as measured by the BET method using nitrogen (N ₂ ) adsorption, in the range of, for example, 3 m ² /g to 200 m ² /g. When the specific surface area of the negative electrode active material is within this range, the affinity between the electrode and the aqueous electrolyte can be further increased. This specific surface area can be determined, for example, by a method similar to the method for measuring the specific surface area of the negative electrode active material-containing layer described later.

It is desirable for the porosity of the negative electrode active material-containing layer to be 20% or more and 50% or less. This allows for an electrode with excellent affinity with the aqueous electrolyte and high density to be obtained. It is more preferable for the porosity of the negative electrode active material-containing layer to be 25% or more and 40% or less.

The porosity of the negative electrode active material-containing layer can be obtained, for example, by mercury intrusion porosimetry. Specifically, first, the pore distribution of the negative electrode active material-containing layer is obtained by mercury intrusion porosimetry. The total pore volume is calculated from this pore distribution. The porosity can be calculated from the ratio of the total pore volume to the volume of the negative electrode active material-containing layer.

The specific surface area of the negative electrode active material-containing layer measured by the BET method using nitrogen (N ₂ ) adsorption is more preferably 3 m ² /g or more and 50 m ² /g or less. If the specific surface area of the negative electrode active material-containing layer is less than 3 m ² /g, the affinity between the active material and the aqueous electrolyte is reduced. As a result, the interface resistance of the electrode increases, and the output performance and charge/discharge cycle performance of the secondary battery may be reduced. On the other hand, if the specific surface area of the negative electrode active material-containing layer exceeds 50 m ² /g, the distribution of ion species ionized from the electrolyte salt contained in the aqueous electrolyte is biased toward the negative electrode, which leads to a shortage of the ion species at the positive electrode, and the output performance and charge/discharge cycle performance may be reduced.

This specific surface area can be determined, for example, by the following method. When the negative electrode containing the negative electrode active material-containing layer to be measured is incorporated in a secondary battery, the secondary battery is disassembled and a part of the negative electrode active material-containing layer is taken. In nitrogen gas at 77K (the boiling point of nitrogen), the nitrogen gas pressure P (mmHg) is gradually increased while measuring the nitrogen gas adsorption amount (mL/g) of the sample for each pressure P. The pressure P (mmHg) is divided by the saturated vapor pressure P0 (mmHg) of nitrogen gas to obtain a relative pressure P/P0, and the nitrogen gas adsorption amount for each relative pressure P/P0 is plotted to obtain an adsorption isotherm. A BET plot is calculated from this nitrogen adsorption isotherm and the BET formula, and the specific surface area is obtained using this BET plot. The BET multipoint method is used to calculate the BET plot.

Conductive agents are blended as necessary to improve current collection performance and reduce contact resistance between the active material and the current collector. Examples of conductive agents include carbonaceous materials such as acetylene black, ketjen black, graphite, and coke. One type of conductive agent may be used, or two or more types may be mixed together.

The binder has the function of binding the active material and the conductive agent. As the binder, for example, at least one selected from the group consisting of polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), cellulose-based polymers such as carboxymethyl cellulose (CMC), fluorine-based rubber, styrene butadiene rubber, acrylic resin or its copolymer, polyacrylic acid, and polyacrylonitrile can be used, but is not limited to these. For example, the polymer material contained in the solid electrolyte layer of the separator can be used as the binder. The binder may be one type, or two or more types may be mixed and used.

The compounding ratios of the negative electrode active material, conductive agent, and binder in the negative electrode active material-containing layer are preferably in the ranges of 70% by mass or more and 95% by mass or less, 3% by mass or more and 20% by mass or less, and 2% by mass or more and 10% by mass or less, respectively. When the compounding ratio of the conductive agent is 3% by mass or more, the conductivity of the negative electrode active material-containing layer can be improved, and when it is 20% by mass or less, the decomposition of the aqueous electrolyte on the conductive agent surface can be reduced. When the compounding ratio of the binder is 2% by mass or more, sufficient electrode strength can be obtained, and when it is 10% by mass or less, the insulating part of the electrode can be reduced.

The negative electrode current collector is in contact with the second surface of the negative electrode active material-containing layer. The negative electrode current collector may be in contact with the second surface of one negative electrode active material-containing layer, or may be in contact with each of the second surfaces of two negative electrode active material-containing layers. For example, the negative electrode current collector may be in contact with the second surface of one negative electrode active material-containing layer on one surface and in contact with the second surface of another negative electrode active material-containing layer on the other surface on the back side. In other words, it may include two negative electrode active material-containing layers sandwiched between one negative electrode current collector.

The negative electrode current collector may include at least a portion that is not in contact with the negative electrode active material-containing layer on either the front or back surface of that portion. This portion may function as a current collecting tab. Alternatively, a current collecting tab separate from the negative electrode current collector may be electrically connected to the electrode. In addition, when a negative electrode active material-containing layer is disposed on each of both surfaces of the negative electrode current collector, at least a portion of one surface of the negative electrode current collector may include a portion that is not in contact with the negative electrode active material-containing layer at that portion. For example, a current collecting tab separate from the negative electrode current collector may be connected to this portion.

The material of the negative electrode current collector is a substance that is electrochemically stable in the electrode potential range when the alkali metal ions are inserted or removed. The negative electrode current collector is, for example, a zinc foil, an aluminum foil, or an aluminum alloy foil containing one or more selected from magnesium (Mg), titanium (Ti), zinc (Zn), manganese (Mn), iron (Fe), copper (Cu) and silicon (Si), and is preferably surface-coated with a metal with a high hydrogen overvoltage such as Zn or Sn. The negative electrode current collector may be in other forms such as a porous body or a mesh. The thickness of the negative electrode current collector is preferably 5 μm or more and 20 μm or less. A negative electrode current collector having such a thickness can balance the strength and weight of the electrode.

The negative electrode can be obtained, for example, by the following method. First, a negative electrode active material, a conductive agent, and a binder are suspended in an appropriate solvent to prepare a slurry. This slurry is applied to one or both sides of a negative electrode current collector. The coating on the negative electrode current collector is dried to form a negative electrode active material-containing layer. Thereafter, the negative electrode current collector and the negative electrode active material-containing layer formed thereon are pressed. The negative electrode active material-containing layer may be a mixture of the negative electrode active material, the conductive agent, and the binder formed into a pellet shape.

(Bipolar electrode)
The bipolar electrode has a positive electrode active material-containing layer on one side and a negative electrode active material-containing layer on the other opposing side. The positive electrode active material-containing layer and the negative electrode active material-containing layer are provided on two sides of a bipolar electrode current collector.

The active material-containing layers contained in the positive electrode active material-containing layer and the negative electrode active material-containing layer can be those described above for the positive electrode and negative electrode, respectively.

A secondary battery can have one or more bipolar electrodes. For example, it can have two or more bipolar electrodes.

The bipolar electrode collector has a metal layer a and a metal layer b. The metal layer a and the metal layer b are electrically connected. The metal layer a has a positive electrode active material-containing layer on the surface thereof, and the metal layer b has a negative electrode active material-containing layer on the surface thereof. For example, FIG. 2 is a schematic diagram of a bipolar electrode collector provided in an aqueous electrolyte bipolar secondary battery. In FIG. 2a, the metal layer a 502a and the metal layer b 502b of the bipolar electrode collector 502 are in direct contact with each other. In FIG. 2b, a conductive adhesive layer 503 is present between the metal layer a 502a and the metal layer b 502b of the bipolar electrode collector. In FIG. 2c, a conductive adhesive layer a 503a and a conductive adhesive layer b 503b are present between the metal layer a 502a and the metal layer b 502b. In FIG. 2d, a conductive adhesive layer a 503a, a conductive adhesive layer b 503b, and a conductive adhesive layer c 503c are present between the metal layer a 502a and the metal layer b 502b. Metal layer a and metal layer b of the bipolar electrode collector need only be electrically connected.

The metal layer a contains at least one element selected from the group consisting of Cr, Ni, Fe, Ti, and Al. The metal layer a is in contact with the positive electrode active material-containing layer, and therefore in contact with the aqueous electrolyte used on the positive electrode side (positive electrode-side aqueous electrolyte). By using the above metal element in the metal layer a, it is possible to provide durability against corrosion from the positive electrode-side aqueous electrolyte. The positive electrode-side aqueous electrolyte will be described in detail later.

Since the metal layer a contains at least one element selected from the group consisting of Cr, Ni, Fe, Ti, and Al, the metal layer a may be a metal foil such as Ni foil or Ti foil that is composed of almost one type of metal element, that is, the metal layer a may contain one element selected from the group consisting of Ni, Fe, Ti, and Al at almost 100 wt. % of the weight of each layer of the metal layer a, or may be an alloy foil such as a stainless steel foil, an Al-Mg alloy, or an Al-Zn-Mg alloy, or the surface of the base material may be provided with at least one element selected from the group consisting of Cr, Ni, Fe, Ti, and Al by plating the base material. In other words, it is sufficient that the part of the metal layer a that contacts the aqueous electrolyte is provided with at least one element selected from the group consisting of Cr, Ni, Fe, Ti, and Al. Therefore, it is preferable that at least one element selected from the group consisting of Cr, Ni, Fe, Ti, and Al is present on all of the surfaces of the metal layer a that contact the aqueous electrolyte. To provide the above elements, it is preferable to make it in foil form or to coat it with plating.

The thickness of metal layer a and metal layer b can be changed depending on the metal type of the surface of the metal layer that comes into contact with the aqueous electrolyte. If the hardness of the metal type used is high, the strength of the electrode can be maintained when it is made into a bipolar electrode. Therefore, if the hardness of the metal type used is high, the thickness of metal layer a and metal layer b can be made thinner than the thickness of the bipolar electrode current collector.

The strength of the bipolar electrode needs to be maintained by at least one of the metal layers a and b.

Depending on the type of metal used in metal layer a and its thickness, it is possible to suppress pinholes and cracks that occur in the bipolar electrode current collector, and when the battery is made, it is also possible to reduce bending of the electrode group including the negative electrode, positive electrode, and bipolar electrode due to external impact.

The thickness of the bipolar electrode collector is preferably 10 μm or more and 100 μm or less. If it is more than 100 μm, the energy density decreases, which is not preferable. If it is less than 10 μm, it is difficult to maintain the strength required as a bipolar electrode because the strength is insufficient.

When Al is used for the metal layer a, the thickness of the metal layer a is preferably 9% to 95% of the thickness of the bipolar electrode collector. The use of Al means, for example, the use of a material composed of almost one type of metal element, such as Al foil. Since the thicknesses of the metal layer a and the metal layer b are in a trade-off relationship with respect to the thickness of the bipolar electrode collector, the corrosion resistance of the bipolar electrode collector can be maintained by having the thickness of the metal layer a be 9% to 95% of the thickness of the bipolar electrode collector. If the thickness of the metal layer a is less than 9% of the thickness of the bipolar electrode collector, when the metal layer a peels off due to pressing or the like, the metal layer b is exposed, which is not preferable because the corrosion resistance is reduced. If the thickness of the metal layer a is more than 95% of the thickness of the bipolar electrode collector, when the metal layer b peels off due to pressing or the like, the metal layer a is exposed, which is not preferable because the corrosion resistance is reduced. More preferably, the thickness of metal layer a is 40% to 60% less than the thickness of the bipolar electrode collector.

