JP7376393B2 - Positive electrode for all-solid-state secondary batteries and all-solid-state secondary batteries - Google Patents

Description

The present invention relates to an all-solid-state secondary battery with excellent load characteristics and high-temperature storage characteristics, and a positive electrode that can constitute the all-solid-state secondary battery.

In recent years, with the development of portable electronic devices such as mobile phones and notebook personal computers, and the practical use of electric vehicles, there has been a need for small, lightweight, high-capacity, and high-energy-density secondary batteries. It has become to.

Currently, lithium secondary batteries that can meet this demand, especially lithium ion secondary batteries, use lithium-containing composite oxides such as lithium cobalt oxide (LiCoO ₂ ) and lithium nickel oxide (LiNiO ₂ ) as positive electrode active materials. Graphite or the like is used as the negative electrode active material, and an organic electrolyte containing an organic solvent and a lithium salt is used as the nonaqueous electrolyte.

With the further development of equipment to which lithium ion secondary batteries are applied, there is a need for lithium ion secondary batteries to have a longer lifespan, higher capacity, and higher energy density. Lithium-ion secondary batteries with higher capacity and higher energy density are also required to be highly reliable.

However, the organic electrolyte used in lithium-ion secondary batteries contains an organic solvent, which is a flammable substance, so when an abnormal situation such as a short circuit occurs in the battery, the organic electrolyte can generate abnormal heat. there is a possibility. Furthermore, with the recent trend towards higher energy densities in lithium ion secondary batteries and an increase in the amount of organic solvents in organic electrolytes, there is a demand for even greater reliability in lithium ion secondary batteries.

Under the above circumstances, all-solid-state lithium secondary batteries (all-solid-state secondary batteries) that do not use organic solvents are attracting attention. All-solid-state secondary batteries use a molded solid electrolyte that does not use organic solvents instead of conventional organic solvent-based electrolytes, and are highly safe as there is no risk of abnormal heat generation of the solid electrolyte. .

Furthermore, various improvements have been attempted in all-solid-state secondary batteries. For example, it is known that a solid electrolyte may form a resistance layer when it comes into direct contact with a positive electrode active material. A technique has been developed in which a layer for suppressing the reaction with the positive electrode active material is provided on the surface of the positive electrode active material (Patent Documents 1 and 2, etc.).

JP2014-22074A (paragraphs [0036] to [0040]) JP2019-106286A (paragraphs [0037], [0041])

By the way, the field of application of all-solid-state secondary batteries is currently expanding rapidly, and for example, applications that require discharging at large current values are considered. It is required to improve the characteristics.

The present invention has been made in view of the above circumstances, and its purpose is to provide an all-solid-state secondary battery with excellent load characteristics and high-temperature storage characteristics, and a positive electrode that can constitute the all-solid-state secondary battery. There is a particular thing.

The positive electrode for an all-solid-state secondary battery of the present invention has a molded positive electrode mixture containing a positive electrode active material, a conductive additive, and a solid electrolyte, and the positive electrode active material has a voltage equivalent to 2.5 V based on Li. When the battery is fully discharged, the following general formula (1)
Li _a Co _1-b-c Mg _b M ¹ _c O ₂ (1)
[In the general formula (1), M ¹ is Al, Ni, Mn, Na, Fe, Cu, Zr, Ti, Bi, Ca, F, P, Sr, W, Ba, Nb, Si, Zn, Mo , V, Sn, Sb, Ta, Ge, Cr, K, S and Er, and a<1, 0<b≦0.1, 0≦c≦0 .05], and a reaction suppression layer is formed on the surface of the positive electrode active material to suppress the reaction between the positive electrode active material and the solid electrolyte. It is characterized by:

Further, the all-solid-state secondary battery of the present invention includes a positive electrode, a negative electrode, and a solid electrolyte layer interposed between the positive electrode and the negative electrode, and the positive electrode for an all-solid-state secondary battery of the present invention is used as the positive electrode. It is characterized by having.

According to the present invention, it is possible to provide an all-solid-state secondary battery with excellent load characteristics and high-temperature storage characteristics, and a positive electrode that can constitute the all-solid-state secondary battery.

FIG. 1 is a cross-sectional view schematically showing an example of an all-solid-state secondary battery of the present invention. FIG. 3 is a plan view schematically showing another example of the all-solid-state secondary battery of the present invention. 3 is a sectional view taken along the line II in FIG. 2. FIG.

<Cathode for all-solid-state secondary batteries>
The positive electrode for an all-solid-state secondary battery of the present invention has a molded body of a positive electrode mixture containing a positive electrode active material, a conductive additive, and a solid electrolyte. Then, a positive electrode active material having a reaction suppression layer for suppressing reaction with the solid electrolyte is used.

When the positive electrode active material and the solid electrolyte come into direct contact in the molded body of the positive electrode mixture, the solid electrolyte is oxidized to form a resistance layer, which may cause a decrease in the ionic conductivity within the molded body. Note that such a deterioration reaction of the solid electrolyte is likely to occur when the battery is placed in a high temperature environment. Therefore, in the present invention, a reaction suppression layer for suppressing the reaction with the solid electrolyte is provided on the surface of the positive electrode active material to prevent direct contact between the positive electrode active material and the solid electrolyte.

In addition, the positive electrode for an all-solid-state secondary battery of the present invention uses a lithium cobalt oxide represented by the general formula (1) as a positive electrode active material when completely discharged to a voltage equivalent to 2.5 V based on Li. Use (A).

