JP7722002B2 - Positive electrodes for lithium-ion secondary batteries - Google Patents

Description

The present invention relates to a positive electrode for a lithium-ion secondary battery.

In recent years, the widespread adoption of various electric vehicles is expected to help solve environmental and energy problems. Secondary batteries are being actively developed as on-board power sources for driving motors and other applications, which hold the key to the widespread adoption of these electric vehicles. Non-aqueous electrolyte secondary batteries, such as lithium-ion secondary batteries, are attracting attention as they are expected to offer high energy density and high output.

Patent Document 1 discloses technology aimed at improving the safety of lithium-ion secondary batteries. Specifically, the technology described in Patent Document 1 is characterized by using a composite PTC (Positive Temperature Coefficient) binder containing polyvinylidene fluoride (PVDF) and a PTC (Positive Temperature Coefficient) functionality-imparting component (e.g., PVDF-HFP) as the binder for the positive electrode active material layer. As a result, when the temperature of the lithium-ion secondary battery rises, the binder melts, increasing the resistance of the positive electrode active material layer and suppressing heat generation in the battery.

International Publication No. 2018/128139

However, after further investigation, the inventors discovered that lithium-ion secondary batteries using the technology described in Patent Document 1 have the problem of insufficient output characteristics.

The present invention therefore aims to provide a means for improving the output characteristics of lithium-ion secondary batteries.

The inventors conducted extensive research to solve the above problems. In the process, they discovered that it is possible to improve the output characteristics of lithium-ion secondary batteries by controlling the roughness factor of the positive electrode active material contained in the positive electrode active material layer and the type and content of the binder within specific ranges, which led to the completion of the present invention.

That is, one aspect of the present invention provides a positive electrode for a lithium-ion secondary battery, having a positive electrode active material layer containing a positive electrode active material, a binder, and a conductive additive. The positive electrode is characterized in that the roughness factor, defined as the ratio of the BET specific surface area of the positive electrode active material to the geometric specific surface area calculated from the average particle diameter, is 3 or less; the binder is a vinylidene fluoride-hexafluoropropylene copolymer, and the proportion of the number of structural units derived from hexafluoropropylene to the total number of structural units of the copolymer is 5 mol% or more; and the binder content is greater than 0.31 mass% and less than 4.85 mass% of the total solid content of the positive electrode active material layer.

The present invention makes it possible to improve the output characteristics of lithium-ion secondary batteries.

1 is a cross-sectional view schematically illustrating a stacked (flat) non-bipolar (internal parallel connection) secondary battery according to one embodiment of the present invention. FIG. 1 is a cross-sectional view schematically illustrating a bipolar secondary battery according to another embodiment of the present invention.

One aspect of the present invention relates to a positive electrode for a lithium-ion secondary battery (hereinafter also referred to simply as "positive electrode") having a positive electrode active material layer containing a positive electrode active material, a binder, and a conductive additive. In this aspect, the positive electrode active material has a roughness factor of 3 or less, defined as the ratio of the BET specific surface area to the geometric specific surface area calculated from the average particle diameter; the binder is a vinylidene fluoride-hexafluoropropylene copolymer (hereinafter also referred to as "PVDF-HFP"), and the ratio of the number of structural units derived from hexafluoropropylene (hereinafter also referred to as "HFP") to the total number of structural units of the copolymer is 5 mol% or more; and the binder content is greater than 0.31 mass% and less than 4.85 mass% of the total solid content of the positive electrode active material layer.

The positive electrode for a lithium ion secondary battery according to this embodiment can improve the output characteristics of the lithium ion secondary battery. The inventors speculate that the mechanism by which this effect is achieved is as follows.

In this embodiment, a positive electrode active material having a roughness factor of 3 or less has a small surface area. Furthermore, PVDF-HFP, which has a ratio of HFP-derived structural units (hereinafter simply referred to as "HFP ratio") of 5 mol % or more as a binder, is a gel-forming polymer and has higher flexibility than PVDF, a homopolymer of vinylidene fluoride, or PVDF-HFP with an HFP ratio of less than 5 mol %. By combining this positive electrode active material with a binder, the binder is more uniformly coated on the surface of the positive electrode active material compared to conventional positive electrodes. This uniform coating can be maintained even after repeated contraction and expansion of the positive electrode active material due to charge and discharge. Furthermore, PVDF-HFP with a high HFP ratio as described above can retain the electrolyte. This improves contact between the surface of the positive electrode active material and the electrolyte, improving lithium ion conductivity. As a result, applying the positive electrode for a lithium ion secondary battery according to this embodiment to a lithium ion secondary battery can improve the output characteristics of the lithium ion secondary battery.

On the other hand, when a cathode active material with a roughness factor greater than 3 is used, as shown in Comparative Examples 3 to 5 below, or when PVDF or PVDF-HFP with an HFP ratio of less than 5 mol% is used, as shown in Comparative Examples 7 to 9 and 11 below, the output characteristics are insufficient. This is thought to be due to the large surface area of the cathode active material and the low flexibility of the binder, which makes it difficult to uniformly coat the surface of the cathode active material with the binder, and the binder's low electrolyte retention capacity. Repeated contraction and expansion of the cathode active material due to charging and discharging under these conditions creates voids on the surface of the cathode active material where no electrolyte is present, reducing lithium ion conductivity and resulting in insufficient output characteristics.

The following describes the above-mentioned embodiment of the present invention with reference to the drawings. However, the technical scope of the present invention should be determined based on the claims and is not limited to the following embodiments. Note that the dimensional proportions in the drawings have been exaggerated for the sake of explanation and may differ from the actual proportions. In this specification, the range "X to Y" means "X or more and Y or less." Furthermore, unless otherwise specified, operations and measurements of physical properties are performed at room temperature (20-25°C) and a relative humidity of 40-50% RH.