When the metal layer a contains Cr, Fe, Ni, Ti, Al-Mg alloy, or Al-Mg-Zn alloy, the thickness of the metal layer a is preferably 7% to 95% of the thickness of the bipolar electrode collector. Cr, Fe, Ni, Ti, Al-Mg alloy, and Al-Mg-Zn alloy may be used as foil or plated. Since there is a trade-off between the thickness of the bipolar electrode collector and the thickness of the metal layer a and the metal layer b, the corrosion resistance of the bipolar electrode collector can be maintained by making the thickness of the metal layer a 7% to 95% of the thickness of the bipolar electrode collector. If the thickness of the metal layer a is less than 7% of the thickness of the bipolar electrode collector, when the metal layer a peels off due to pressing, the metal layer b is exposed, and the corrosion resistance is reduced, which is not preferable. If the thickness of metal layer a is greater than 95% of the thickness of the bipolar electrode current collector, when metal layer b peels off due to pressing or the like, metal layer a will be exposed, resulting in a decrease in corrosion resistance, which is undesirable. More preferably, the thickness of metal layer a is 30% to 70% of the thickness of the bipolar electrode current collector. Even more preferably, the thickness of metal layer a is 40% to 60% of the thickness of the bipolar electrode current collector.

When stainless steel is used for the metal layer a, the thickness of the metal layer a is preferably 5% to 95% of the thickness of the bipolar electrode collector. For example, stainless steel foil is used. Since there is a trade-off between the thickness of the bipolar electrode collector and the thickness of the metal layer a, the thickness of the metal layer a is 5% to 95% of the thickness of the bipolar electrode collector, so that the corrosion resistance of the bipolar electrode collector can be maintained. If the thickness of the metal layer a is less than 5% of the thickness of the bipolar electrode collector, when the metal layer a peels off due to pressing or the like, the metal layer b is exposed, which is not preferable because the corrosion resistance is reduced. If the thickness of the metal layer a is more than 95% of the thickness of the bipolar electrode collector, when the metal layer b peels off due to pressing or the like, the metal layer a is exposed, which is not preferable because the corrosion resistance is reduced.

When metal layer a is plated to include at least one element selected from the group consisting of Cr, Ni, Fe, Ti, and Al, the thickness of metal layer a is the sum of the plating thickness and the substrate thickness. The plating thickness is preferably 5% to 95% of the thickness of the bipolar electrode collector. This is because the corrosion resistance of the bipolar electrode collector can be maintained by having the plating thickness in the above range.

The metal layer b contains one of the elements Zn and Sn in an amount of 10 wt. % or more and 100 wt. % or less based on the weight of each layer of the metal layer b. Each layer of the metal layer b refers to the metal layer b of the bipolar electrode current collector of one bipolar electrode. The metal layer b contacts the negative electrode active material-containing layer, and therefore contacts the aqueous electrolyte (negative electrode-side aqueous electrolyte) used on the negative electrode side. The metal layer b contains one of the elements Zn and Sn in an amount of 10 wt. % or more and 100 wt. % or less based on the weight of each layer of the metal layer b, thereby making the metal layer b durable against corrosion from the negative electrode-side aqueous electrolyte. The negative electrode-side aqueous electrolyte will be described in detail later. The metal layer b contains one of the elements Zn and Sn in an amount of at least 10 wt. % or more based on the weight of each layer of the metal layer b. % or more means that the metal layer b is a metal foil such as Zn foil or Sn foil, or an alloy foil, or that the surface of the base material is provided with either Zn or Sn by plating the base material. When plating is applied, the metal layer b is a combination of the plated portion and the base material portion. In other words, it is sufficient that the portion of the metal layer b that contacts the aqueous electrolyte contains either Zn or Sn in an amount of at least 10 wt. % or more relative to the weight of each layer of the metal layer b.

More preferably, it is 15 wt. % or more, and even more preferably, it is 20 wt. % or more. This is because by being in this range, side reactions can be suppressed and corrosion resistance can be further improved.

More preferably, metal layer b is a Cu-Zn alloy or a Cu-Sn alloy. When metal layer b is either a Cu-Zn alloy or a Cu-Sn alloy, corrosion resistance is further improved.

The thickness of metal layer b relative to the bipolar electrode collector can be changed as appropriate according to the type and thickness of metal layer a.

When metal layer a and metal layer b of the bipolar electrode collector are in contact with each other, these two layers may be fabricated as a clad material, or may be a laminate of metal layer a and metal layer b.

The bipolar electrode current collector may include a conductive adhesive layer. The conductive adhesive layer is between metal layer a and metal layer b, and is in contact with metal layer a and metal layer b. The conductive adhesive layer may be provided in multiple layers, such as two or three layers, as shown in Figures 2c and 2d. When there are multiple conductive adhesive layers, the materials used for each layer may be different or the same.

The conductive adhesive layer may be of any type as long as it is conductive and can maintain electrical connection between the metal layer a and the metal layer b. Therefore, the conductive adhesive layer may be made of a conductive resin or a metal. The metal may be, for example, that used in solder. Specifically, a resin with Pb-Zn alloy or conductive carbon dispersed therein is preferred. A conductive resin and a metal may be used as one conductive adhesive layer, or when there are multiple conductive adhesive layers, each layer may be provided with a different type of conductive resin, or each layer may be provided with different types of metals to form a conductive adhesive layer. Multiple conductive adhesive layers may be provided by using a layer with conductive resin and a layer with metal. Also, multiple types of materials may be used in one conductive adhesive layer.

The thickness of the conductive adhesive layer is not particularly important, but it is preferable that the thickness be 0.1 μm or more and 10 μm or less.

This section describes how to measure the thicknesses of metal layer a, metal layer b, and the conductive adhesive layer, and the proportions of elements contained in metal layer a, metal layer b, and the conductive adhesive layer.

(Checking the conductivity of bipolar electrodes)
First, it is confirmed whether the bipolar electrode has conductivity. If it has conductivity, and if there is a layer between the metal layer a and the metal layer b in the subsequent measurement, the layer is determined to be a conductive adhesive layer.

(Method of measuring thickness of metal layer a, metal layer b and conductive adhesive layer)
The cross section is observed and measured using a scanning electron microscope (SEM) at a magnification of, for example, 100 to 5000 times. The cross section is subjected to elemental analysis using energy dispersive X-ray spectroscopy (EDX) to measure the elements and layer thicknesses contained in the metal layer a, the metal layer b, and the conductive adhesive layer.

(Separator)
A separator can be disposed between the positive electrode and the negative electrode. The first separator and the second separator are collectively called the separator.

By constructing the separator from an insulating material, electrical contact between the positive and negative electrodes can be prevented. It is also desirable to use a separator with a shape that allows the electrolyte to move within it.

Examples of separators include nonwoven fabrics, films, and papers. Examples of separator materials include polyolefins such as polyethylene and polypropylene, and cellulose. Examples of preferred separators include nonwoven fabrics containing cellulose fibers and porous films containing polyolefin fibers.

The separator can be either porous or non-porous, but non-porous is preferred. Using a non-porous separator can prevent the aqueous electrolyte from mixing.

The fiber diameter of the separator containing fibers is preferably 10 μm or less. By making the fiber diameter 10 μm or less, the affinity of the separator to the electrolyte is improved, thereby reducing the battery resistance. A more preferable range for the fiber diameter is 3 μm or less.

The separator preferably has a thickness of 20 μm or more and 100 μm or less and a density of 0.2 g/cm ³ or more and 0.9 g/cm ³ or less. In this range, a balance between mechanical strength and reduction in battery resistance can be achieved, and a secondary battery with high output and suppressed internal short circuit can be provided. In addition, the separator suffers little thermal shrinkage in a high-temperature environment, and good high-temperature storage performance can be achieved.

A solid electrolyte layer containing solid electrolyte particles can also be used as the separator. The solid electrolyte layer may contain one type of solid electrolyte particles, or may contain multiple types of solid electrolyte particles. The solid electrolyte layer may be a solid electrolyte composite membrane containing solid electrolyte particles. The solid electrolyte composite membrane is, for example, a membrane formed from solid electrolyte particles using a polymer binder. The solid electrolyte layer may contain at least one selected from the group consisting of a plasticizer and an electrolyte salt. If the solid electrolyte layer contains an electrolyte salt, for example, the alkali metal ion conductivity of the solid electrolyte layer can be further increased.

Examples of inorganic solid particles contained in the solid electrolyte layer include oxide-based ceramics such as alumina, silica, zirconia, yttria, magnesium oxide, calcium oxide, barium oxide, strontium oxide, and vanadium oxide; carbonates and sulfates such as sodium carbonate, potassium carbonate, magnesium carbonate, calcium carbonate, barium carbonate, lanthanum carbonate, cerium carbonate, calcium sulfate, magnesium sulfate, aluminum sulfate, gypsum, and barium sulfate; phosphates such as hydroxyapatite, lithium phosphate, zirconium phosphate, and titanium phosphate; and nitride-based ceramics such as silicon nitride, titanium nitride, and boron nitride. The inorganic particles listed above may be in the form of a hydrate.

The inorganic solid particles preferably include solid electrolyte particles having ionic conductivity for alkali metal ions. Specifically, inorganic solid particles having ionic conductivity for lithium ions and sodium ions are more preferable.

Examples of inorganic solid particles having lithium ion conductivity include oxide-based solid electrolytes and sulfide-based solid electrolytes. As the oxide-based solid electrolyte, it is preferable to use a lithium phosphate solid electrolyte having a NASICON (Sodium (Na) Super Ionic Conductor) type structure and represented by the general formula Li _1+x M ₂ (PO ₄ ) _3. In the general formula, M is, for example, one or more selected from the group consisting of titanium (Ti), germanium (Ge), strontium (Sr), zirconium (Zr), tin (Sn), aluminum (Al), and calcium (Ca). The subscript x is within the range of 0≦x≦2. The ionic conductivity of the lithium phosphate solid electrolyte represented by the general formula LiM ₂ (PO ₄ ) ₃ is, for example, 1×10 ⁻⁵ S/cm or more and 1×10 ⁻³ S/cm or less.