Lithium cobalt oxide (A) has a limited amount of Li, and its use makes it possible to improve the load characteristics of the battery. In addition, in a secondary battery having an organic electrolyte, if lithium cobalt oxide with a limited amount of Li is used as the positive electrode active material, the battery characteristics tend to deteriorate when placed in a high-temperature environment ( In other words, high-temperature storage characteristics tend to deteriorate). However, although the reason is not clear, when lithium cobalt oxide (A) is combined with a solid electrolyte, the load characteristics of the battery can be improved without causing the above-mentioned deterioration of high-temperature storage characteristics.

As described above, in the present invention, the effect of suppressing the deterioration of battery characteristics (especially the deterioration of battery characteristics during high-temperature storage) by suppressing the deterioration of the solid electrolyte of the reaction suppression layer in the positive electrode active material, and the effect of suppressing the deterioration of battery characteristics during high-temperature storage, and ) and the effect of the combination with the solid electrolyte, it is possible to simultaneously improve the load characteristics and high-temperature storage characteristics of the all-solid-state secondary battery.

As a positive electrode for an all-solid-state secondary battery, a molded body (pellet, etc.) formed by molding a positive electrode mixture, or a layer (positive electrode mixture layer) made of a molded positive electrode mixture formed on a current collector. Examples include those with a structure such as

The lithium cobalt oxide (A) satisfies the above general formula (1) when completely discharged to a voltage equivalent to 2.5V on Li basis, but the lithium cobalt oxide (A) here The composition is determined by the following method (the composition of lithium cobalt oxide, which is a positive electrode active material described in Examples below, is determined by this method). As charge/discharge conditions, the battery is discharged at a current value equivalent to 0.02C up to 2.5V based on Li. After discharging, the battery is disassembled, the positive electrode mixture molded body is taken out, and the solid electrolyte is separated by dissolving it in water. Thereafter, a classification process is performed to separate only lithium cobalt oxide (A), and the composition of the active material is calculated by ICP analysis. In addition, in the ICP analysis for measuring the amount of metal elements in the positive electrode active material in this specification, approximately 0.5 g of the active material is accurately weighed, placed in a 100 ml beaker, and 10 ml of aqua regia is added, so that the liquid volume is approximately 10 ml. After cooling, solid matter was separated using quantitative filter paper "No. 5A" manufactured by Advantech Corporation. A method of measurement is adopted using the ICP analyzer "iCAP7600" manufactured by Manufacturer.

In the lithium cobalt oxide (A) used as the positive electrode active material, the amount a of Li is less than 1, and preferably 0.98 or less, from the viewpoint of improving the load characteristics of the all-solid-state secondary battery. Further, the amount a of Li in the lithium cobalt oxide (A) is preferably 0.9 or more from the viewpoint of increasing the true density and reversibility of the lithium cobalt oxide (A).

In the lithium cobalt oxide (A), Mg and element ^M1 have the effect of increasing the stability (stability in a high voltage region and thermal stability) of the lithium cobalt oxide (A) and suppressing the elution of Co ions. have. Furthermore, element M ¹ also has the effect of enhancing the continuous charging characteristics of the battery (characteristics in which a minute short circuit takes a very long time even if the battery continues to be charged).

In the lithium cobalt oxide (A), the amount b of Mg is more than 0, but is preferably 0.01 or more from the viewpoint of exhibiting the above-mentioned effect more effectively.

On the other hand, the lithium cobalt oxide (A) does not need to contain the element M ¹ (the amount c may be 0), but from the viewpoint of effectively exerting the above effect, the amount of the element M ¹ It is preferable that c is 0.01 or more.

However, if the amount of Mg or element ^M1 in the lithium cobalt oxide (A) is too large, the amount of Co will be too small, and there is a possibility that its effect cannot be sufficiently ensured. Therefore, in lithium cobalt oxide (A), the amount b of Mg is 0.1 or less, preferably 0.05 or less. Further, in the lithium cobalt oxide (A), the amount c of element ^M1 is 0.05 or less, preferably 0.02 or less.

As the positive electrode active material, lithium cobalt oxide (A) alone may be used, or lithium cobalt oxide (A) and other positive electrode active materials may be used together. Other positive electrode active materials that can be used in combination with lithium cobalt oxide (A) include those that absorb and release lithium ions, such as those used as positive electrode active materials in conventionally known lithium ion secondary batteries. Possible active materials include: However, when using lithium cobalt oxide (A) and other positive electrode active materials together as the positive electrode active material, the proportion of lithium cobalt oxide (A) in the total positive electrode active material must be 60% by mass or more. is preferred. Note that since all of the positive electrode active material may be lithium cobalt oxide (A), the upper limit of the preferable content of lithium cobalt oxide (A) in all the positive electrode active materials is 100% by mass.

The positive electrode active material has a reaction suppression layer on its surface for suppressing reaction with the solid electrolyte.

The reaction suppression layer may be made of a material that has ionic conductivity and can suppress the reaction between the positive electrode active material and the solid electrolyte. Examples of materials that can constitute the reaction suppression layer include, for example, oxides containing Li and at least one element selected from the group consisting of Nb, P, B, Si, Ge, Ti, and Zr; Examples include Nb-containing oxides such as LiNbO ₃ , Li ₃ PO ₄ , Li ₃ BO ₃ , Li ₄ SiO ₄ , Li ₄ GeO ₄ , LiTiO ₃ , LiZrO ₃ and the like. The reaction suppression layer may contain only one type of these oxides, or may contain two or more types of these oxides, and may further contain multiple types of these oxides in a composite compound. may be formed. Among these oxides, it is preferable to use Nb-containing oxides, and it is more preferable to use LiNbO ₃ .