Figure 1 is a cross-sectional view schematically illustrating a flat (stacked) non-bipolar (internal parallel connection) secondary battery (hereinafter simply referred to as a "stacked secondary battery") according to one embodiment of the present invention.

As shown in FIG. 1, the stacked secondary battery 10a of this embodiment has a structure in which a substantially rectangular power generating element 21, where the actual charge/discharge reaction takes place, is sealed inside a laminate film 29. Here, the power generating element 21 has a laminated configuration comprising a positive electrode in which a positive electrode active material layer 13 is disposed on both sides of a positive electrode current collector 11', an electrolyte layer 17 made of a separator containing an electrolytic solution, and a negative electrode in which a negative electrode active material layer 15 is disposed on both sides of a negative electrode current collector 12. Specifically, the positive electrode, electrolyte layer, and negative electrode are stacked in this order, with one positive electrode active material layer 13 and the adjacent negative electrode active material layer 15 facing each other with the electrolyte layer 17 interposed therebetween.

As a result, the positive electrode, electrolyte layer, and negative electrode constitute a single cell layer 19. Therefore, the stacked secondary battery 10a shown in FIG. 1 can be said to have a configuration in which multiple cell layers 19 are stacked and electrically connected in parallel. Note that while the positive electrode current collectors located on both outermost layers of the power generating element 21 each have a positive electrode active material layer 13 disposed on only one side, active material layers may be disposed on both sides. That is, instead of using a current collector dedicated to the outermost layer with an active material layer disposed on only one side, a current collector with active material layers on both sides may be used as the outermost current collector. Furthermore, by reversing the arrangement of the positive and negative electrodes from FIG. 1, the outermost negative electrode current collectors may be located on both outermost layers of the power generating element 21, and negative electrode active material layers may be disposed on one or both sides of the outermost negative electrode current collectors.

A positive electrode current collector 25 and a negative electrode current collector 27, which are electrically connected to the respective electrodes (positive and negative electrodes), are attached to the positive electrode current collector 11' and the negative electrode current collector 12, respectively, and are configured to be sandwiched between the edges of the laminate film 29 and extend to the outside of the laminate film 29. If necessary, the positive electrode current collector 25 and the negative electrode current collector 27 may be attached to the positive electrode current collector 11' and the negative electrode current collector 12 of each electrode via positive electrode terminal leads and negative electrode terminal leads (not shown), respectively, by ultrasonic welding, resistance welding, or the like.

Figure 2 is a cross-sectional view schematically illustrating a bipolar secondary battery according to another embodiment of the present invention. The bipolar secondary battery 10b shown in Figure 2 has a structure in which a substantially rectangular power generating element 21, where the actual charge/discharge reactions take place, is sealed inside a laminate film 29, which is the battery exterior. Note that in this specification, a bipolar lithium-ion secondary battery may also be referred to simply as a "bipolar secondary battery," and an electrode for a bipolar lithium-ion secondary battery may also be referred to simply as a "bipolar electrode."

As shown in FIG. 2 , the power generating element 21 of the bipolar secondary battery 10b of this embodiment has multiple bipolar electrodes 23, each having a positive electrode active material layer 13 electrically coupled to one surface of a current collector 11 and a negative electrode active material layer 15 electrically coupled to the other surface of the current collector 11. The bipolar electrodes 23 are stacked with an electrolyte layer 17 interposed between them to form the power generating element 21. The bipolar electrodes 23 and the electrolyte layers 17 are alternately stacked so that the positive electrode active material layer 13 of one bipolar electrode 23 faces the negative electrode active material layer 15 of another bipolar electrode 23 adjacent to the first bipolar electrode 23 with the electrolyte layer 17 interposed between them. In other words, the electrolyte layer 17 is sandwiched between the positive electrode active material layer 13 of one bipolar electrode 23 and the negative electrode active material layer 15 of the other bipolar electrode 23 adjacent to the first bipolar electrode 23.

Adjacent positive electrode active material layers 13, electrolyte layers 17, and negative electrode active material layers 15 constitute a single cell layer 19. Therefore, the bipolar secondary battery 10b can be said to have a configuration in which the cell layers 19 are stacked one on top of the other. A seal (insulating layer) 31 is disposed around the outer periphery of the cell layer 19. This prevents liquid junctions due to leakage of electrolyte from the electrolyte layer 17, and prevents contact between adjacent current collectors 11 within the battery and short circuits caused by slight misalignment of the edges of the cell layers 19 in the power-generating element 21. The positive electrode side outermost current collector 11a, which is the outermost layer of the power-generating element 21, has a positive electrode active material layer 13 formed on only one side. The negative electrode side outermost current collector 11b, which is the outermost layer of the power-generating element 21, has a negative electrode active material layer 15 formed on only one side.

Furthermore, in the bipolar secondary battery 10b shown in Figure 2, a positive electrode current collector (positive electrode tab) 25 is arranged adjacent to the outermost current collector 11a on the positive electrode side, and this extends out from the laminate film 29 that is the battery outer casing. On the other hand, a negative electrode current collector (negative electrode tab) 27 is arranged adjacent to the outermost current collector 11b on the negative electrode side, and similarly extends out from the laminate film 29.

The number of times the cell layers 19 are stacked is adjusted according to the desired voltage. Furthermore, in the case of bipolar secondary battery 10b, the number of times the cell layers 19 are stacked may be reduced if sufficient output can be ensured even when the battery thickness is made as thin as possible. In order to protect bipolar secondary battery 10b from external impacts and environmental degradation during use, it is also preferable to evacuate and seal the power generating element 21 in a laminate film 29, which is the battery outer casing, and to extend the positive electrode current collector 25 and negative electrode current collector 27 outside the laminate film 29.

The main components of the positive electrode for a lithium-ion secondary battery according to this embodiment are described below. The positive electrode for a lithium-ion secondary battery according to this embodiment has a positive electrode active material layer containing a positive electrode active material, a binder, and a conductive additive. The positive electrode active material layer is formed on the surface of an optional current collector.