Specific examples of lithium phosphate solid electrolytes having a NASICON structure include an LATP compound represented by Li _1+w Al _w Ti _2-w (PO ₄ ) ₃ where 0.1≦w≦0.5; a compound represented by Li _1+y Al _z M1 _2-z (PO ₄ ) ₃ where M1 is one or more selected from the group consisting of Ti, Ge, Sr, Zr, Sn, and Ca, and 0≦y≦1 and 0≦z≦1; a compound represented by Li _1+x Al _x Ge _2-x (PO ₄ ) ₃ where 0≦x≦2; and a compound represented by Li _1+x Al _x Zr _2-x (PO ₄ ) ₃ where 0≦x≦2; Li _1+u+v Al _u Mα _2-u Si _v P _3-v O ₁₂ , where Mα is 1 or more selected from the group consisting of Ti and Ge, and 0<u≦2, 0≦v<3; and a compound represented by Li1 ₊ 2tZr1 _{-tCat ( PO4} ) ₃ , where 0≦t<1. _{Li1+2tZr1 - tCat} ( _PO4 ) ₃ is preferably used as inorganic solid electrolyte particles because it has high water resistance, low reducing properties, and low cost.

Examples of oxide-based solid electrolytes include, in addition to the lithium phosphate solid electrolyte, amorphous LIPON compounds (e.g., Li _2.9 PO _3.3 N _0.46 ) represented by Li _p PO _q N _r , where 2.6≦p≦3.5, 1.9≦q≦3.8, and 0.1≦r≦1.3; compounds having a garnet structure represented by La _{5+s As} La _3-s Mβ ₂ O _12, where A is 1 or more selected from the group consisting of Ca, Sr, and Ba, Mβ is 1 or more selected from the group consisting of Nb and Ta, and 0≦s≦0.5; compounds represented by Li ₃ Mγ _2-s L _{2 O 12} , where Mγ is 1 or more selected from the group consisting of Ta and Nb, L may contain Zr, and 0≦ _s ≦0.5; and a LLZ compound (e.g., Li 7 _{La 3} Zr ₂ O ₁₂ ) represented by Li _5+x La ₃ M2 _2-x Zr _x O ₁₂ , where M2 is 1 or more selected from the group consisting of Nb and Ta, and 0≦x≦2. The solid electrolyte may be one type, or two or more types may be mixed and used. The ionic conductivity of LIPON is, for example, ₁ × _{10 −6} S/ _cm or more and 5×10 ⁻⁶ S/cm or less. The ionic conductivity of LLZ is, for example, 1× ^{10 −4 S} /cm or more and 5×10 ⁻⁴ S/cm or less.

As the inorganic solid particles having ionic conductivity of sodium ions, a sodium-containing solid electrolyte may be used. The sodium-containing solid electrolyte has excellent ionic conductivity of sodium ions. Examples of the sodium-containing solid electrolyte include β-alumina, sodium phosphorus sulfide, and sodium phosphate. The sodium-ion-containing solid electrolyte is preferably in the form of glass ceramics.

The inorganic solid particles are preferably a solid electrolyte having a lithium ion conductivity of 1×10 ⁻⁵ S/cm or more at 25° C. The lithium ion conductivity can be measured, for example, by an AC impedance method, as will be described in detail later.

The shape of the inorganic solid particles is not particularly limited, but may be, for example, spherical, elliptical, flat, or fibrous.

The average particle size of the inorganic solid particles is preferably 15 μm or less, and more preferably 12 μm or less. If the average particle size of the inorganic solid particles is small, the density of the solid electrolyte layer can be increased.

The average particle size of the inorganic solid particles is preferably 0.01 μm or more, and more preferably 0.1 μm or more. If the average particle size of the inorganic solid particles is large, aggregation of the particles tends to be suppressed.

The average particle size of inorganic solid particles means the particle size at which the volumetric cumulative value is 50% in the particle size distribution determined by a laser diffraction particle size distribution measuring device. The sample used for this particle size distribution measurement is a dispersion diluted with ethanol so that the concentration of inorganic solid particles is 0.01% by mass to 5% by mass.

In the solid electrolyte layer, inorganic solid particles are preferably the main component. From the viewpoint of increasing the ionic conductivity of the solid electrolyte layer, the proportion of inorganic solid particles in the solid electrolyte layer is preferably 70 mass% or more, more preferably 80 mass% or more, and even more preferably 85 mass% or more. From the viewpoint of increasing the membrane strength of the solid electrolyte layer, the proportion of inorganic solid particles in the solid electrolyte layer is preferably 98 mass% or less, more preferably 95 mass% or less, and even more preferably 90 mass% or less. The proportion of inorganic solid particles in the solid electrolyte layer can be calculated by thermogravimetric analysis (TG) analysis.

The polymer material contained in the solid electrolyte layer enhances the adhesion between inorganic solid particles. The weight average molecular weight of the polymer material is, for example, 3000 or more. When the weight average molecular weight of the polymer material is 3000 or more, the adhesion between the inorganic solid particles can be further enhanced. The weight average molecular weight of the polymer material is preferably 3000 or more and 5,000,000 or less, more preferably 5000 or more and 2,000,000 or less, and even more preferably 10,000 or more and 1,000,000 or less. The weight average molecular weight of the polymer material can be determined by gel permeation chromatography (GPC).

The polymeric material may be a polymer made of a single monomer unit, a copolymer made of multiple monomer units, or a mixture thereof. The polymeric material preferably contains a monomer unit made of a hydrocarbon having a functional group containing one or more selected from the group consisting of oxygen (O), sulfur (S), nitrogen (N), and fluorine (F). In the polymeric material, the proportion of the portion made of the monomer unit is preferably 70 mol % or more. Hereinafter, this monomer unit is referred to as a first monomer unit. In addition, in the copolymer, the unit other than the first monomer unit is referred to as a second monomer unit. The copolymer of the first monomer unit and the second monomer unit may be an alternating copolymer, a random copolymer, or a block copolymer.

If the proportion of the portion composed of the first monomer unit in the polymer material is less than 70 mol %, the water-blocking properties of the first and second solid electrolyte layers may be reduced. In the polymer material, the proportion of the portion composed of the first monomer unit is preferably 90 mol % or more. It is most preferable that the proportion of the portion composed of the first monomer unit is 100 mol %, that is, the polymer is composed only of the first monomer unit.

The first monomer unit may be a compound having a functional group in a side chain containing one or more elements selected from the group consisting of oxygen (O), sulfur (S), nitrogen (N), and fluorine (F), and a main chain formed by a carbon-carbon bond. The hydrocarbon may have one or more functional groups containing one or more elements selected from the group consisting of oxygen (O), sulfur (S), nitrogen (N), and fluorine (F). The functional group in the first monomer unit increases the conductivity of alkali metal ions passing through the solid electrolyte layer.

The hydrocarbon constituting the first monomer unit preferably has a functional group containing one or more selected from the group consisting of oxygen (O), sulfur (S), and nitrogen (N). When the first monomer unit has such a functional group, the conductivity of the alkali metal ions in the solid electrolyte layer tends to be higher and the internal resistance tends to be lower.

The functional group contained in the first monomer unit preferably contains one or more selected from the group consisting of a formal group, a butyral group, a carboxymethyl ester group, an acetyl group, a carbonyl group, a hydroxyl group, and a fluoro group. Moreover, the first monomer unit more preferably contains at least one of a carbonyl group and a hydroxyl group in the functional group, and even more preferably contains both.

The first monomer unit can be represented by the following formula:

In the above formula, R1 is preferably selected from the group consisting of hydrogen (H), an alkyl group, and an amino group, and R2 is preferably selected from the group consisting of a hydroxyl group (-OH), -OR1, -COOR1, -OCOR1, -OCH(R1)O-, -CN, -N(R1) ₃ , and SO ₂ R1.

The first monomer unit may be, for example, one or more selected from the group consisting of vinyl formal, vinyl alcohol, vinyl acetate, vinyl acetal, vinyl butyral, acrylic acid and its derivatives, methacrylic acid and its derivatives, acrylonitrile, acrylamide and its derivatives, styrene sulfonic acid, polyvinylidene fluoride, and tetrafluoroethylene.

The polymer material preferably includes one or more selected from the group consisting of polyvinyl formal, polyvinyl alcohol, polyvinyl acetal, polyvinyl butyral, polymethyl methacrylate, polyvinylidene fluoride, and polytetrafluoroethylene.

Below is an example of the structural formula of a compound that can be used as a polymer material.

The structural formula of polyvinyl formal is as follows: In the following formula, it is preferable that a is 50 or more and 80 or less, b is 0 or more and 5 or less, and c is 15 or more and 50 or less.

The structural formula of polyvinyl butyral is as follows: In the following formula, it is preferable that l is 50 or more and 80 or less, m is 0 or more and 10 or less, and n is 10 or more and 50 or less.

The structural formula of polyvinyl alcohol is as follows: In the following formula, n is preferably 70 or more and 20,000 or less.

The structural formula of polymethyl methacrylate is as follows: In the following formula, n is preferably 30 or more and 10,000 or less.

The second monomer unit is a compound other than the first monomer unit, i.e., a compound that does not have a functional group containing one or more selected from the group consisting of oxygen (O), sulfur (S), nitrogen (N), and fluorine (F), or a compound that has such a functional group but is not a hydrocarbon. Examples of the second monomer unit include ethylene oxide and styrene. Examples of the polymer composed of the second monomer unit include polyethylene oxide (PEO) and polystyrene (PS).

The types of functional groups contained in the first monomer unit and the second monomer unit can be identified by infrared spectroscopy (Fourier Transform Infrared Spectroscopy; FT-IR). Furthermore, it can be determined by nuclear magnetic resonance (NMR) that the first monomer unit is composed of a hydrocarbon. Furthermore, the proportion of the portion composed of the first monomer unit in a copolymer of the first monomer unit and the second monomer unit can be calculated by NMR.

The polymer material may contain an electrolyte. The proportion of electrolytes that may be contained in a polymer material can be determined from its water absorption rate. Here, the water absorption rate of a polymer material is the value obtained by subtracting the mass Mp of the polymer material before immersion in water at a temperature of 23°C for 24 hours from the mass Mp' of the polymer material after immersion in water at a temperature of 23°C for 24 hours, and dividing the result by the mass Mp of the polymer material before immersion ([Mp'-Mp]/Mp x 100). The water absorption rate of a polymer material is thought to be related to the polarity of the polymer material.

Other examples of polymeric binders include polyvinyl-based, polyether-based, polyester-based, polyamine-based, polyethylene-based, silicone-based and polysulfide-based.

When a polymeric material with high water absorption is used, the alkali metal ion conductivity of the solid electrolyte layer tends to increase. In addition, when a polymeric material with high water absorption is used, the binding strength between the inorganic solid particles and the polymeric material is increased, so that the flexibility of the solid electrolyte layer can be increased. The water absorption of the polymeric material is preferably 0.01% or more, more preferably 0.5% or more, and even more preferably 2% or more.

The strength of the solid electrolyte layer can be increased by using a polymer material with low water absorption. That is, if the polymer material has too high a water absorption rate, the solid electrolyte layer may swell due to the electrolyte. Also, if the polymer material has too high a water absorption rate, the polymer material in the solid electrolyte layer may flow into the electrolyte. The water absorption rate of the polymer material is preferably 15% or less, more preferably 10% or less, even more preferably 7% or less, and particularly preferably 3% or less.