The reaction suppression layer is preferably present on the surface in an amount of 0.1 to 1.0 parts by mass based on 100 parts by mass of the positive electrode active material. Within this range, the reaction between the lithium cobalt oxide and the solid electrolyte can be suppressed well, and deterioration of load characteristics can be prevented.

Examples of methods for forming a reaction suppression layer on the surface of the positive electrode active material include a sol-gel method, a mechanofusion method, a CVD method, and a PVD method.

The content of the positive electrode active material in the positive electrode mixture is preferably 60 to 95% by mass.

Carbon materials such as graphite (natural graphite, artificial graphite), graphene, carbon black, carbon nanofibers, carbon nanotubes, etc. can be used as the conductive agent of the positive electrode. The content of the conductive additive in the positive electrode mixture is preferably 1 to 10% by mass.

The solid electrolyte of the positive electrode is not particularly limited as long as it has lithium ion conductivity, and for example, sulfide-based solid electrolytes, hydride-based solid electrolytes, oxide-based solid electrolytes, etc. can be used.

Examples of sulfide-based solid electrolytes include Li ₂ S-P ₂ S ₅ , Li ₂ S-SiS ₂ , Li ₂ S-P ₂ S ₅ -GeS ₂ , Li ₂ S-B ₂ S _3- based glass, and the like. In addition to particles, there are also LGPS-based particles (such as Li ₁₀ GeP ₂ S ₁₂ ), which have recently attracted attention as having high lithium ion conductivity, and argyrodite-based particles [such as Li ₆ PS ₅ Cl, Li _{7 -x+y} PS _6-x Cl _x+y (0.05≦y≦0.9, -3.0x+1.8≦y≦-3.0x+5.7), Li _7-a PS _{6- a} Cl _b Br _c (wherein a=b+c, 0<a≦1.8, 0.1≦b/c≦10.0) can also be used. Among these, argyrodite-based materials, which have particularly high lithium ion conductivity and high chemical stability, are preferably used.

Examples of the hydride solid electrolyte include LiBH ₄ and a solid solution of LIBH ₄ and the following alkali metal compound (for example, one in which the molar ratio of LiBH ₄ and the alkali metal compound is 1:1 to 20:1). Can be mentioned. Examples of the alkali metal compounds in the solid solution include lithium halides (LiI, LiBr, LiF, LiCl, etc.), rubidium halides (RbI, RbBr, RbiF, RbCl, etc.), and cesium halides (CsI, CsBr, CsF, CsCl, etc.). , lithium amide, rubidium amide, and cesium amide.

Examples of the oxide solid electrolyte include Li ₇ La ₃ Zr ₂ O ₁₂ , LiTi(PO ₄ ) ₃ , LiGe(PO ₄ ) ₃ , and LiLaTiO ₃ .

The solid electrolyte can be one or more of the ones listed above.Among the solid electrolytes listed above, it has a high lithium ion conductivity and also has a function of improving the formability of the positive electrode mixture. It is more preferable to use a sulfide-based solid electrolyte.

The content of the solid electrolyte in the positive electrode mixture is preferably 4 to 30% by mass.

The positive electrode mixture does not need to contain a resin binder, or may contain it. Examples of the resin binder include fluororesins such as polyvinylidene fluoride (PVDF). However, since the resin binder acts as a resistance component in the positive electrode mixture, it is desirable that its amount be as small as possible. Therefore, it is preferable that the positive electrode mixture does not contain a resin binder, or if it contains a resin binder, the content thereof is 0.5% by mass or less. The content of the resinous binder in the positive electrode mixture is more preferably 0.3% by mass or less, and even more preferably 0% by mass (that is, no resinous binder is contained).

When a current collector is used for the positive electrode, metal foil such as aluminum or stainless steel, punched metal, net, expanded metal, foamed metal, carbon sheet, etc. can be used as the current collector.

A positive electrode mixture molded body is produced by, for example, compressing a positive electrode mixture prepared by mixing a positive electrode active material, a conductive additive, a solid electrolyte, and a binder added as necessary, etc. by pressure molding or the like. It can be formed by

In the case of a positive electrode having a current collector, it can be manufactured by bonding a molded body of the positive electrode mixture formed by the method described above to the current collector by pressing or the like.

The thickness of the positive electrode mixture molded body (in the case of a positive electrode with a current collector, the thickness of the positive electrode mixture molded body per one side of the current collector; the same applies hereinafter) is determined from the viewpoint of increasing the capacity of the battery. , 200 μm or more. Note that the load characteristics of a battery are generally easily improved by making the positive electrode and negative electrode thinner, but according to the present invention, the load characteristics can be improved even when the molded body of the positive electrode mixture is as thick as 200 μm or more. It is possible. Therefore, in the present invention, the effect becomes more pronounced when the thickness of the molded body of the positive electrode mixture is, for example, 200 μm or more. Further, the thickness of the molded body of the positive electrode mixture is usually 2000 μm or less.

<All-solid-state secondary battery>
The all-solid-state secondary battery of the present invention has a positive electrode, a negative electrode, and a solid electrolyte layer interposed between the positive electrode and the negative electrode, and the positive electrode is the positive electrode for an all-solid-state secondary battery of the present invention.

(Negative electrode)
The negative electrode of an all-solid-state secondary battery has a molded body of a negative electrode mixture containing a negative electrode active material, a conductive aid, a solid electrolyte, etc. For example, a negative electrode consisting only of the molded body, or a negative electrode composed of the molded body and a mixture thereof. Examples include a negative electrode that is integrated with an electric body.