[Current collector]
The current collector has a function of mediating the transfer of electrons from the positive electrode active material layer and the negative electrode active material layer described later. There are no particular limitations on the material constituting the current collector. For example, metals and conductive resins can be used as the material constituting the current collector.

Specific examples of metals include aluminum, nickel, iron, stainless steel, titanium, and copper. Other materials that may be used include clad materials of nickel and aluminum, and clad materials of copper and aluminum. Foils in which the metal surface is coated with aluminum may also be used. Among these, aluminum, stainless steel, copper, and nickel are preferred from the standpoints of electronic conductivity, battery operating potential, and adhesion of the negative electrode active material to the current collector by sputtering.

An example of the latter conductive resin is a resin in which a conductive filler is added to a non-conductive polymer material.

Examples of non-conductive polymeric materials include polyethylene (PE; high-density polyethylene (HDPE), low-density polyethylene (LDPE), etc.), polypropylene (PP), polyethylene terephthalate (PET), polyethernitrile (PEN), polyimide (PI), polyamideimide (PAI), polyamide (PA), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polymethyl acrylate (PMA), polymethyl methacrylate (PMMA), polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), and polystyrene (PS).

Any conductive filler can be used without particular restrictions, as long as it is a conductive material. Examples of materials with excellent conductivity, potential resistance, or lithium ion blocking properties include metals and conductive carbon. There are no particular restrictions on the metal, but it is preferable that it contains at least one metal selected from the group consisting of Ni, Ti, Al, Cu, Pt, Fe, Cr, Sn, Zn, In, and Sb, or an alloy or metal oxide containing such a metal. There are also no particular restrictions on the conductive carbon. It is preferable that it contains at least one material selected from the group consisting of acetylene black, Vulcan (registered trademark), Black Pearl (registered trademark), carbon nanofiber, Ketjen Black (registered trademark), carbon nanotubes, carbon nanohorns, carbon nanoballoons, and fullerenes.

There are no particular restrictions on the amount of conductive filler added, as long as it is an amount that can impart sufficient conductivity to the current collector, and it is generally around 5 to 80% by mass.

The current collector may have a single layer structure made of a single material, or a laminate structure made of an appropriate combination of layers made of these materials. From the perspective of reducing the weight of the current collector, it is preferable for it to contain at least a conductive resin layer made of a resin with electrical conductivity. Furthermore, from the perspective of blocking the movement of lithium ions between the cell layers, a metal layer may be provided on part of the current collector. Furthermore, if the positive electrode active material layer and negative electrode active material layer described below are themselves conductive and can perform a current collecting function, it is not necessary to use a current collector as a separate component from these electrode active material layers. In such a configuration, the positive electrode active material layer described below directly constitutes the negative electrode, and the negative electrode active material layer described below directly constitutes the positive electrode.

[Cathode active material layer]
The positive electrode active material layer contains a positive electrode active material, a binder, and a conductive additive, and may further contain optional components such as an electrolytic solution (liquid electrolyte).

(Cathode active material)
The positive electrode active material has the function of releasing ions such as lithium ions during charging and absorbing ions such as lithium ions during discharging. In the positive electrode for a lithium ion secondary battery of this embodiment, the type of positive electrode active material is not particularly limited, but it is preferable that it be made of a space group R3m because it has a higher capacity. Positive electrode active materials belonging to the space group R3m have a layered structure (layered rock salt structure) in which lithium atomic layers and transition metal atomic layers are alternately stacked. Therefore, the use of such a positive electrode active material can improve the battery capacity of a lithium ion secondary battery.

Examples of positive electrode active materials belonging to the space group R3m include lithium-transition metal composite oxides such as LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , Li(Ni—Mn—Co)O ₂ , Li(Ni—Co—Al)O ₂ , and those in which a portion of the transition metal is substituted with another element. In some cases, two or more positive electrode active materials may be used in combination. More preferably, a composite oxide containing lithium and nickel is used. Even more preferably, Li(Ni—Mn—Co)O ₂ and those in which a portion of the transition metal is substituted with another element (hereinafter simply referred to as "NMC composite oxide") or Li(Ni—Co—Al)O ₂ and those in which a portion of the transition metal is substituted with another element (hereinafter simply referred to as "NCA composite oxide") is used. NMC composite oxide is particularly preferred. That is, according to a preferred embodiment of the present invention, the positive electrode active material is a lithium-nickel-manganese-cobalt composite oxide having a layered structure. NMC composite oxide and NCA composite oxide have a layered crystal structure in which lithium atomic layers and transition metal atomic layers are alternately stacked with oxygen atomic layers interposed therebetween, and each transition metal atom contains one Li atom, so the amount of Li that can be extracted is twice that of spinel-based lithium manganese oxide, i.e., the supply capacity is doubled, resulting in high capacity.

As mentioned above, NMC composite oxides and NCA composite oxides also include composite oxides in which a portion of the transition metal elements is replaced with another metal element. In such cases, other elements include Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, Cr, Fe, B, Ga, In, Si, Mo, Y, Sn, V, Cu, Ag, and Zn. Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, and Cr are preferred, and Ti, Zr, P, Al, Mg, and Cr are more preferred. From the perspective of improving cycle characteristics, Ti, Zr, Al, Mg, and Cr are even more preferred. However, the other metal elements that can replace the transition metal elements of the NCA composite oxide are those other than Al.

Since the NMC composite oxide has a high theoretical discharge capacity, it preferably has a composition represented by the general formula (1): Li _a Ni _b Mn _c Co _d M _x O ₂ (wherein, a, b, c, d, and x satisfy 0.9≦a≦1.2, 0<b<1, 0<c≦0.5, 0<d≦0.5, 0≦x≦0.3, and b+c+d+x=1. M is at least one element selected from Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, and Cr). Here, a represents the atomic ratio of Li, b represents the atomic ratio of Ni, c represents the atomic ratio of Mn, d represents the atomic ratio of Co, and x represents the atomic ratio of M. From the viewpoint of cycle characteristics, it is preferable that 0.4≦b≦0.92 in the general formula (1). The composition of each element can be measured by, for example, plasma (ICP) emission spectrometry.