From the viewpoint of increasing the flexibility of the solid electrolyte layer, the proportion of the polymer material in the solid electrolyte layer is preferably 1% by mass or more, more preferably 3% by mass or more, and even more preferably 10% by mass or more. Furthermore, the higher the proportion of the polymer material, the more likely it is that the solid electrolyte layer will have a high density.

In addition, from the viewpoint of increasing the lithium ion conductivity of the solid electrolyte layer, the proportion of the polymer material in the solid electrolyte layer is preferably 20 mass% or less, more preferably 10 mass% or less, and even more preferably 5 mass% or less. The proportion of the polymer material in the solid electrolyte layer can be calculated by thermogravimetry (TG) analysis.

The separator may be composed of only a solid electrolyte layer, or may be composed of a substrate layer and a solid electrolyte layer. The substrate layer may support a solid electrolyte layer (first solid electrolyte layer) on one side. Alternatively, the substrate layer may support a first solid electrolyte layer and a second solid electrolyte layer on both sides. When the first solid electrolyte layer and the second solid electrolyte layer are provided on both the front and back sides of the substrate layer, respectively, the thicknesses of the solid electrolyte layers may be the same or different. When the thicknesses are different, it is preferable that the first solid electrolyte layer in contact with the first side of the negative electrode active material-containing layer is thicker than the second solid electrolyte layer on the other side. By providing the first solid electrolyte layer with a larger thickness, water impermeability can be improved. In addition, the solid electrolyte layers provided on both the front and back sides of the substrate layer may have the same configuration or different configurations.

When the first solid electrolyte layer and the second solid electrolyte layer are included, the polymer materials included in each solid electrolyte layer may be the same as each other, or different types may be used. In addition, a single type of polymer material may be used, or multiple types may be mixed and used.

From the viewpoint of preventing internal short circuits, the thickness of the solid electrolyte layer is preferably 3 μm or more, more preferably 5 μm or more, and even more preferably 7 μm or more. From the viewpoint of increasing ion conductivity and energy density, the thickness of the solid electrolyte layer is preferably 50 μm or less, more preferably 30 μm or less, and even more preferably 20 μm or less.

The above thicknesses are preferred for each of the first solid electrolyte layer and the second solid electrolyte layer, but both solid electrolyte layers do not necessarily need to meet these requirements.

The substrate layer is, for example, a nonwoven fabric or a self-supporting porous membrane. Examples of materials used for the nonwoven fabric or self-supporting porous membrane include polyethylene (PE), polypropylene (PP), cellulose, or polyvinylidenefluoride (PVdF). The substrate layer is preferably a cellulose nonwoven fabric.

The substrate layer can contain many pores and can be impregnated with a large amount of electrolyte. The substrate layer typically does not contain inorganic solid particles. For example, the proportion of the area of the cross section of the substrate layer that is occupied by inorganic solid particles can be 5% or less.

The thickness of the substrate layer is, for example, 1 μm or more, and preferably 3 μm or more. If the substrate layer is thick, the mechanical strength of the separator increases, and the secondary battery is less likely to have an internal short circuit. The thickness of the substrate layer is, for example, 30 μm or less, and preferably 10 μm or less. If the substrate layer is thin, the internal resistance of the secondary battery decreases, and the volumetric energy density of the secondary battery tends to increase. The thickness of the substrate layer can be measured, for example, by a scanning electron microscope.

The electrolyte salt to be contained in the solid electrolyte layer can be a lithium salt and/or a sodium salt that can be contained in an aqueous electrolyte.

The proportion of electrolyte salt in the solid electrolyte layer is preferably 0.01% by mass or more and 10% by mass or less, and more preferably 0.05% by mass or more and 5% by mass or less. The proportion of electrolyte salt in the solid electrolyte layer can be calculated by thermogravimetry (TG) analysis.

The fact that the solid electrolyte layer contains electrolyte salt can be confirmed, for example, by the distribution of alkali metal ions obtained by energy dispersive X-ray spectrometry (EDX) on a cross section of the solid electrolyte layer. That is, when the solid electrolyte layer is made of a material that does not contain electrolyte salt, the alkali metal ions remain on the surface layer of the polymer material in the solid electrolyte layer, and are hardly present inside the solid electrolyte layer. Therefore, a concentration gradient can be observed in which the concentration of alkali metal ions is high on the surface layer of the solid electrolyte layer and low inside the solid electrolyte layer. On the other hand, when the solid electrolyte layer is made of a material that contains electrolyte salt, it can be confirmed that the alkali metal ions are uniformly present even inside the solid electrolyte layer.

Alternatively, when the electrolyte salt contained in the solid electrolyte layer and the electrolyte salt contained in the aqueous electrolyte are different types, the difference in the types of ions present indicates that the solid electrolyte layer contains an electrolyte salt different from that of the aqueous electrolyte. For example, when lithium chloride (LiCl) is used as the aqueous electrolyte and LiTFSI (lithium bis(fluorosulfonyl)imide) is used in the solid electrolyte layer, the presence of (fluorosulfonyl)imide ions can be confirmed in the solid electrolyte layer. On the other hand, the presence of (fluorosulfonylimide) ions cannot be confirmed in the aqueous electrolyte, or they are present at extremely low concentrations.

The separator may have an air permeability coefficient of 1×10 ⁻¹³ m ² or less. For example, the air permeability coefficient of a nonwoven fabric containing cellulose fibers is about 5×10 ⁻¹⁴ m ^2. The air permeability coefficient of the solid electrolyte composite membrane may be 1×10 ⁻¹⁶ m ² or less, for example, 1×10 ⁻¹⁷ m ^2. By using a separator with a low air permeability coefficient, when different aqueous electrolytes are used on the positive electrode side and the negative electrode side, the mixing of these aqueous electrolytes can be further suppressed. Even when the separator includes a substrate layer, the air permeability coefficient of the substrate layer is larger than that of the solid electrolyte layer, so that the air permeability coefficient of the solid electrolyte layer can be measured without separating the solid electrolyte layer from the substrate layer.

The air permeability coefficient KT (m ² ) of a separator can be calculated as follows. In calculating the air permeability coefficient KT, for example, when a separator with a thickness L (m) is the measurement target, a gas with a dynamic viscosity coefficient σ (Pa·s) is allowed to permeate within a range of a measurement area A (m ² ). In this case, the gas is allowed to permeate under a plurality of conditions in which the pressure p (Pa) of the gas introduced is different from one another, and the amount of gas Q (m ³ /s) that permeates the separator is measured under each of the plurality of conditions. Then, from the measurement results, the amount of gas Q against the pressure p is plotted, and the slope dQ/dp is obtained. Then, the air permeability coefficient KT is calculated from the thickness L, the measurement area A, the dynamic viscosity coefficient σ, and the slope dQ/dp according to formula (1):
KT = ((σ・l)/A)×(dQ/dp) (1)

In one example of a method for calculating the air permeability coefficient KT, a separator is sandwiched between a pair of stainless steel plates, each with a hole of 10 mm in diameter. Air is then pumped through the hole in one of the stainless steel plates at a pressure p. The amount of air Q leaking through the hole in the other stainless steel plate is then measured. Therefore, the area of the hole (25πmm ² ) is used as the measurement area A, and 0.000018 Pa·s is used as the dynamic viscosity coefficient σ. The amount of air Q is calculated by measuring the amount δ (m ³ ) leaking through the hole in 100 seconds and dividing the measured amount δ by 100.

Then, the amount of gas Q with respect to the pressure p is measured as described above at four points where the pressure p is at least 1000 Pa apart from each other. For example, the amount of gas Q with respect to the pressure p is measured at each of four points where the pressure p is 1000 Pa, 2500 Pa, 4000 Pa, and 6000 Pa. Then, the amount of gas Q with respect to the pressure p is plotted for the four measured points, and the slope (dQ/dp) of the amount of gas Q with respect to the pressure p is calculated by linear fitting (the least squares method). The calculated slope (dQ/dp) is then multiplied by (σ·L)/A to calculate the air permeability coefficient KT.

When measuring the air permeability coefficient of the separator, it is disassembled from the battery and the separator is separated from the other components of the battery. After rinsing both sides of the separator with pure water, it is immersed in pure water and left to stand for 48 hours or more. After that, both sides are further rinsed with pure water and dried in a vacuum drying oven at 100°C for 48 hours or more before measuring the air permeability coefficient. The air permeability coefficient is also measured at multiple arbitrary locations on the separator. The value at the location with the lowest air permeability coefficient among the multiple arbitrary locations is regarded as the air permeability coefficient of the separator.

(Method for measuring lithium ion conductivity of inorganic solid particles)
Measurement of the lithium ion conductivity of inorganic solid particles by the AC impedance method will be described. First, the inorganic solid particles are molded using a tablet molding machine to obtain a powder compact. Gold (Au) is deposited on both sides of this powder compact to obtain a measurement sample. The AC impedance of the measurement sample is measured using an impedance measuring device. For example, a Solartron Frequency Response Analyzer Model 1260 can be used as the measurement device. The measurement is performed at a measurement frequency of 5 Hz to 32 MHz, at a measurement temperature of 25° C., and in an argon atmosphere.

Next, a complex impedance plot is created based on the measured AC impedance. The complex impedance plot is a plot in which the horizontal axis represents the real component and the vertical axis represents the imaginary component. The ionic conductivity σLi of the inorganic solid particles is calculated by the following formula (2). In the formula, ZLi is a resistance value calculated from the diameter of an arc of the complex impedance plot, S is an area, and d is a thickness.
σLi=(1/ZLi)×(d/S) (2)

(Aqueous electrolyte)
The aqueous electrolyte contains an aqueous solvent and an electrolyte salt. The aqueous electrolyte is, for example, liquid. The liquid aqueous electrolyte is prepared by dissolving an electrolyte salt as a solute in an aqueous solvent.

The aqueous electrolyte includes a positive electrode aqueous electrolyte that contacts metal layer a, and a negative electrode aqueous electrolyte that contacts metal layer b, and the positive electrode aqueous electrolyte and the negative electrode aqueous electrolyte are different. "Different" here indicates that the electrolyte salts, electrolyte salt concentrations, and pHs contained in the positive electrode aqueous electrolyte and the negative electrode aqueous electrolyte are different. Unless otherwise specified, the positive electrode aqueous electrolyte and the negative electrode aqueous electrolyte are collectively referred to simply as "aqueous electrolytes."

As the electrolyte salt, for example, a lithium salt, a sodium salt, or a mixture thereof is used. One type or two or more types of electrolyte salts can be used.