Examples of negative electrode active materials include graphite, pyrolytic carbons, cokes, glassy carbons, fired bodies of organic polymer compounds, mesocarbon microbeads (MCMB), carbon fibers, and other materials that can occlude and release lithium. One or a mixture of two or more carbon-based materials may be used. Also, simple substances, compounds, and alloys thereof containing elements such as Si, Sn, Ge, Bi, Sb, and In; compounds that can be charged and discharged at low voltages close to those of lithium metal, such as lithium-containing nitrides or lithium-containing oxides; lithium metal ; Lithium/aluminum alloy; can also be used as the negative electrode active material.

The content of the negative electrode active material in the negative electrode mixture is preferably 50 to 95% by mass.

Carbon materials such as graphite (natural graphite, artificial graphite), graphene, carbon black, carbon nanofibers, carbon nanotubes, etc. can be used as the conductive agent for the negative electrode. The content of the conductive additive in the negative electrode mixture is preferably 1 to 10% by mass.

As the solid electrolyte of the negative electrode, one or more of the solid electrolytes exemplified above as solid electrolytes that can be included in the positive electrode mixture can be used. Among the solid electrolytes mentioned above, it is more preferable to use a sulfide-based solid electrolyte because it has high lithium ion conductivity and also has a function of improving the moldability of the negative electrode mixture.

The content of the solid electrolyte in the negative electrode mixture is preferably 4 to 49% by mass.

The negative electrode mixture does not need to contain a resin binder, or may contain it. Examples of the resin binder include fluororesins such as polyvinylidene fluoride (PVDF). However, since the resin binder also acts as a resistance component in the negative electrode mixture, it is desirable that its amount be as small as possible. Therefore, it is preferable that the negative electrode mixture does not contain a resin binder, or if it contains a resin binder, the content thereof is 0.5% by mass or less. The content of the resinous binder in the negative electrode mixture is more preferably 0.3% by mass or less, and even more preferably 0% by mass (that is, no resinous binder is contained).

When a current collector is used for the negative electrode, copper or nickel foil, punched metal, net, expanded metal, foamed metal, carbon sheet, etc. can be used as the current collector.

A negative electrode mixture molded body is produced by, for example, compressing a negative electrode mixture prepared by mixing a negative electrode active material, a conductive additive, a solid electrolyte, and a binder added as necessary, etc. by pressure molding or the like. It can be formed by

In the case of a negative electrode having a current collector, it can be manufactured by bonding a molded negative electrode mixture formed by the method described above to the current collector by pressing or the like.

The thickness of the molded body of the negative electrode mixture (in the case of a negative electrode with a current collector, the thickness of the molded body of the positive electrode mixture per one side of the current collector; the same applies hereinafter) is determined from the viewpoint of increasing the capacity of the battery. , 200 μm or more. Note that the load characteristics of a battery are generally easily improved by making the positive electrode and the negative electrode thinner, but according to the present invention, the load characteristics can be improved even when the molded body of the negative electrode mixture is as thick as 200 μm or more. It is possible. Therefore, in the present invention, the effect becomes more pronounced when the thickness of the molded body of the negative electrode mixture is, for example, 200 μm or more. In the present invention, the effect becomes particularly remarkable when the thickness of the molded body of the positive electrode mixture is 200 μm or more and the thickness of the molded body of the negative electrode mixture is 200 μm or more. Further, the thickness of the molded body of the negative electrode mixture is usually 3000 μm or less.

(solid electrolyte layer)
As the solid electrolyte in the solid electrolyte layer, one or more of the same solid electrolytes as those exemplified above as the solid electrolyte of the positive electrode can be used. However, in order to improve battery characteristics, it is desirable to contain a sulfide-based solid electrolyte, and it is more desirable to contain a sulfide-based solid electrolyte in all of the positive electrode, negative electrode, and solid electrolyte layer.

The solid electrolyte layer may have a porous material such as a resin nonwoven fabric as a support.

A solid electrolyte layer is formed by compressing a solid electrolyte by pressure molding or the like; a solid electrolyte layer-forming composition prepared by dispersing a solid electrolyte in a solvent is applied onto a base material, a positive electrode, and a negative electrode, and dried. It can be formed by a method of performing pressure forming such as press treatment as necessary.

The solvent used in the solid electrolyte layer forming composition is preferably selected from a solvent that does not easily deteriorate the solid electrolyte. In particular, sulfide-based solid electrolytes and hydride-based solid electrolytes cause chemical reactions with minute amounts of water, so hydrocarbon solvents such as hexane, heptane, octane, nonane, decane, decalin, toluene, and xylene are typical. Preference is given to using non-polar aprotic solvents. In particular, it is more preferable to use a super dehydrated solvent with a water content of 0.001% by mass (10 ppm) or less. In addition, fluorinated solvents such as "Vertrell (registered trademark)" manufactured by Mitsui-DuPont Fluorochemicals, "Zeorolla (registered trademark)" manufactured by Nippon Zeon, and "Novec (registered trademark)" manufactured by Sumitomo 3M, Non-aqueous organic solvents such as , dichloromethane and diethyl ether can also be used.

The thickness of the solid electrolyte layer is preferably 100 to 300 μm.

(electrode body)
The positive electrode and the negative electrode can be used in a battery in the form of a laminated electrode body laminated with a solid electrolyte layer in between, or a wound electrode body in which this laminated electrode body is further wound.