Nickel (Ni), cobalt (Co), and manganese (Mn) are generally known to contribute to capacity and output characteristics by improving the purity and electronic conductivity of the material. Elements such as Ti partially substitute for transition metals in the crystal lattice. From the perspective of cycle characteristics, some of the transition metal elements may be replaced by other metal elements. The solid solution of at least one element selected from the group consisting of Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, and Cr stabilizes the crystal structure, which is thought to prevent a decrease in battery capacity even after repeated charge and discharge, thereby achieving excellent cycle characteristics.

In a more preferred embodiment, in general formula (1), b, c, and d are preferably 0.44≦b≦0.92, 0.05≦c≦0.31, and _{0.03 ≦} d≦ _0.26 , from the viewpoint of improving the balance between capacity and life characteristics. For example, _{LiNi0.5Mn0.3Co0.2O2} has the advantage of being larger in capacity per unit mass and enabling improved energy density, thereby enabling the production of compact, high-capacity batteries, compared with _LiCoO2 , _LiMn2O4 , LiNi1 _{/3Mn1 / 3Co1 / 3O2} , which have a proven track record in general consumer batteries, and is also preferred from the viewpoint of range. In terms of larger capacity, _{LiNi0.8Co0.1Al0.1O2} , _{LiNi0.8Mn0.1Co0.1O2 , LiNi0.88Mn0.06Co0.06O2 , and LiNi0.86Mn0.08Co0.06O2} are _more advantageous. _{On the other hand , LiNi0.5Mn0.3Co0.2O2} has excellent life characteristics _{comparable to LiNi1 / 3Mn1 / 3Co1 / 3O2 .}

The positive electrode for a lithium-ion secondary battery according to this embodiment is characterized in that the roughness factor of the positive electrode active material is 3 or less. Here, the "roughness factor" is a parameter that indicates the surface smoothness of the positive electrode active material. It is calculated as the ratio of the "BET specific surface area" of the positive electrode active material to the "geometric specific surface area calculated from the average particle diameter" (= BET specific surface area/geometric specific surface area) using the method described in the Examples section below. According to this embodiment, by controlling the roughness factor to 3 or less, the surface of the positive electrode active material can be uniformly coated with a binder (described below). Because the binder functions to retain the electrolyte, uniformly coating the binder is thought to improve lithium ion conductivity near the surface of the positive electrode active material. As a result, this embodiment enables improved output characteristics in lithium-ion secondary batteries. The roughness factor is preferably 2.7 or less, more preferably 2.4 or less. Meanwhile, the lower limit of the roughness factor is 1 or greater. Furthermore, when the positive electrode active material is a mixture of two or more materials, the BET specific surface area and geometric specific surface area of each positive electrode active material constituting the mixture are first measured using the above-described method, and the obtained values are added together in accordance with the mixing ratio to calculate the BET specific surface area and geometric specific surface area of the mixture. The roughness factor is then calculated from these calculated values using the same method as above.

There are no particular limitations on the method for controlling the roughness factor of the positive electrode active material to 3 or less, and conventionally known knowledge that can control the surface smoothness of positive electrode active material particles can be referenced as appropriate. For example, as an example of a method for producing an _NMC composite oxide, first, (1) a precursor ( _{NixCoyMnz )} (OH) ₂ is synthesized by coprecipitation. Specifically, an aqueous solution containing raw materials such as nickel sulfate, cobalt sulfate, manganese sulfate, sodium hydroxide, and ammonium hydroxide in the desired stoichiometric ratio is stirred for a predetermined period of time. The precipitate is separated by filtration and then dried at a temperature of about 80°C for about 8 to _{15 hours} to obtain a precursor. Next, (2) the obtained precursor is baked at a temperature of about 600 to 800°C for about 3 to 7 hours to obtain _NixCoyMnzO2 . Thereafter, (3) a lithium salt ₍ e.g., lithium carbonate) is added to the obtained oxide so that the stoichiometric ratio is 1 to 1.2, and the mixture is pulverized and mixed in a ball mill. Then, (4) the powder is calcined in air or an oxygen atmosphere at a temperature of about 600 to 800°C for about 3 to 5 hours, followed by further calcination at a temperature of about 750 to 1000°C for about 2 to 12 hours. Finally, (5) the calcined powder is washed with water to remove residual lithium salt, and then dried at a temperature of about 80°C for about 10 to 15 hours to obtain an NMC composite oxide. (6) If necessary, the sample may be pulverized in a ball mill and then re-calcined in air or an oxygen atmosphere at 500 to 1000°C for 3 to 5 hours. In this case, the primary particle size can be controlled by adjusting the calcination temperature and time in (2), the degree of pulverization in (3), the calcination time and temperature in (4), and the degree of pulverization in (6). Furthermore, by performing the re-calcination in (6) for a long period of time and at a high temperature, the surface after pulverization becomes smoother, thereby reducing the roughness factor.

As described above, methods for controlling the crystallite diameter of the positive electrode active material to a large value, for example, 1 μm or more, or for controlling the proportion of particles consisting of a single crystallite among the particles constituting the positive electrode active material to within the above-mentioned preferred range, can be found in patent documents such as Japanese Patent No. 6574222 and Japanese Patent No. 5702289, and non-patent documents such as Solid State Ionics, Volume 345, February 2020, 115200, and Journal of the Electrochemical Society, 165(5) A1038-A1045 (2018).