Examples of lithium salts that can be used include lithium chloride (LiCl), lithium bromide (LiBr), lithium hydroxide (LiOH), lithium sulfate (Li2SO4), lithium nitrate (LiNO3), lithium acetate ( _CH3COOLi ), lithium oxalate ( _{Li2C2O4 )} , lithium carbonate ( _Li2CO3 ), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI; LiN(SO2CF3)2) _, lithium bis(fluorosulfonyl)imide (LiFSI; LiN(SO2F)2), and lithium bisoxalate borate (LiBOB: LiB[(OCO)2]2).

Examples of sodium salts that can be used include sodium chloride (NaCl), sodium sulfate (Na2SO4), sodium hydroxide (NaOH), sodium nitrate (NaNO3), and sodium trifluoromethanesulfonylamide (NaTFSA).

The lithium salt preferably contains LiCl. The use of LiCl can increase the lithium ion concentration in the aqueous electrolyte. The lithium salt preferably contains at least one of LiSO4 and LiOH in addition to LiCl.

In addition to the lithium salt, a zinc salt such as zinc chloride or zinc sulfate may be added to the aqueous electrolyte. By adding such a compound to the aqueous electrolyte solution, a zinc-containing coating layer and/or an oxidized zinc-containing region can be formed on the negative electrode.

The molar concentration of lithium ions in the aqueous electrolyte is preferably 3 mol/L or more, more preferably 6 mol/L or more, and even more preferably 12 mol/L or more. If the concentration of lithium ions in the aqueous electrolyte is high, electrolysis of the aqueous solvent at the negative electrode is easily suppressed, and hydrogen generation from the negative electrode tends to be low.

The aqueous electrolyte preferably contains 1 mol or more of aqueous solvent per mol of solute salt. More preferably, the amount of aqueous solvent per mol of solute salt is 3.5 mol or more. The aqueous solvent contains, for example, water at a ratio of 50% by volume or more. The aqueous electrolyte preferably contains at least one anion selected from chloride ions (Cl ⁻ ), hydroxide ions (OH ⁻ ), sulfate ions (SO ₄ ²⁻ ), and nitrate ions (NO ₃ ⁻ ).

The pH of the aqueous electrolyte is preferably 3 or more and 14 or less, and more preferably 4 or more and 13 or less.

If the pH of the aqueous electrolyte differs between the negative electrode side and the positive electrode side after the initial charge, in the secondary battery after the initial charge, the pH of the aqueous electrolyte on the negative electrode side is preferably 3 or more, more preferably 5 or more, and even more preferably 7 or more. In addition, in the secondary battery after the initial charge, the pH of the aqueous electrolyte on the positive electrode side is preferably in the range of 0 to 7, and more preferably in the range of 0 to 6.

The pH of the aqueous electrolyte on the negative and positive electrode sides can be obtained, for example, by dismantling the secondary battery and measuring the pH of the aqueous electrolyte present between the separator and the negative and positive electrodes, respectively.

As the aqueous solvent, a solution containing water can be used. The aqueous solution may be pure water or a mixed solvent of water and an organic solvent.

The aqueous electrolyte may be a gel electrolyte. The gel electrolyte is prepared by mixing the above-mentioned liquid aqueous electrolyte with a polymer compound to form a composite. Examples of the polymer compound include polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), and polyethylene oxide (PEO). The aqueous electrolyte contained in the first positive electrode active material-containing layer and the second positive electrode active material-containing layer, and the aqueous electrolyte contained in the first negative electrode active material-containing layer and the second negative electrode active material-containing layer can be appropriately selected to be either a liquid or a gel electrolyte.

The presence of water in an aqueous electrolyte can be confirmed by GC-MS (Gas Chromatography-Mass Spectrometry) measurement. The salt concentration and water content in an aqueous electrolyte can be measured, for example, by ICP (Inductively Coupled Plasma) emission spectrometry. The molar concentration (mol/L) can be calculated by weighing a specified amount of aqueous electrolyte and calculating the salt concentration contained therein. The number of moles of solute and solvent can also be calculated by measuring the specific gravity of the aqueous electrolyte.

(Method of measuring pH of aqueous electrolyte)
The method for measuring the pH of the aqueous electrolyte is as follows.

The secondary battery is disassembled, the aqueous electrolyte contained in the electrodes and separators is extracted, the volume of the liquid is measured, and the pH value is measured with a pH meter. The pH measurement is performed, for example, as follows. For this measurement, for example, an F-74 made by Horiba Ltd. is used, and the measurement is performed in an environment of 25±2°C. First, standard solutions of pH 4.0, 7.0, and 9.0 are prepared. Next, the F-74 is calibrated using these standard solutions. An appropriate amount of the aqueous electrolyte (aqueous electrolyte solution) to be measured is prepared and placed in a container, and the pH is measured. After the pH measurement, the sensor part of the F-74 is cleaned. When measuring a different object, the above-mentioned procedure, i.e., calibration, measurement, and cleaning, is performed each time.

The secondary battery according to the first embodiment may further include an electrode group including a negative electrode, a positive electrode, and a bipolar electrode, and an outer layer member that contains an aqueous electrolyte.

(Outer layer member)
For the outer layer member in which the electrode group and the aqueous electrolyte are housed, a metal container, a laminate film container, or a resin container can be used.

Metal containers that can be used are rectangular or cylindrical metal cans made of nickel, iron, stainless steel, etc. Resin containers that can be used are made of polyethylene or polypropylene, etc.

The thickness of each of the resin container and the metal container is preferably in the range of 0.05 mm to 1 mm. The thickness is more preferably 0.5 mm or less, and even more preferably 0.3 mm or less.

Examples of laminate films include multilayer films in which a metal layer is coated with a resin layer. Examples of metal layers include stainless steel foil, aluminum foil, and aluminum alloy foil. Polymers such as polypropylene (PP), polyethylene (PE), nylon, and polyethylene terephthalate (PET) can be used for the resin layer. The thickness of the laminate film is preferably in the range of 0.01 mm or more and 0.5 mm or less. The thickness of the laminate film is more preferably 0.2 mm or less.

Secondary batteries can be used in a variety of shapes, including rectangular, cylindrical, flat, thin, and coin-shaped.

(Terminal)
The negative electrode terminal can be formed from a material that is electrochemically stable and conductive in a potential range of 1 V to 3 V with respect to the oxidation-reduction potential of lithium (vs. Li/Li ⁺ ). Specifically, the material of the negative electrode terminal can be zinc, copper, nickel, stainless steel, aluminum, or an aluminum alloy containing at least one element selected from the group consisting of Mg, Ti, Zn, Mn, Fe, Cu, and Si. As the material of the negative electrode terminal, zinc or a zinc alloy is preferably used. The negative electrode terminal is preferably made of the same material as the negative electrode collector in order to reduce the contact resistance with the negative electrode collector (for example, the current collecting layer included in the negative electrode structure).

The positive electrode terminal can be formed from a material that is electrically stable and conductive in a potential range of 2.5 V to 4.5 V (vs. Li/Li ⁺ ) relative to the oxidation-reduction potential of lithium. Examples of the material for the positive electrode terminal include titanium, aluminum, and aluminum alloys containing at least one element selected from the group consisting of Mg, Ti, Zn, Mn, Fe, Cu, and Si. The positive electrode terminal is preferably formed from the same material as the positive electrode collector in order to reduce the contact resistance with the positive electrode collector.

The secondary battery according to the embodiment will be described in detail below with reference to Figs. 3 and 4. Fig. 3 is a cross-sectional view showing an example of a secondary battery according to the embodiment. Fig. 4 is a cross-sectional view taken along line IV-IV of the secondary battery shown in Fig. 3.

The electrode group 1 is housed in an exterior member 2 made of a rectangular cylindrical metal container. The electrode group 1 includes a negative electrode 3, a positive electrode 5, and a bipolar electrode. The electrode group 1 has a structure in which a separator 4 is arranged between the positive electrode 5, the negative electrode 3, and the bipolar electrode, and the electrode is wound in a spiral shape to form a flat shape. An aqueous electrolyte (not shown) is held in the electrode group 1. As shown in FIG. 3, a strip-shaped negative electrode current collector tab 16 is electrically connected to each of multiple points on the end of the negative electrode 3 located on the end face of the electrode group 1. In addition, a strip-shaped positive electrode current collector tab 17 is electrically connected to each of multiple points on the end of the positive electrode 5 located on this end face. The multiple negative electrode current collector tabs 16 are connected to the negative electrode terminal 6 in a bundled state as shown in FIG. 4. In addition, although not shown, the positive electrode current collector tabs 17 are also electrically connected to the positive electrode terminal 7 in a bundled state.

The metal sealing plate 10 is fixed to the opening of the metal exterior member 2 by welding or the like. The negative electrode terminal 6 and the positive electrode terminal 7 are each pulled out to the outside through an extraction hole provided in the sealing plate 10. A negative electrode gasket 8 and a positive electrode gasket 9 are arranged on the inner circumferential surface of each extraction hole of the sealing plate 10 to prevent short circuits due to contact with the negative electrode terminal 6 and the positive electrode terminal 7, respectively. By arranging the negative electrode gasket 8 and the positive electrode gasket 9, the airtightness of the secondary battery 100 can be maintained.

A control valve 11 (safety valve) is arranged on the sealing plate 10. When the internal pressure in the battery cell increases due to gas generated by electrolysis of the aqueous solvent, the generated gas can be released to the outside from the control valve 11. The control valve 11 can be, for example, a reset type that operates when the internal pressure becomes higher than a set value and functions as a sealing plug when the internal pressure decreases. Alternatively, a non-reset type control valve that does not recover its function as a sealing plug once it is activated may be used. In FIG. 3, the control valve 11 is arranged in the center of the sealing plate 10, but the control valve 11 may be located at the end of the sealing plate 10. The control valve 11 may be omitted.

The sealing plate 10 is also provided with a liquid inlet 12. The aqueous electrolyte can be injected through this liquid inlet 12. After the aqueous electrolyte is injected, the liquid inlet 12 can be closed by a sealing plug 13. The liquid inlet 12 and the sealing plug 13 may be omitted.

Figure 5 is a partially cutaway perspective view that shows a schematic diagram of another example of a secondary battery according to an embodiment. Figure 6 is an enlarged cross-sectional view of part B of the secondary battery shown in Figure 5. Figures 5 and 6 show an example of a secondary battery 100 that uses an exterior member made of a laminate film as a container.

The secondary battery 100 shown in Figs. 5 and 6 includes an electrode group 1 shown in Figs. 5 and 6, an exterior member 2 shown in Fig. 5, and an aqueous electrolyte (not shown). The electrode group 1 and the aqueous electrolyte are housed in the exterior member 2. The aqueous electrolyte is held in the electrode group 1.

The exterior member 2 is made of a laminate film that includes two resin layers and a metal layer interposed between them.

Figure 6 is a schematic diagram of an aqueous electrolyte bipolar secondary battery with multiple bipolar electrodes stacked on top of each other.