Note that when forming the electrode body, it is preferable to press-mold the positive electrode, the negative electrode, and the solid electrolyte layer in a laminated state from the viewpoint of increasing the mechanical strength of the electrode body.

(Battery form)
A cross-sectional view schematically showing an example of the all-solid-state secondary battery of the present invention is shown in FIG. The battery 1 shown in FIG. 1 includes a positive electrode 10, a negative electrode 20, and a positive electrode 10 and a negative electrode 20 in an external case formed of an external can 40, a sealed can 50, and a resin gasket 60 interposed between these. A solid electrolyte layer 30 interposed therebetween is enclosed.

The sealing can 50 is fitted into the opening of the outer can 40 via a gasket 60, and the open end of the outer can 40 is tightened inward, causing the gasket 60 to come into contact with the sealing can 50. The opening of the outer can 40 is sealed, so that the inside of the element has a hermetically sealed structure.

Stainless steel can be used for the outer can and sealed can. In addition, polypropylene, nylon, etc. can be used as the material for the gasket, and if heat resistance is required due to battery usage, materials such as tetrafluoroethylene-perfluoroalkoxyethylene copolymer (PFA) can be used. Heat-resistant materials with melting points exceeding 240°C such as fluororesin, polyphenylene ether (PEE), polysulfone (PSF), polyarylate (PAR), polyether sulfone (PES), polyphenylene sulfide (PPS), and polyether ether ketone (PEEK) Resins can also be used. Furthermore, when the battery is used in applications requiring heat resistance, a glass hermetic seal can also be used to seal the battery.

Further, FIGS. 2 and 3 are diagrams schematically showing other examples of the all-solid-state secondary battery of the present invention. FIG. 2 is a plan view of the all-solid-state secondary battery, and FIG. 3 is a sectional view taken along line II in FIG. 2.

The all-solid-state secondary battery 100 shown in FIGS. 2 and 3 houses an electrode body 200 comprising a positive electrode, a solid electrolyte layer, and a negative electrode of the present invention in a laminate film exterior body 500 comprised of two metal laminate films. The laminate film exterior body 500 is sealed at its outer periphery by heat-sealing the upper and lower metal laminate films. In addition, in FIG. 3, in order to avoid complicating the drawing, each layer constituting the laminate film exterior body 500 and the positive electrode, negative electrode, and separator constituting the electrode body are not shown separately.

The positive electrode of the electrode body 200 is connected to the positive external terminal 300 within the battery 100, and although not shown, the negative electrode of the electrode body 200 is also connected to the negative external terminal 400 within the battery 100. There is. The positive external terminal 300 and the negative external terminal 400 have one end pulled out to the outside of the laminate film exterior body 500 so that they can be connected to an external device or the like.

The form of an all-solid-state secondary battery is one that has an exterior body composed of an exterior can, a sealed can, and a gasket, as shown in Figure 1, that is, a type that is generally referred to as a coin-shaped battery or a button-shaped battery. In addition to those with exterior bodies made of resin films or metal-resin laminate films as shown in Figures 2 and 3, there are It may have an exterior body that includes an exterior can and a sealing structure that seals the opening of the exterior can.

The all-solid-state secondary battery of the present invention uses lithium cobalt oxide (A), which has excellent stability in a high-voltage charging state, as a positive electrode active material. Therefore, the all-solid-state secondary battery of the present invention can be used with the final voltage set at about 4.2V during charging, similar to conventional lithium-ion secondary batteries having organic electrolytes. , it may be used in a charging method with a higher final voltage of 4.3 V or higher, and even if it is charged in this way to increase the capacity and is used at high temperatures, it will maintain good discharge characteristics. and storage properties. However, the final voltage during charging of the all-solid-state secondary battery is preferably 4.6V or less.

The all-solid-state secondary battery of the present invention can be applied to the same applications as conventionally known secondary batteries, but it has excellent heat resistance because it has a solid electrolyte instead of an organic electrolyte. It can be preferably used in applications that are exposed to high temperatures.

Hereinafter, the present invention will be described in detail based on Examples. However, the following examples do not limit the present invention.

Example 1
<Preparation of positive electrode>
Li _0.98 Co _0.95 Mg _0.05 O ₂ (positive electrode active material) having a layer consisting of LiNbO ₃ on the surface, and a sulfide-based solid electrolyte (Li ₆ PS) having an argyrodite structure with an average particle size of 4 μm. ₅ Cl) and acetylene black (conductive support agent) were mixed at a mass ratio of 60:35:5 and thoroughly kneaded to prepare a positive electrode mixture. Next, 15 mg of the positive electrode mixture was placed in a powder molding mold with a diameter of 10 mm, and pressure molded using a press to produce a positive electrode made of a cylindrical positive electrode mixture molded body. Note that the amount of the layer made of LiNbO ₃ on the surface of Li _0.98 Co _0.95 Mg _0.05 O ₂ is 0 per 100 parts by mass of Li _0.98 Co _0.95 Mg _0.05 O ₂ It was .5 parts by mass.

<Formation of solid electrolyte layer>
Next, 15 mg of the same sulfide solid electrolyte as that used for producing the positive electrode was placed on top of the positive electrode mixture molded body in the powder molding mold, and pressure molding was performed using a press machine. A solid electrolyte layer was formed on the positive electrode mixture molded body.

<Formation of laminated electrode body>
As a counter electrode, a stack of Li and In metal formed into a cylindrical shape was used. This counter electrode was placed on the surface of the solid electrolyte layer in the powder molding die opposite to the positive electrode, and pressure molded using a press machine to produce a laminated electrode body.