In the positive electrode for a lithium-ion secondary battery according to this embodiment, the positive electrode active material preferably satisfies Y/X≦70, where X (nm) is the crystallite diameter calculated by the Williamson-Hall method and Y (nm) is the average particle diameter (D50) calculated by laser diffraction. Here, Y/X is an index of the number of crystal grains constituting the positive electrode active material particles; a smaller value indicates fewer crystal grains constituting the particles. The crystallite diameter X (nm) and the average particle diameter (D50) can be measured by the methods described in the Examples section below. Y/X is more preferably 50 or less, and even more preferably 40 or less. Meanwhile, the lower limit of Y/X is 1 or greater. A ratio of Y/X within the above range allows for more uniform coating of the binder (described below) on the surface of the positive electrode active material. Because the binder functions to retain the electrolyte, more uniform coating of the binder is believed to further improve lithium ion conductivity near the surface of the positive electrode active material. As a result, it is possible to further improve the output characteristics of a lithium-ion secondary battery using the positive electrode according to this embodiment.

In the positive electrode for a lithium-ion secondary battery according to this embodiment, the positive electrode active material preferably has the aforementioned R3m space group and, in addition, the peak intensity ratio ((003)/(104)) of the diffraction peak of the (003) plane to the diffraction peak of the (104) plane obtained by X-ray diffraction measurement is preferably 1.35 or greater. The peak intensity ratio ((003)/(104)) is more preferably 1.4 or greater, even more preferably 1.45 or greater, and particularly preferably 1.48 or greater. The upper limit of the peak intensity ratio ((003)/(104)) is not particularly limited, but is preferably 2.1 or less. This peak intensity ratio is an indicator of the crystallinity of the positive electrode active material; a higher peak intensity ratio indicates higher crystallinity. A peak intensity ratio within the above range reduces defects within the crystal, thereby suppressing decreases in battery charge/discharge capacity and durability. The peak intensity ratio can be controlled by the raw materials, composition, firing conditions, etc. The crystal structure and peak intensity ratio of the above positive electrode active material can be determined by the measurement methods described in the Examples below.

In the positive electrode for a lithium ion secondary battery according to this embodiment, the tap density of the positive electrode active material is preferably 2 g/cm ³ or less, more preferably 1.9 g/cm ³ or less, and even more preferably 1.8 g/cm ³ or less. The lower limit of the tap density is not particularly limited, but is preferably 1.2 g/cm ³ or more. A smaller tap density value means a smaller volume of the space in the unevenness of the particle surface and the volume of the gaps between particles. When the tap density is within the above range, the binder described below can be more uniformly coated on the surface of the positive electrode active material. Since the binder has the function of retaining the electrolyte, it is thought that more uniform coating of the binder further improves the lithium ion conductivity near the surface of the positive electrode active material. As a result, it is possible to further improve the output characteristics of a lithium ion secondary battery using the positive electrode according to this embodiment.

The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but is preferably in the range of 60 to 99% by mass, and more preferably in the range of 80 to 98% by mass.

(binder)
The binder (binding agent) functions to maintain the structure of the positive electrode active material layer by binding the components contained in the positive electrode active material layer together. In the positive electrode for a lithium ion secondary battery according to this embodiment, the binder is a vinylidene fluoride-hexafluoropropylene copolymer, and the ratio of the number of hexafluoropropylene-derived structural units to the total number of structural units of the copolymer is 5 mol% or more. This ratio is preferably 6 mol% to 20 mol%, more preferably 6.5 mol% to 10 mol%, and even more preferably 6.9 mol% to 8 mol%. If this ratio is less than 5 mol%, the flexibility of the binder and its swelling in the electrolyte may be insufficient, which may result in an inability to uniformly coat the surface of the positive electrode active material with the binder or a decrease in the amount of electrolyte retained. As a result, the lithium ion conductivity near the surface of the positive electrode active material may decrease, potentially preventing sufficient output characteristics from being exhibited.

Furthermore, in the positive electrode for a lithium ion secondary battery according to this embodiment, the binder content is greater than 0.31% by mass and less than 4.85% by mass, relative to the total solid content of the positive electrode active material layer. The binder content is preferably 0.50% by mass or more and 2.97% by mass or less, and more preferably 1.01% by mass or more and 2.00% by mass or less. If the binder content is 0.31% by mass or less, the electrode structure may not be maintained due to insufficient binding. On the other hand, if the binder content is 4.85% by mass or more, the binder may not cover the surface of the positive electrode active material uniformly, and the effect of improving output characteristics may not be achieved.

(Conductive additive)
The conductive additive has the function of forming an electron conduction path (conductive passage) in the positive electrode active material layer. When such an electron conduction path is formed in the positive electrode active material layer, the internal resistance of the battery is reduced and the output characteristics at high rates can be improved. In the positive electrode for a lithium ion secondary battery according to this embodiment, the surface of the positive electrode active material is uniformly coated with the binder, so the conductive additive is essential from the viewpoint of ensuring an electron conduction path between adjacent positive electrode active materials and between the current collector and the positive electrode active material.

Conductive additives include particulate carbon materials such as acetylene black, carbon black, channel black, thermal black, and Ketjen Black (registered trademark), as well as fibrous carbon materials such as carbon nanotubes (single-walled carbon nanotubes and multi-walled carbon nanotubes), carbon nanofibers, vapor-grown carbon fibers, electrospun carbon fibers, polyacrylonitrile-based carbon fibers, and pitch-based carbon fibers. One type of conductive additive may be used alone, or two or more types may be used in combination. Among these, fibrous carbon materials are preferred as the conductive additive, and carbon nanotubes are more preferred, as they are capable of forming good electron conduction paths.

The content of the conductive additive in the positive electrode active material layer (the total amount when two or more types are included) is preferably 2% by mass or less, and more preferably 1% by mass or less, relative to 100% by mass of the total solids content of the positive electrode active material layer. At such an upper limit, aggregation of the conductive additives is suppressed, resulting in favorable formation of electron conduction paths, further improving output characteristics. It also enables further improvement of the energy density of the lithium-ion secondary battery. While there is no particular lower limit for the conductive additive content, it is preferably greater than 0% by mass, and is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, and more preferably 0.3% by mass or more. At such a lower limit, sufficient conductive additive is present to form electron conduction paths, further improving output characteristics.