The aqueous electrolyte bipolar secondary battery, which is made by stacking multiple bipolar electrodes, has a bipolar electrode 501 having a structure in which a negative electrode active material-containing layer 3b2, a bipolar electrode current collector 502, and a positive electrode active material-containing layer 5b2 are stacked in this order, and multiple separators 4 arranged to face the negative electrode active material-containing layer 3b2 and the positive electrode active material-containing layer 5b2 of the bipolar electrode 501, and the outermost layer includes a positive electrode active material-containing layer 5b1 supported on the positive electrode current collector 5a and a negative electrode active material-containing layer 3b1 supported on the negative electrode current collector 3a.

The positive electrode collector and the negative electrode collector, which are the outermost layers when stacked, are not in contact with the positive electrode active material-containing layer and the negative electrode active material-containing layer, and have portions that are pulled out. These portions become leads. Although not shown, these leads are electrically connected to the strip-shaped negative electrode terminal 6 and the strip-shaped positive electrode terminal 7. The tip of the strip-shaped negative electrode terminal 6 is pulled out to the outside of the exterior member 2. The tip of the strip-shaped positive electrode terminal 7 is located on the opposite side to the negative electrode terminal 6, and is pulled out to the outside of the exterior member 2.

The aqueous electrolyte bipolar secondary battery according to the first embodiment is an aqueous electrolyte bipolar secondary battery comprising: an anode having a first anode active material-containing layer provided on at least one surface of an anode current collector; a cathode having a first cathode active material-containing layer provided on at least one surface of a cathode current collector; a bipolar electrode having a bipolar electrode current collector having a second cathode active material-containing layer provided on one surface and a second anode active material-containing layer provided on the other opposing surface; and an aqueous electrolyte, wherein the bipolar electrode current collector has a metal layer a and a metal layer b, the metal layer a contains at least one element selected from the group consisting of Cr, Ni, Fe, Ti, and Al, and the metal layer b contains 10 wt. % or more and 100 wt. % or more of metal layer b relative to the weight of each layer. % or less of either one of the elements Zn and Sn, the metal layer a and the metal layer b are electrically connected, a second positive electrode active material-containing layer is provided on the surface of the metal layer a, and a second negative electrode active material-containing layer is provided on the surface of the metal layer b. Therefore, the bipolar electrode collector can be matched to the type of positive electrode side aqueous electrolyte and the negative electrode side aqueous electrolyte, so that the secondary battery has excellent life characteristics.

Second Embodiment
According to a second embodiment, there is provided a battery pack including a plurality of secondary batteries according to the first embodiment.

In the battery pack according to the embodiment, the individual cells may be electrically connected in series or parallel, or may be arranged in a combination of series and parallel connections.

Next, an example of a battery pack will be described with reference to the drawings.

Figure 7 is a perspective view that shows a schematic diagram of an example of a battery pack according to an embodiment. The battery pack 200 shown in Figure 7 includes five cells 100a to 100e, four bus bars 21, a positive electrode lead 22, and a negative electrode lead 23. Each of the five cells 100a to 100e is a secondary battery according to the first embodiment.

The bus bar 21 connects, for example, the negative terminal 6 of one cell 100a to the positive terminal 7 of the adjacent cell 100b. In this way, the five cells 100 are connected in series by the four bus bars 21. That is, the battery pack 200 in FIG. 7 is a five-cell battery pack. Although no example is shown, in a battery pack including a plurality of cells electrically connected in parallel, for example, the plurality of negative terminals are connected to each other by a bus bar and the plurality of positive terminals are connected to each other by a bus bar, so that the plurality of cells are electrically connected.

The positive terminal 7 of at least one of the five cells 100a to 100e is electrically connected to a positive lead 22 for external connection. The negative terminal 6 of at least one of the five cells 100a to 100e is electrically connected to a negative lead 23 for external connection.

The battery pack according to the embodiment includes the secondary battery according to the embodiment. Therefore, the battery pack can exhibit excellent life characteristics.

[Third embodiment]
According to a third embodiment, there is provided a battery pack including the secondary battery according to the first embodiment. This battery pack can include the battery assembly according to the second embodiment. This battery pack may include a single secondary battery according to the first embodiment instead of the battery assembly according to the second embodiment.

Such a battery pack may further include a protection circuit. The protection circuit has a function of controlling charging and discharging of the secondary battery. Alternatively, a circuit included in a device that uses the battery pack as a power source (e.g., electronic equipment, automobiles, etc.) may be used as the protection circuit for the battery pack.

The battery pack may further include an external terminal for current flow. The external terminal for current flow is for outputting current from the secondary battery to the outside and/or for inputting current from the outside to the secondary battery. In other words, when the battery pack is used as a power source, current is supplied to the outside through the external terminal for current flow. When the battery pack is charged, charging current (including regenerative energy from the power of an automobile or the like) is supplied to the battery pack through the external terminal for current flow.

Next, an example of a battery pack according to an embodiment will be described with reference to the drawings.

Figure 8 is a perspective view showing an example of a battery pack according to an embodiment.

The battery pack 300 includes a battery assembly consisting of secondary batteries as shown in FIG. 5 and FIG. 6. The battery pack 300 includes a housing 310 and a battery assembly 200 housed in the housing 310. The battery assembly 200 is a battery assembly in which a plurality of (e.g., five) secondary batteries 100 are electrically connected in series. The secondary batteries 100 are stacked in the thickness direction. The housing 310 has openings 320 on the top and on each of the four sides. The side from which the negative terminal 6 and the positive terminal 7 of the secondary battery 100 protrude is exposed to the opening 320 of the housing 310. The output positive terminal 332 of the battery assembly 200 is strip-shaped, with one end electrically connected to one of the positive terminals 7 of the secondary batteries 100 and the other end protruding from the opening 320 of the housing 310 and protruding from the top of the housing 310. On the other hand, the output negative electrode terminal 333 of the battery pack 200 is strip-shaped, with one end electrically connected to one of the negative electrode terminals 6 of the secondary batteries 100, and the other end protruding from the opening 320 of the housing 310 and protruding from the top of the housing 310.

Another example of such a battery pack will be described in detail with reference to Figs. 9 and 10. Fig. 9 is an exploded perspective view showing a schematic diagram of another example of a battery pack according to an embodiment. Fig. 10 is a block diagram showing an example of an electrical circuit of the battery pack shown in Fig. 9.

The battery pack 300 shown in Figures 9 and 10 includes a container 31, a lid 32, a protective sheet 33, a battery pack 200, a printed wiring board 34, wiring 35, and an insulating plate (not shown).

The storage container 31 shown in FIG. 9 is a bottomed, square container having a rectangular bottom. The storage container 31 is configured to be able to accommodate a protective sheet 33, a battery pack 200, a printed wiring board 34, and wiring 35. The lid 32 has a rectangular shape. The lid 32 covers the storage container 31 to accommodate the battery pack 200 and the like. Although not shown, the storage container 31 and the lid 32 are provided with openings or connection terminals for connection to external devices and the like.

The battery pack 200 includes a plurality of cells 100, a positive electrode lead 22, a negative electrode lead 23, and an adhesive tape 24.

At least one of the multiple single cells 100 is a secondary battery according to the embodiment. Each of the multiple single cells 100 is electrically connected in series as shown in FIG. 10. The multiple single cells 100 may be electrically connected in parallel, or may be connected in a combination of series and parallel connections. When the multiple single cells 100 are connected in parallel, the battery capacity is increased compared to when they are connected in series.

The adhesive tape 24 fastens the multiple cells 100 together. Instead of the adhesive tape 24, heat shrink tape may be used to secure the multiple cells 100 together. In this case, protective sheets 33 are placed on both sides of the battery pack 200, the heat shrink tape is wrapped around the battery pack 200, and the heat shrink tape is then thermally shrunk to bind the multiple cells 100 together.

One end of the positive electrode lead 22 is connected to the battery pack 200. One end of the positive electrode lead 22 is electrically connected to the positive electrode of one or more single cells 100. One end of the negative electrode lead 23 is connected to the battery pack 200. One end of the negative electrode lead 23 is electrically connected to the negative electrode of one or more single cells 100.

The printed wiring board 34 is installed along one of the short sides of the inner surface of the container 31. The printed wiring board 34 includes a positive connector 342, a negative connector 343, a thermistor 345, a protection circuit 346, wiring 342a and 343a, an external terminal 350 for supplying electricity, a positive wiring (positive wiring) 348a, and a negative wiring (negative wiring) 348b. One surface of the printed wiring board 34 faces one side of the battery pack 200. An insulating plate (not shown) is interposed between the printed wiring board 34 and the battery pack 200.

The other end 22a of the positive electrode lead 22 is electrically connected to the positive electrode connector 342. The other end 23a of the negative electrode lead 23 is electrically connected to the negative electrode connector 343.

The thermistor 345 is fixed to one side of the printed wiring board 34. The thermistor 345 detects the temperature of each of the cells 100 and transmits the detection signal to the protection circuit 346.

The external terminals 350 for electrical current flow are fixed to the other surface of the printed wiring board 34. The external terminals 350 for electrical current flow are electrically connected to a device that is external to the battery pack 300. The external terminals 350 for electrical current flow include a positive terminal 352 and a negative terminal 353.

The protection circuit 346 is fixed to the other surface of the printed wiring board 34. The protection circuit 346 is connected to the positive terminal 352 via the positive wiring 348a. The protection circuit 346 is connected to the negative terminal 353 via the negative wiring 348b. The protection circuit 346 is also electrically connected to the positive connector 342 via the wiring 342a. The protection circuit 346 is electrically connected to the negative connector 343 via the wiring 343a. Furthermore, the protection circuit 346 is electrically connected to each of the multiple single cells 100 via the wiring 35.

The protective sheet 33 is disposed on both inner surfaces of the long sides of the container 31 and on the inner surface of the short side that faces the printed wiring board 34 via the battery pack 200. The protective sheet 33 is made of, for example, resin or rubber.

The protection circuit 346 controls the charging and discharging of the multiple cells 100. The protection circuit 346 also cuts off the electrical connection between the protection circuit 346 and the external terminals 350 (positive terminal 352, negative terminal 353) for supplying electricity to an external device based on a detection signal transmitted from the thermistor 345 or a detection signal transmitted from each cell 100 or the battery pack 200.

The detection signal transmitted from the thermistor 345 may be, for example, a signal that detects that the temperature of the cell 100 is equal to or higher than a predetermined temperature. The detection signal transmitted from each cell 100 or the battery pack 200 may be, for example, a signal that detects overcharging, overdischarging, or overcurrent of the cell 100. When detecting overcharging or the like for each cell 100, the battery voltage may be detected, or the positive electrode potential or negative electrode potential may be detected. In the latter case, a lithium electrode used as a reference electrode is inserted into each cell 100.

The protection circuit 346 may be a circuit included in a device (e.g., electronic device, automobile, etc.) that uses the battery pack 300 as a power source.