<Assembling the model cell>
A model cell having a planar structure similar to that shown in FIG. 2 was manufactured using the laminated electrode body described above. A positive electrode current collector foil (SUS foil) and a counter electrode current collector foil (SUS foil) are placed horizontally on the inside surface of the exterior body of the aluminum laminate film that constitutes the laminate film exterior body, with a certain amount of space between them. I pasted them side by side. Each of the current collector foils has a main body portion facing the positive electrode side surface or the counter electrode side surface of the laminated electrode body, and a positive electrode external terminal 300 and a counter electrode external terminal 400 protruding from the main body portion toward the outside of the cell. A piece cut into a shape with two parts was used.

The laminated electrode body is placed on the counter electrode current collector foil of the laminated film exterior body, and the laminated electrode body is wrapped with the laminated film exterior body so that the positive electrode current collector foil is disposed on the positive electrode of the laminated electrode body, The remaining three sides of the laminate film exterior body were sealed by heat sealing under vacuum to obtain a model cell.

Example 2
The positive electrode active material is made of Li _0.98 Co _0.99 Mg _0.01 O ₂ with a layer made of LiNbO ₃ on the surface (consisting of LiNbO ₃ on the surface of Li _0.98 Co _0.99 Mg _0.01 O ₂ ). A model cell was produced in the same manner as in Example 1, except that the amount of the layer was changed to 0.5 parts by mass per 100 parts by mass of Li _0.98 Co _0.99 Mg _0.01 O ₂ .

Example 3
The positive electrode active material is made of Li _0.98 Co _0.9 Mg _0.1 O ₂ with a layer made of LiNbO ₃ on the surface (consisting of LiNbO ₃ on the surface of Li _0.98 Co _0.9 Mg _0.1 O _{2 )} . A model cell was produced in the same manner as in Example 1, except that the amount of the layer was changed to 0.5 parts by mass per 100 parts by mass of Li _0.98 Co _0.9 Mg _0.1 O ₂ .

Example 4
The positive electrode active material is made of Li _0.95 Co _0.99 Mg _0.01 O ₂ with a layer made of LiNbO ₃ on the surface (consisting of LiNbO ₃ on the surface of Li _0.95 Co _0.99 Mg _0.01 O ₂ ). A model cell was produced in the same manner as in Example 1, except that the amount of the layer was changed to 0.5 parts by mass per 100 parts by mass of Li _0.95 Co _0.99 Mg _0.01 O ₂ .

Example 5
The positive electrode active material is made of Li _0.95 Co _0.95 Mg _0.05 O ₂ (Li _0.95 Co _0.95 Mg _0.05 O ₂ with _{a layer made of LiNbO 3 on} the surface). A model cell was produced in the same manner as in Example 1, except that the amount of the layer was changed to 0.5 parts by mass per 100 parts by mass of Li _0.95 Co _0.95 Mg _0.05 O ₂ .

Example 6
The positive electrode active material is made of Li _0.95 Co _0.9 Mg _0.1 O ₂ with a layer made of LiNbO ₃ on the surface (consisting of LiNbO ₃ on the surface of Li _0.95 Co _0.9 Mg _0.1 O _{2 )} . A model cell was produced in the same manner as in Example 1, except that the amount of the layer was changed to 0.5 parts by mass per 100 parts by mass of Li _0.95 Co _0.9 Mg _0.1 O ₂ .

Example 7
The positive electrode active material was made of Li _0.98 Co _0.9 Mg _0.05 Al _0.05 O ₂ (Li _0.98 Co _0.9 Mg _0.05 Al _0.05 O) having a layer of LiNbO ₃ on the surface. Except that the amount of the layer consisting of LiNbO ₃ on the surface of ₂ was changed to 0.5 parts by mass per 100 parts by mass of Li _0.98 Co _0.9 Mg _0.05 Al _0.05 O ₂ A model cell was produced in the same manner as in Example 1.

Example 8
The positive electrode active material was made of Li _0.98 Co _0.94 Mg _0.01 Al _0.05 O ₂ (Li _0.98 Co _0.94 Mg _0.01 Al _0.05 O) having a layer of LiNbO ₃ on the surface. Except that the amount of the layer consisting of LiNbO ₃ on the surface of _{No. 2} was changed to 0.5 parts by mass relative to 100 parts by mass of Li _0.98 Co _0.94 Mg _0.01 Al _0.05 O ₂ A model cell was produced in the same manner as in Example 1.

Example 9
The positive electrode active material was made of Li _0.98 Co _0.85 Mg _0.1 Al _0.05 O ₂ (Li _0.98 Co _0.85 Mg _0.1 Al _0.05 O) having a layer made of LiNbO ₃ on the surface. Except that the amount of the layer consisting of LiNbO ₃ on the surface of _{No. 2} was changed to 0.5 parts by mass relative to 100 parts by mass of Li _0.98 Co _0.85 Mg _0.1 Al _0.05 O ₂ A model cell was produced in the same manner as in Example 1.

Example 10
The positive electrode active material was made of Li _0.98 Co _0.9 Mg _0.05 Al _0.04 Ti _0.01 O ₂ (Li _0.98 Co _0.9 Mg _0.05 Al) having a layer of LiNbO ₃ on the surface. The amount of the layer consisting of LiNbO ₃ on the surface of _0.04 Ti _0.01 O ₂ is based on 100 parts by mass of Li _0.98 Co _0.9 Mg _0.05 Al _0.04 Ti _0.01 O ₂ A model cell was produced in the same manner as in Example 1, except that the amount was changed to 0.5 parts by mass).