In the positive electrode for a lithium-ion secondary battery of this embodiment, the thickness of the positive electrode active material layer is not particularly limited, and conventionally known knowledge about batteries can be referenced as appropriate. For example, the thickness of the positive electrode active material layer is typically approximately 1 to 1000 μm, preferably 20 to 800 μm, more preferably 30 to 500 μm, and even more preferably 40 to 200 μm. The thicker the positive electrode active material layer, the more positive electrode active material can be retained to achieve sufficient capacity (energy density). On the other hand, the thinner the positive electrode active material layer, the more improved the discharge rate characteristics can be.

The porosity of the positive electrode active material layer is preferably 20 to 50%, and more preferably 20 to 45%. When the porosity of the positive electrode active material layer is within this range, sufficient contact between the electron conductive materials (conductive additive, positive electrode active material, etc.) in the positive electrode active material layer can be maintained, preventing an increase in electron transfer resistance. In addition, because sufficient electrolyte is present between the positive electrode active material particles, an increase in lithium ion transfer resistance can be prevented. As a result, it is possible to further improve the output characteristics of lithium ion secondary batteries.

The density of the positive electrode active material layer is preferably 2.10 to 3.00 g/cm ³ , more preferably 2.15 to 2.85 g/cm ³ , and even more preferably 2.20 to 2.80 g/cm ³ . When the density of the positive electrode active material layer is equal to or greater than the above-mentioned lower limit, a battery having sufficient energy density can be obtained. On the other hand, when the density of the positive electrode active material layer is equal to or less than the above-mentioned upper limit, a sufficient amount of electrolyte is secured to fill the voids, preventing an increase in lithium ion migration resistance in the positive electrode active material layer. As a result, it is possible to further improve the output characteristics of the lithium ion secondary battery. In this specification, the density of the positive electrode active material layer is measured by the following method.

The density of the positive electrode active material layer is calculated according to the following formula.
Positive electrode active material layer density (g/cm ³ )=solid material mass (g)/positive electrode active material layer volume (cm ³ ).

The mass of solid materials is calculated by adding up only the masses of the solid materials among the masses of each material in the positive electrode active material layer. The volume of the positive electrode active material layer is calculated from the thickness and application area of the positive electrode active material layer.

When applied to a lithium ion secondary battery, the positive electrode for a lithium ion secondary battery according to this embodiment can improve the output characteristics of the lithium ion secondary battery. Therefore, according to another embodiment of the present invention, a lithium ion secondary battery is provided that includes a power generating element having the positive electrode for a lithium ion secondary battery according to the above-described embodiment of the present invention. The components of the lithium ion secondary battery other than the positive electrode will be briefly described below.

[Negative electrode active material layer]
(Negative electrode active material)
The negative electrode active material has the function of releasing ions such as lithium ions during discharge and absorbing ions such as lithium ions during charge.

Examples of negative electrode active materials include carbon materials such as graphite, soft carbon, and hard carbon, lithium-transition metal composite oxides (e.g., Li ₄ Ti ₅ O ₁₂ ), metal materials (tin, silicon), silicon-containing alloy-based negative electrode materials (e.g., Si ₆₀ Sn ₁₀ Ti ₃₀ ), and lithium alloy-based negative electrode materials (e.g., lithium-tin alloy, lithium-silicon alloy, lithium-aluminum alloy, lithium-aluminum-manganese alloy, etc.). In some cases, two or more negative electrode active materials may be used in combination. Preferably, from the viewpoint of capacity and output characteristics, silicon-containing alloy-based negative electrode materials, carbon materials, lithium-transition metal composite oxides, and lithium alloy-based negative electrode materials are preferably used as the negative electrode active material. Of course, negative electrode active materials other than those mentioned above may also be used.

The average particle size (D ₅₀ ) of the negative electrode active material is not particularly limited, but from the viewpoint of achieving high output, it is preferably 1 to 100 μm, more preferably 1 to 20 μm.

The content of the negative electrode active material in the negative electrode active material layer is, for example, 60% by mass or more but less than 100% by mass, preferably 80% by mass or more but less than 99.5% by mass, more preferably more than 95% by mass but less than 99.0% by mass, and even more preferably 97% by mass or more but less than 98.5% by mass, relative to 100% by mass of the total solids content. If the content of the negative electrode active material is within the above range, it is possible to achieve both good battery capacity and good output characteristics.

The negative electrode active material layer may also contain other additives, such as conductive additives and binders, as described above for the positive electrode active material layer, as needed.

The thickness of the negative electrode active material layer is not particularly limited, and the same thickness as described above for the positive electrode active material layer can be used.

[Electrolyte layer]
The electrolyte layer preferably has a configuration in which a separator is impregnated with an electrolytic solution (liquid electrolyte).

(electrolyte)
The electrolyte functions as a carrier of lithium ions and has a form in which a lithium salt is dissolved in a non-aqueous solvent.

Examples of non-aqueous solvents include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propionate (MP), methyl acetate (MA), methyl formate (MF), 4-methyldioxolane (4MeDOL), dioxolane (DOL), 2-methyltetrahydrofuran (2MeTHF), tetrahydrofuran (THF), dimethoxyethane (DME), propylene carbonate (PC), butylene carbonate (BC), dimethyl sulfoxide (DMSO), and gamma-butyrolactone (GBL). Among these, from the viewpoint of further improving the rapid charging characteristics and output characteristics, the non-aqueous solvent is preferably a chain carbonate, more preferably at least one selected from the group consisting of diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC), and more preferably selected from ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC).