As described above, the battery pack 300 is provided with an external terminal 350 for current flow. Therefore, the battery pack 300 can output current from the battery pack 200 to an external device and input current from the external device to the battery pack 200 via the external terminal 350 for current flow. In other words, when the battery pack 300 is used as a power source, the current from the battery pack 200 is supplied to the external device through the external terminal 350 for current flow. When the battery pack 300 is charged, a charging current from the external device is supplied to the battery pack 300 through the external terminal 350 for current flow. When the battery pack 300 is used as an in-vehicle battery, the regenerative energy of the vehicle's power can be used as the charging current from the external device.

The battery pack 300 may include a plurality of assembled batteries 200. In this case, the assembled batteries 200 may be connected in series, in parallel, or in a combination of series and parallel connections. The printed wiring board 34 and the wiring 35 may be omitted. In this case, the positive electrode lead 22 and the negative electrode lead 23 may be used as the positive and negative terminals of the external terminals for current supply, respectively.

Such battery packs are used in applications that require excellent cycle performance, for example, when drawing a large current. Specifically, these battery packs are used, for example, as power sources for electronic devices, stationary batteries, and on-board batteries for various vehicles. Examples of electronic devices include digital cameras. These battery packs are particularly suitable for use as on-board batteries.

The battery pack according to the third embodiment includes the secondary battery according to the first embodiment or the battery pack according to the second embodiment. Therefore, the battery pack can exhibit excellent life characteristics.

[Fourth embodiment]
According to a fourth embodiment, a vehicle including the battery pack according to the third embodiment is provided.

In such a vehicle, the battery pack, for example, recovers regenerative energy from the vehicle's motive power. The vehicle may also include a mechanism (regenerator) that converts the kinetic energy of the vehicle into regenerative energy.

Examples of vehicles according to the embodiment include, for example, two- to four-wheel hybrid electric vehicles, two- to four-wheel electric vehicles, power-assisted bicycles, and rail vehicles.

The mounting position of the battery pack in the vehicle according to the embodiment is not particularly limited. For example, when the battery pack is mounted in an automobile, the battery pack can be mounted in the engine compartment, the rear of the vehicle body, or under the seat of the vehicle.

The vehicle according to the embodiment may be equipped with a plurality of battery packs. In this case, the batteries included in each battery pack may be electrically connected in series, electrically connected in parallel, or electrically connected by a combination of series and parallel connections. For example, when each battery pack includes a battery pack, the battery packs may be electrically connected in series, electrically connected in parallel, or electrically connected by a combination of series and parallel connections. Alternatively, when each battery pack includes a single battery, the batteries may be electrically connected in series, electrically connected in parallel, or electrically connected by a combination of series and parallel connections.

Next, an example of a vehicle according to an embodiment will be described with reference to the drawings.

FIG. 11 is a partially see-through view that illustrates an example of a vehicle according to an embodiment.
A vehicle 400 shown in Fig. 11 includes a vehicle body 40 and a battery pack 300 according to the fourth embodiment. In the example shown in Fig. 11, the vehicle 400 is a four-wheeled automobile.

The vehicle 400 may be equipped with multiple battery packs 300. In this case, the batteries (e.g., single cells or battery packs) included in the battery packs 300 may be connected in series, in parallel, or in a combination of series and parallel connections.

Figure 11 illustrates an example in which the battery pack 300 is mounted in an engine compartment located in front of the vehicle body 40. As described above, the battery pack 300 may be mounted, for example, at the rear of the vehicle body 40 or under a seat. This battery pack 300 can be used as a power source for the vehicle 400. In addition, this battery pack 300 can recover regenerative energy for the power of the vehicle 400.

The vehicle according to the fourth embodiment is equipped with a battery pack according to the fourth embodiment. Therefore, the vehicle has excellent driving performance and reliability.

[Fifth embodiment]
According to a fifth embodiment, there is provided a stationary power source including the battery pack according to the fourth embodiment.

Instead of the battery pack according to the fourth embodiment, the stationary power source may be equipped with the battery pack according to the second embodiment or the secondary battery according to the first embodiment. The stationary power source according to the embodiment can achieve a long life.

FIG. 12 is a block diagram showing an example of a system including a stationary power source according to the embodiment. FIG. 12 is a diagram showing an example of application to stationary power sources 112 and 123 as an example of use of battery packs 300A and 300B according to the embodiment. In the example shown in FIG. 12, a system 110 in which stationary power sources 112 and 123 are used is shown. The system 110 includes a power plant 111, a stationary power source 112, a consumer-side power system 113, and an energy management system (EMS) 115. In addition, a power grid 116 and a communication network 117 are formed in the system 110, and the power plant 111, the stationary power source 112, the consumer-side power system 113, and the EMS 115 are connected via the power grid 116 and the communication network 117. The EMS 115 uses the power grid 116 and the communication network 117 to perform control to stabilize the entire system 110.

The power plant 111 generates a large amount of electricity using fuel sources such as thermal power and nuclear power. Electricity is supplied from the power plant 111 through the power grid 116, etc. The stationary power source 112 is equipped with a battery pack 300A. The battery pack 300A can store the electricity supplied from the power plant 111. The stationary power source 112 can supply the electricity stored in the battery pack 300A through the power grid 116, etc. The system 110 is provided with a power conversion device 118. The power conversion device 118 includes a converter, an inverter, a transformer, etc. Therefore, the power conversion device 118 can convert between direct current and alternating current, convert between alternating currents with different frequencies, and perform voltage transformation (boosting and bucking), etc. Therefore, the power conversion device 118 can convert the electricity from the power plant 111 into electricity that can be stored in the battery pack 300A.

The consumer-side power system 113 includes a power system for factories, a power system for buildings, and a power system for homes. The consumer-side power system 113 includes a consumer-side EMS 121, a power conversion device 122, and a stationary power source 123. The stationary power source 123 is equipped with a battery pack 300B. The consumer-side EMS 121 performs control to stabilize the consumer-side power system 113.

The consumer-side power system 113 is supplied with power from the power plant 111 and from the battery pack 300A through the power grid 116. The battery pack 300B can store the power supplied to the consumer-side power system 113. Similarly to the power conversion device 118, the power conversion device 122 includes a converter, an inverter, a transformer, and the like. Therefore, the power conversion device 122 can convert between DC and AC, convert between AC with different frequencies, and perform voltage transformation (boost and step-down), etc. Therefore, the power conversion device 122 can convert the power supplied to the consumer-side power system 113 into power that can be stored in the battery pack 300B.

The power stored in the battery pack 300B can be used, for example, to charge a vehicle such as an electric car. The system 110 may also be provided with a natural energy source. In this case, the natural energy source generates power using natural energy such as wind power and solar power. Power is supplied from the natural energy source in addition to the power plant 111 through the power grid 116.

[Example]
Example 1
<Manufacture of aqueous electrolyte bipolar secondary battery>
An aqueous electrolyte bipolar secondary battery was produced by the following method.

(Preparation of bipolar electrodes)
Preparation of Positive Electrode Active Material Slurry A positive electrode active material slurry was prepared to form a positive electrode active material-containing layer. The positive electrode active material slurry was prepared by mixing LiMn ₂ O ₄ as a positive electrode active material, acetylene black as a conductive additive, and PVDF as a binder in a ratio of 100:5:5.

Preparation of negative electrode active material slurry To form a negative _{electrode active material-containing layer, a negative electrode active material slurry was prepared. The negative electrode active material slurry was prepared by mixing Li4Ti5O12 as a} negative electrode active material, graphite as a conductive additive, and PVDF as a binder in a ratio of 100:5:1.

A Ti foil in which the metal layer a contains 100 wt. % Ti relative to the weight of each layer of the metal layer a, and a Zn foil in which the metal layer b contains 100 wt. % Zn relative to the weight of each layer of the metal layer b are used. The laminate of the metal layers a and b was used as a bipolar electrode current collector. A positive electrode active material slurry was applied to one side of the Ti foil, and a negative electrode active material slurry was applied to one side of the Zn foil so that the non-coated portion remained, and the laminate was dried at 120°C for 12 hours to form a positive electrode active material-containing layer and a negative electrode active material-containing layer. A PVdF solution in which acetylene black was dispersed was applied to the other side of the metal layer Ti foil that was not coated with the positive electrode active material slurry, and the surface of the Zn foil that was not coated with the negative electrode active material slurry was joined, and the laminate was vacuum-dried at 120°C for 12 hours to produce a bipolar electrode with one side as the positive electrode surface and the other side as the negative electrode surface.

(Preparation of aqueous electrolyte bipolar secondary battery)
A Ti foil was welded to the non-coated portion of the Ti foil surface of the bipolar electrode, and this was used as a lead. A 12M-LiCl electrolyte was applied to the negative electrode surface of the bipolar electrode, and a solid electrolyte sheet containing Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ was placed from above as a solid electrolyte, and then a 2M-Li ₂ SO ₄ electrolyte was applied from above, and the positive electrode surface of another bipolar electrode prepared in the same manner as above was placed so as to face each other from above. At that time, the bipolar electrodes were placed so that the lead direction was the same. This operation was repeated to stack three bipolar electrodes.

The stacked bipolar electrodes were placed in a polyethylene container, the leads were removed from the container, and the space between the bipolar electrodes and the container was filled with silicone sealant to eliminate any gaps, and the container was then covered with a lid to create an aqueous electrolyte bipolar secondary battery.

To evaluate the number of cycles and the self-discharge characteristics, the following tests were performed.

<Number of cycles>
After a waiting time of 12 hours from the assembly of the secondary battery, the battery was charged to 2.55 V at a constant current of 0.2 C (converted into negative electrode active material) in a 25° C. environment, and then discharged to 1.8 V at 0.2 C to measure the initial discharge capacity.

<Self-discharge characteristics>
(Initial discharge capacity)
After a standby time of 12 hours from the assembly of the cell, the secondary battery was charged to 2.6 V at a constant current of 0.1 C (as negative electrode active material) in a 25° C. environment, and then discharged to 2.2 V at 0.1 C to measure the initial discharge capacity (mAh/g (per weight of negative electrode active material)).

(Self-discharge rate)
After measuring the initial discharge capacity, a predetermined number of charge/discharge cycles were repeated at a rate of 0.25 C, and then the battery was charged to 2.6 V at a constant current of a rate of 0.1 C (as negative electrode active material), and then the battery was left for 24 hours before being discharged at 0.1 C to 2.2 V. As an index of storage performance, the self-discharge rate (%) was calculated from the following formula.
(Self-discharge rate) = 100 - (discharge capacity after 24 hours ÷ initial discharge capacity) x 100

Example 2
The same method as in Example 1 was used to prepare and evaluate the sample, except that the metal layer a was a stainless steel foil, and the metal layer b was a Sn foil containing 100 wt. % Sn relative to the weight of each layer of the metal layer b.

Example 3
The same method as in Example 1 was used to prepare and evaluate the sample, except that an Al-Mg alloy foil was used for the metal layer a, and an Sn foil containing 100 wt. % Sn relative to the weight of each metal layer b was used for the metal layer b.