Comparative example 1
The positive electrode active material is LiCo _0.95 Mg _0.05 O ₂ having a layer made of LiNbO ₃ on the surface (the amount of the layer made of LiNbO ₃ on the surface of LiCo _0.95 Mg _0.05 O ₂ is LiCo _{0.95 Mg 0.05 O 2 ).} A model cell was produced in the same manner as in Example 1, except that the amount of ₉₅ Mg _0.05 O ₂ was changed to 0.5 parts by mass per 100 parts by mass).

Comparative example 2
The positive electrode active material is made of Li _1.02 Co _0.95 Mg _0.05 O ₂ with a layer made of LiNbO ₃ on the surface (consisting of LiNbO ₃ on the surface of Li _1.02 Co _0.95 Mg _0.05 O ₂ ). A model cell was produced in the same manner as in Example 1, except that the amount of the layer was changed to 0.5 parts by mass per 100 parts by mass of Li _1.02 Co _0.95 Mg _0.05 O ₂ .

Comparative example 3
The positive electrode active material was made of Li _0.98 CoO ₂ having a layer made of _{LiNbO 3} on the surface (the amount of the layer made of LiNbO ₃ on the surface of Li 0.98 CoO 2 was 100 parts by mass of Li _0.98 CoO _{2 )} . A model cell was produced in the same manner as in Example 1, except that the amount was changed to 0.5 parts by mass).

Comparative example 4
Except that the positive electrode active material was changed to LiCoO ₂ having a layer made of LiNbO ₃ on the surface (the amount of the layer made of LiNbO ₃ on the surface of LiCoO ₂ was 0.5 parts by mass per 100 parts by mass of LiCoO ₂ ). A model cell was prepared in the same manner as in Example 1.

Comparative example 5
_The positive electrode active material was made of Li _1.02 CoO ₂ having a layer made of LiNbO ₃ on the surface (the amount of the layer made of LiNbO ₃ on the surface of Li _1.02 CoO 2 was 100 parts by mass of Li _1.02 CoO ₂ ). A model cell was produced in the same manner as in Example 1, except that the amount was changed to 0.5 parts by mass).

Comparative example 6
The positive electrode active material is made of Li _0.98 Co _0.95 Al _0.05 O ₂ with a layer made of LiNbO ₃ on the surface (consisting of LiNbO ₃ on the surface of Li _0.98 Co _0.95 Al _0.05 O ₂ ). A model cell was produced in the same manner as in Example 1, except that the amount of the layer was changed to 0.5 parts by mass per 100 parts by mass of Li _0.98 Co _0.95 Al _0.05 O ₂ .

Comparative example 7
The positive electrode active material is LiCo _0.95 Al _0.05 O ₂ having a layer of LiNbO ₃ on the surface (the amount of the layer of LiNbO ₃ on the surface of LiCo _0.95 Al _0.05 O ₂ is LiCo 0.95 Al 0.05 _{O 2 ).} A model cell was produced in the same manner as in Example 1, except that the amount of ₉₅ Al _0.05 O ₂ was changed to 0.5 parts by mass per 100 parts by mass).

Comparative example 8
The positive electrode active material is made of Li _1.02 Co _0.95 Al _0.05 O ₂ with a layer made of LiNbO ₃ on the surface (consisting of LiNbO ₃ on the surface of Li _1.02 Co _0.95 Al _0.05 O ₂ ). A model cell was produced in the same manner as in Example 1, except that the amount of the layer was changed to 0.5 parts by mass per 100 parts by mass of Li _1.02 Co _0.95 Al _0.05 O ₂ .

Comparative example 9
The positive electrode active material is made of Li _0.98 Co _0.85 Mg _0.15 O ₂ (Li _0.98 Co _0.85 Mg _0.15 O ₂ with _{a layer made of LiNbO 3 on} the surface). A model cell was produced in the same manner as in Example 1, except that the amount of the layer was changed to 0.5 parts by mass per 100 parts by mass of Li _0.98 Co _0.85 Mg _0.15 O ₂ .

Comparative example 10
The positive electrode active material was made of Li _0.98 Co _0.87 Mg _0.05 Al _0.08 O ₂ (Li _0.98 Co _0.87 Mg _0.05 Al _0.08 O) having a layer made of LiNbO ₃ on the surface. Except that the amount of the layer consisting of LiNbO ₃ on the surface of ₂ was changed to 0.5 parts by mass per 100 parts by mass of Li _0.98 Co _0.87 Mg _0.05 Al _0.08 O ₂ A model cell was produced in the same manner as in Example 1.

Comparative example 11
A model cell was produced in the same manner as in Example 1, except that the positive electrode active material was changed to Li _0.98 Co _0.95 Mg _0.05 O ₂ which does not have a layer made of LiNbO ₃ on the surface.

Reference example 1
A positive electrode produced in the same manner as in Example 1 and a counter electrode identical to that used in Example 1 were stacked with a PE separator having a thickness of 20 μm sandwiched therebetween to produce a laminated electrode body. This laminated electrode body and an organic electrolyte (a solution of 1M LiPF ₆ dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate at a volume ratio of 1:2) were mixed into the same laminate as used in Example 1. A model cell was produced by encapsulating it in a film exterior.

Reference example 2
The positive electrode active material is made of Li _1.02 Co _0.95 Mg _0.05 O ₂ with a layer made of LiNbO ₃ on the surface (consisting of LiNbO ₃ on the surface of Li _1.02 Co _0.95 Mg _0.05 O ₂ ). A model cell was produced in the same manner as in Reference Example 1, except that the amount of the layer was changed to 0.5 parts by mass per 100 parts by mass of Li _1.02 Co _0.95 Mg _0.05 O ₂ .