Examples of lithium salts include Li( _FSO2 ) _2N (lithium bis(fluorosulfonyl)imide; _LiFSI ), _Li ( _C2F5SO2 ) _2N , _LiPF6 , _LiBF4 , _LiClO4 , _LiAsF6 , and _LiCF3SO3 . Of these, the lithium salt is preferably _{Li(FSO2)2N from the} viewpoints of battery output and charge/discharge cycle characteristics.

The concentration of lithium salt in the electrolyte is preferably 0.1 to 3.0 mol/L, and more preferably 0.8 to 2.2 mol/L.

The electrolyte may further contain additives other than the components described above. Specific examples of such compounds include ethylene carbonate, vinylene carbonate, methyl vinylene carbonate, dimethyl vinylene carbonate, phenyl vinylene carbonate, diphenyl vinylene carbonate, ethyl vinylene carbonate, diethyl vinylene carbonate, vinyl ethylene carbonate, 1,2-divinyl ethylene carbonate, 1-methyl-1-vinyl ethylene carbonate, 1-methyl-2-vinyl ethylene carbonate, 1-ethyl-1-vinyl ethylene carbonate, and 1-ethyl-2-vinyl ethylene carbonate. Examples of additives include ethylene carbonate, vinyl vinylene carbonate, allyl ethylene carbonate, vinyloxymethyl ethylene carbonate, allyloxymethyl ethylene carbonate, acryloxymethyl ethylene carbonate, methacryloxymethyl ethylene carbonate, ethynyl ethylene carbonate, propargyl ethylene carbonate, ethynyloxymethyl ethylene carbonate, propargyloxyethylene carbonate, methylene ethylene carbonate, and 1,1-dimethyl-2-methylene ethylene carbonate. These additives may be used alone or in combination of two or more. Furthermore, when an additive is used in the electrolyte, the amount used can be adjusted as appropriate.

(separator)
The separator constituting the electrolyte layer has the function of retaining the electrolyte to ensure lithium ion conductivity between the positive electrode and the negative electrode, and also functions as a partition wall between the positive electrode and the negative electrode.

Examples of the separator's form include a porous sheet separator made of polymer or fiber that absorbs and retains the electrolyte, and a nonwoven fabric separator.

As a porous sheet separator made of a polymer or fiber, for example, a microporous material (microporous membrane) can be used. Specific forms of the porous sheet made of polymer or fiber include microporous (microporous membrane) separators made of polyolefins such as polyethylene (PE) and polypropylene (PP); laminates of multiple layers of these (e.g., a laminate with a three-layer structure of PP/PE/PP); hydrocarbon resins such as polyimide, aramid, and polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP); and glass fibers.

The thickness of a microporous (microporous membrane) separator cannot be uniquely defined, as it varies depending on the intended use. For example, for applications such as motor-driven secondary batteries for electric vehicles (EVs), hybrid electric vehicles (HEVs), and fuel cell vehicles (FCVs), a single-layer or multi-layer thickness of 4 to 60 μm is desirable. The micropore diameter of the microporous (microporous membrane) separator is desirably a maximum of 1 μm or less (typically a pore diameter of several tens of nanometers).

Nonwoven fabric separators can be made from conventional materials such as cotton, rayon, acetate, nylon, polyester; polyolefins such as PP and PE; polyimide, and aramid, either alone or in combination. The bulk density of the nonwoven fabric is not particularly limited, as long as it provides sufficient battery characteristics with the impregnated polymer gel electrolyte. The thickness of the nonwoven fabric separator should be the same as that of the electrolyte layer, and is preferably 5 to 200 μm, and particularly preferably 10 to 100 μm.

The separator is preferably one in which a heat-resistant insulating layer is laminated on a porous substrate (a separator with a heat-resistant insulating layer). The heat-resistant insulating layer is a ceramic layer containing inorganic particles and a binder. A separator with a heat-resistant insulating layer is used that has high heat resistance, with a melting point or softening point of 150°C or higher, preferably 200°C or higher. The presence of a heat-resistant insulating layer alleviates the internal stress of the separator that increases with temperature rise, thereby suppressing thermal shrinkage. As a result, short circuits between battery electrodes can be prevented, resulting in a battery configuration that is less susceptible to performance degradation due to temperature rise. Furthermore, the presence of a heat-resistant insulating layer improves the mechanical strength of the separator with a heat-resistant insulating layer, making it less likely to rupture. Furthermore, the heat-shrinkage suppression effect and high mechanical strength make the separator less likely to curl during the battery manufacturing process.

[Positive electrode current collector plate and negative electrode current collector plate]
The material constituting the current collector plates (25, 27) is not particularly limited, and known highly conductive materials conventionally used as current collector plates for lithium-ion secondary batteries can be used. Metal materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferred as constituent materials of the current collector plates. From the viewpoints of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. The positive electrode current collector plate 25 and the negative electrode current collector plate 27 may be made of the same material or different materials.

[Positive electrode lead and negative electrode lead]
Although not shown, the current collector 11 and the current collector plates (25, 27) may be electrically connected via a positive electrode lead or a negative electrode lead. Materials used in known lithium-ion secondary batteries may be used as the constituent materials of the positive and negative electrode leads. It is preferable that the portion removed from the outer casing be covered with a heat-resistant, insulating heat-shrinkable tube or the like to prevent contact with peripheral devices or wiring, resulting in electrical leakage and affecting products (e.g., automotive parts, particularly electronic devices).