Example 4
The same method as in Example 1 was used to prepare and evaluate the sample, except that the metal layer a was a Ti foil containing 100 wt. % Ti relative to the weight of each layer of the metal layer a, and the metal layer b was a Zn-Sn plated copper foil containing 15 wt. % Zn relative to the weight of each layer of the metal layer b.

Example 5
The same method as in Example 1 was used to prepare and evaluate the sample, except that the metal layer a was a Ti foil containing 100 wt. % Ti relative to the weight of each layer of the metal layer a, and the metal layer b was a Cu-Zn alloy foil containing 30 wt. % Zn relative to the weight of each layer of the metal layer b.

Example 6
The same method as in Example 1 was used to prepare and evaluate the specimen, except that the metal layer a was a Ti foil containing 100 wt. % Ti relative to the weight of each layer of the metal layer a, and the metal layer b was a Sn-plated copper foil containing 15 wt. % Sn relative to the weight of each layer of the metal layer b.

Comparative Example 1
The fabrication and evaluation were carried out in the same manner as in Example 1, except that stainless steel foil was used for metal layer a and stainless steel foil was also used for metal layer b.

(Comparative Example 2)
The metal layer a was made of Al foil containing 100 wt. % Al relative to the weight of each layer of the metal layer a, the same Al foil as that used for the metal layer a was used for the metal layer b, and 12M-LiCl was used as the electrolyte in contact with the positive electrode surface. The Al foil was 100 wt. % relative to the weight of each layer of the metal layer b.

(Comparative Example 3)
The same method as in Example 1 was used to prepare and evaluate the sample, except that the metal layer a was a Ti foil containing 100 wt. % Ti relative to the weight of each metal layer a, and the metal layer b was a stainless steel foil.

(Comparative Example 4)
An aqueous electrolyte bipolar secondary battery was fabricated and evaluated in the same manner as in Example 1, except that a Cu-Sn alloy foil containing 3 wt. % Sn relative to the weight of each layer of the metal layer b was used for the metal layer b.

From the above results, an aqueous electrolyte bipolar secondary battery is provided with a negative electrode having a first negative electrode active material-containing layer on at least one surface of a negative electrode current collector, a positive electrode having a first positive electrode active material-containing layer on at least one surface of a positive electrode current collector, a bipolar electrode having a bipolar electrode current collector having a second positive electrode active material-containing layer on one surface and a second negative electrode active material-containing layer on the other opposing surface, and an aqueous electrolyte, in which the bipolar electrode current collector has a metal layer a and a metal layer b, the metal layer a contains at least one element selected from the group consisting of Cr, Ni, Fe, Ti and Al, and the metal layer b contains 10 wt. % or more and 100 wt. % or less of the weight of each metal layer b. % or less of either one of the elements Zn and Sn, metal layer a and metal layer b are electrically connected, a second positive electrode active material-containing layer is provided on the surface of metal layer a, and a second negative electrode active material-containing layer is provided on the surface of metal layer b. This aqueous electrolyte bipolar secondary battery was able to suppress corrosion of the bipolar electrode current collector, and therefore the capacity retention rate and initial discharge capacity were improved and self-discharge was improved compared to the comparative example. It can be seen that the life performance is excellent even when an aqueous electrolyte is used.

Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the gist of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are included in the scope of the invention and its equivalents described in the claims.

1...electrode group, 2...exterior member, 3...negative electrode, 3a...negative electrode current collector, 3b...negative electrode active material-containing layer, 3b1...first negative electrode active material-containing layer, 3b2...second negative electrode active material-containing layer, 4...separator, 4a...first separator, 4b...second separator, 5...positive electrode, 5a...positive electrode current collector, 5b...positive electrode active material-containing layer, 5b1...first positive electrode active material-containing layer, 5b2...second positive electrode active material-containing layer, 6...negative electrode terminal, 7...positive electrode terminal, 8...negative electrode gasket, 9...positive electrode Gasket, 10... sealing plate, 11... control valve, 12... liquid injection port, 13... sealing plug, 16... negative electrode current collecting tab, 17... positive electrode current collecting tab, 21... bus bar, 22... positive electrode side lead, 23... negative electrode side lead, 24... adhesive tape, 31... storage container, 32... lid, 33... protective sheet, 34... printed wiring board, 35... wiring, 40... vehicle body, 100... secondary battery, 110... system, 111... power plant, 112... stationary power source, 113... consumer side power system, 11 5...Energy management system, 116...Power grid, 117...Communication network, 118...Power conversion device, 121...Demand side EMS, 122...Power conversion device, 123...Stationary power source, 200...Battery pack, 300...Battery pack, 300A...Battery pack, 300B...Battery pack, 310...Housing, 320...Opening, 332...Output positive terminal, 333...Output negative terminal, 342...Positive side connector, 343...Negative side connector, 345...Thermistor, 34 6...protection circuit, 342a...wiring, 343a...wiring, 350...external terminal for current supply, 352...positive terminal, 353...negative terminal, 348a...positive wiring, 348b...negative wiring, 400...vehicle, 500...aqueous electrolyte bipolar secondary battery, 501...bipolar electrode, 502...bipolar electrode current collector, 502a...metal layer a, 502b...metal layer b, 503...conductive adhesive layer, 504...negative electrode aqueous electrolyte, 505...positive electrode aqueous electrolyte.

Claims (22)

a negative electrode having a first negative electrode active material-containing layer provided on at least one surface of a negative electrode current collector;
a positive electrode having a first positive electrode active material-containing layer provided on at least one surface of a positive electrode current collector;
a bipolar electrode having a bipolar electrode current collector provided with a second positive electrode active material-containing layer on one side and a second negative electrode active material-containing layer on the other opposing side;
An aqueous electrolyte bipolar secondary battery comprising an aqueous electrolyte,
The bipolar electrode current collector has a metal layer a and a metal layer b,
The metal layer a contains at least one element selected from the group consisting of Cr, Ni, Fe, Ti, and Al,
The metal layer b contains one of elements Zn and Sn in an amount of 10 wt. % or more and 100 wt. % or less based on the weight of each layer of the metal layer b,
The metal layer a and the metal layer b are electrically connected to each other,
The second positive electrode active material-containing layer is provided on a surface of the metal layer a,
a water-based electrolyte bipolar secondary battery having the second negative electrode active material-containing layer on a surface of the metal layer b; A conductive adhesive layer is provided between the metal layer a and the metal layer b,
The conductive adhesive layer is a conductive resin or a metal.
2. The aqueous electrolyte bipolar secondary battery according to claim 1. The aqueous electrolyte bipolar secondary battery according to claim 1, wherein the metal layer a and the metal layer b are in direct contact. The aqueous electrolyte bipolar secondary battery according to any one of claims 1 to 3, wherein the metal layer b is a Cu-Zn alloy or a Cu-Sn alloy. The bipolar electrode current collector is
5. The aqueous electrolyte bipolar secondary battery according to claim 1, wherein when the metal layer a is made of Al, the thickness of the metal layer a is 9% or more and 95% or less of the thickness of the bipolar electrode current collector. The bipolar electrode current collector is
5. The aqueous electrolyte bipolar secondary battery according to claim 1, wherein when the metal layer a contains Cr, Fe, Ni, Ti , an Al-Mg alloy, or an Al-Mg-Zn alloy, the thickness of the metal layer a is 7% or more and 95% or less of the thickness of the bipolar electrode current collector. The bipolar electrode current collector is
5. The aqueous electrolyte bipolar secondary battery according to claim 1, wherein when the metal layer a is made of stainless steel, the thickness of the metal layer a is 5% or more and 95% or less of the thickness of the bipolar electrode current collector. The aqueous electrolyte bipolar secondary battery according to any one of claims 1 to 7, wherein the aqueous electrolyte in contact with the metal layer a is different from the aqueous electrolyte in contact with the metal layer b. The aqueous electrolyte bipolar secondary battery according to any one of claims 1 to 8, wherein the aqueous electrolyte in contact with the metal layer a and the aqueous electrolyte in contact with the metal layer b are liquid or gel-like. The aqueous electrolyte bipolar secondary battery according to any one of claims 1 to 9, wherein the aqueous electrolyte bipolar secondary battery includes a separator, and the separator contains inorganic solid particles. 11. The aqueous electrolyte bipolar secondary battery according to claim 10, wherein the separator has an air permeability coefficient of 1×10 ⁻¹³ m ² or less. The aqueous electrolyte bipolar secondary battery according to claim 10 or 11, wherein the inorganic solid particles include solid electrolyte particles having ionic conductivity for alkali metal ions. 13. The aqueous electrolyte bipolar secondary battery according to claim 10, wherein the inorganic solid particles include at least one selected from the group consisting of a compound represented by Li1 _{+ wAlwTi2 -w} ( _PO4 ) ₃ where 0.1≦w≦0.5, a compound represented by Li1 _{+ yAlzM12 -z} ( _PO4 ) ₃ where M1 is 1 or more selected from the group consisting of Ti, Ge, _Sr , Zr, Sn, and _Ca , and 0≦y≦1 and 0≦z≦1, and a compound represented by Li5 ₊ xLa3M22 _{- xZrxO12} where M2 is 1 or more selected from the group consisting of Nb and Ta, and 0≦x≦2. The aqueous electrolyte bipolar secondary battery according to any one of claims 10 to 13, wherein the separator includes a polymeric material, the polymeric material includes a monomer unit made of a hydrocarbon having a functional group containing one or more selected from the group consisting of oxygen (O), sulfur (S), nitrogen (N) and fluorine (F), and the proportion of the monomer unit in the polymeric material is 70 mol % or more. 15. The aqueous electrolyte bipolar secondary battery according to claim 1, wherein the first negative electrode active material-containing layer or the second negative electrode active material-containing layer contains a negative electrode active material containing a compound having a lithium ion insertion-extraction potential of 1 V or more and 3 V or less relative to the oxidation-reduction potential of lithium (vs. Li/Li ⁺ ). 16. The aqueous electrolyte bipolar secondary battery according to claim 1, wherein the first positive electrode active material-containing layer or the second positive electrode active material-containing layer contains a positive electrode active material containing a compound having a lithium ion insertion-extraction potential of 2.5 V or more and 5.5 V or less relative to the oxidation-reduction potential of lithium (vs. Li/Li ⁺ ). A battery pack comprising a secondary battery according to any one of claims 1 to 16. The battery pack according to claim 17, further comprising an external terminal for energizing and a protection circuit. The battery pack according to claim 17 or 18, comprising a plurality of the secondary batteries, the plurality of secondary batteries being electrically connected in series, in parallel, or in a combination of series and parallel. A vehicle equipped with a battery pack according to any one of claims 17 to 19. The vehicle according to claim 20, including a mechanism for converting the kinetic energy of the vehicle into regenerative energy. A stationary power source comprising a battery pack according to any one of claims 17 to 19.

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