The model cells produced in Examples, Comparative Examples, and Reference Examples were evaluated for load characteristics and high-temperature storage characteristics using the following methods.

[Load characteristics evaluation]
For each model cell, constant current charging was performed at a current value of 0.1C until the voltage reached 4.3V based on Li, then constant voltage charging was performed until the current value reached 0.01C, and then 0.1C The initial capacity was measured by discharging until the current value reached a voltage of 2.5 V based on Li.

After the initial capacity measurement, each model cell was charged and discharged under the same conditions as the initial capacity measurement, except that the current value during discharge was changed to 1C, and the 1C discharge capacity was measured. Then, the value obtained by dividing the 1C discharge capacity of each battery by the initial capacity (0.1C discharge capacity) was expressed as a percentage to determine the capacity retention rate, and the load characteristics of each model cell were evaluated.

For the model cells of Example 1 and Reference Example 1, the initial capacity and 1C discharge capacity were determined under the same conditions as above, except that the end-of-charge voltage was changed to 4.5V, and the capacity retention rate under these conditions was also calculated. did.

[Evaluation of high temperature storage characteristics]
For each model cell, the initial capacity was determined under the same conditions as when evaluating load characteristics. Next, each model cell was charged under the same conditions as when measuring the initial capacity, stored for 5 days at 60°C, and then the temperature of each battery was returned to room temperature, and then discharged under the same conditions as the initial capacity. I let it happen. Furthermore, each battery after discharge was charged and discharged under the same conditions as the initial capacity to determine the recovery capacity. Then, for each battery, the recovered capacity retention rate was determined by dividing the recovered capacity by the initial capacity and expressed as a percentage, and the high temperature storage characteristics were evaluated.

Table 1 shows the configuration of each model cell, and Table 2 shows the evaluation results. Note that the voltages in parentheses in the column of Example 1 and Reference Examples in Table 2 mean the end-of-charge voltage.

As shown in Tables 1 and 2, the model cells produced in Examples 1 to 10 had positive electrodes using lithium cobalt oxide (A) as a positive electrode active material with a reaction suppression layer on the surface, and had load characteristics. Both the capacity retention rate at the time of evaluation and the recovery capacity retention rate at the time of high-temperature storage characteristics evaluation were high, and it had excellent load characteristics and high-temperature storage characteristics. Furthermore, the model cell produced in Example 1 had good load characteristics and high-temperature storage characteristics even when the end-of-charge voltage was increased to 4.5V.

On the other hand, the model cells prepared in Comparative Examples 1 to 10 have positive electrodes using lithium cobalt oxide as a positive electrode active material in which the amount of constituent elements does not satisfy the general formula (1), and the load characteristics were evaluated. The capacity retention rate was low and the load characteristics were poor. In addition, the model cell prepared in Comparative Example 11 has a positive electrode using a positive electrode active material that does not have a reaction suppression layer on the surface, and has a capacity retention rate when evaluating load characteristics and a recovery capacity when evaluating high temperature storage characteristics. Both retention rates were low, and both load characteristics and high temperature storage characteristics were poor.

Note that among the model cells using an organic electrolyte instead of a solid electrolyte, the model cell of Reference Example 1, which has a positive electrode using lithium cobalt oxide (A) having a reaction suppression layer on the surface as a positive electrode active material, Although the capacity retention rate during property evaluation was high, the recovery capacity retention rate during high-temperature storage property evaluation was low. In addition, the model cell of Reference Example 2, which has a positive electrode using lithium cobalt oxide as a positive electrode active material whose amount of constituent elements does not satisfy the above general formula (1), has a capacity retention rate during load characteristic evaluation and high-temperature storage characteristic evaluation. Both recovery capacity maintenance rates were low.

1,100 All-solid secondary battery 10 Positive electrode 20 Negative electrode 30 Solid electrolyte layer 40 Exterior can 50 Sealing can 60 Gasket 200 Electrode body 300 Positive electrode external terminal 400 Negative electrode external terminal 500 Laminated film exterior body

Claims (3)

A positive electrode for an all-solid-state secondary battery having a molded positive electrode mixture containing a positive electrode active material, a conductive additive, and a solid electrolyte,
As the positive electrode active material, when fully discharged to a voltage equivalent to 2.5V based on Li, the following general formula (1)
Li _a Co _1-b-c Mg _b M ¹ _c O ₂ (1)
[In the general formula (1), M ¹ is Al, Ni, Mn, Na, Fe, Cu, Zr, Ti, Bi, Ca, F, P, Sr, W, Ba, Nb, Si, Zn, Mo , V, Sn, Sb, Ta, Ge, Cr, K, S and Er, and a<1, 0<b≦0.1, 0≦c≦0 contains a lithium cobalt oxide represented by
A positive electrode for an all-solid-state secondary battery, characterized in that a reaction suppression layer for suppressing a reaction between the positive electrode active material and the solid electrolyte is formed on the surface of the positive electrode active material. The positive electrode for an all-solid-state secondary battery according to claim 1, wherein the reaction suppression layer contains an Nb-containing oxide. An all-solid-state secondary battery comprising a positive electrode, a negative electrode, and a solid electrolyte layer interposed between the positive electrode and the negative electrode,
An all-solid-state secondary battery comprising, as the positive electrode, the positive electrode for an all-solid-state secondary battery according to claim 1 or 2.

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