[Sealing part]
The sealing portion (insulating layer) 31 is a component specific to bipolar secondary batteries (series-stacked batteries) and has the function of preventing leakage of the electrolyte solution from the electrolyte layer. The sealing portion 31 also has the function of preventing contact between current collectors and short-circuiting at the ends of the unit cell layers. The material constituting the sealing portion may be any material that has insulating properties, sealing properties against the detachment of the solid electrolyte, sealing properties against moisture penetration from the outside (sealing properties), and heat resistance at the battery operating temperature. Examples of materials that can be used include acrylic resin, urethane resin, epoxy resin, polyethylene resin, polypropylene resin, polyimide resin, and rubber (ethylene-propylene-diene rubber: EPDM). Alternatively, an isocyanate-based adhesive, an acrylic resin-based adhesive, a cyanoacrylate-based adhesive, or a hot-melt adhesive (urethane resin, polyamide resin, polyolefin resin) may be used. Among these, polyethylene resin and polypropylene resin are preferably used as the constituent material of the sealing portion from the viewpoints of corrosion resistance, chemical resistance, ease of production (film-forming ability), economy, etc., and it is more preferable to use a resin obtained by copolymerizing ethylene, propylene, and butene with amorphous polypropylene resin as the main component.

[Battery exterior]
As the battery exterior, a known metal can case can be used, or a bag-shaped case using an aluminum-containing laminate film 29 that can cover the power-generating element, as shown in Figures 1 and 2, can be used. The laminate film can be, for example, a three-layer laminate film formed by laminating PP, aluminum, and nylon in this order, but is not limited thereto. A laminate film is desirable from the viewpoint of its high output and excellent cooling performance, making it suitable for use in batteries for large devices such as EVs and HEVs. Furthermore, an aluminate laminate is more preferred for the exterior because it allows for easy adjustment of the collective pressure applied to the power-generating element from the outside and allows for easy adjustment of the electrolyte layer thickness to the desired value.

The lithium-ion secondary battery according to this embodiment can exhibit excellent output characteristics. Therefore, the lithium-ion secondary battery according to this embodiment is suitable for use as a power source for driving EVs and HEVs.

[Battery assembly]
A battery pack is made up of multiple batteries connected together. Specifically, it is made up of at least two batteries connected in series, parallel, or both. By connecting them in series or parallel, it is possible to freely adjust the capacity and voltage.

Multiple batteries can be connected in series or parallel to form a small, detachable battery pack. Then, multiple such small, detachable battery packs can be connected in series or parallel to form a large-capacity, high-output battery pack (battery module, battery pack, etc.) suitable for vehicle drive power sources and auxiliary power sources that require high volumetric energy density and high volumetric output density. The number of batteries connected to form a battery pack, and the number of stacked small battery packs to form a large-capacity battery pack, can be determined based on the battery capacity and output of the vehicle (electric vehicle) in which it will be installed.

[vehicle]
The lithium ion secondary battery of this embodiment has excellent output characteristics. Furthermore, it has a high volumetric energy density. In vehicle applications such as electric vehicles, hybrid electric vehicles, fuel cell vehicles, and hybrid fuel cell vehicles, higher capacity and larger current are required compared to applications in electrical and portable electronic devices. Therefore, the lithium ion secondary battery can be suitably used as a power source for vehicles, for example, as a vehicle drive power source or auxiliary power source.

Specifically, a battery or a battery pack consisting of a combination of multiple batteries can be installed in a vehicle. The present invention allows for the construction of a battery with large capacity and excellent output characteristics, and installing such a battery in a vehicle allows the construction of plug-in hybrid electric vehicles with long EV driving distances and electric vehicles with long driving distances per charge. Examples of vehicles include hybrid vehicles, fuel cell vehicles, and electric vehicles (all of which include four-wheeled vehicles (commercial vehicles such as passenger cars, trucks, and buses, and light vehicles), as well as two-wheeled vehicles (motorcycles) and three-wheeled vehicles). However, the application is not limited to automobiles, and the battery can also be applied to various power sources for other vehicles, such as trains, and can also be used as an on-board power source for uninterruptible power supplies and the like.

The present invention will be explained in more detail below using examples and comparative examples, but the present invention is not limited to the following examples.

<Cathode active material>
The following two types of commercially available positive electrode active materials were prepared.

Single crystal NMC composite oxide (manufactured by Easpring, ME88SC, _{LiNi0.86Mn0.08Co0.06O2} , roughness factor: 2.4, crystallite size ( _X ): 110 nm, average particle size ₍ D50) ( _Y ): 4000 nm, Y/X: 36.4, crystal structure: space group R3m, peak intensity ratio (003)/(104): 1.48, tap density: 1.76 g/ ^cm3 ).

Polycrystalline NMC composite oxide (manufactured by Ecopro, NMC811, _{LiNi0.8Mn0.1Co0.1O2} , roughness factor: 16.5, crystallite size ( _X ): 80 nm, average particle size ₍ D50) ( _Y ): 11,500 nm, Y/X: 144, crystal structure: space group R3m, peak intensity ratio (003)/(104): 1.29, tap density: 2.68 g/ ^cm3 ).

The physical properties of each positive electrode active material were measured using the following methods.

[Roughness factor]
First, the powder of the positive electrode active material was observed using a scanning electron microscope (SEM). From the obtained SEM image, the maximum distance between any two points on the contour line of the active material particle was measured as the particle diameter, and the arithmetic mean value of the particle diameters of particles observed within several dozen fields of view was calculated as the average particle diameter.

Next, the particle volume [m ³ ] and particle surface area [m ² ] of a spherical particle having the average particle diameter calculated above were calculated from the calculated average particle diameter, and the particle volume was multiplied by the specific gravity of the active material (here, 4.78 [g/cm ³ ]) to calculate the particle mass [g] of the spherical particle. The particle surface area calculated above was then divided by the particle mass also calculated above to calculate the "geometric specific surface area [m ² /g] calculated from the average particle diameter."

On the other hand, in accordance with the "Method for measuring specific surface area of powder (solid) by gas adsorption" described in JIS Z8830:2013 (ISO 9277:2010), measurement was performed by the static volumetric method using nitrogen gas as the adsorption gas, and the BET specific surface area [ ^m2 /g] was calculated by analyzing using the multipoint method. Then, the roughness factor was calculated as the ratio of the "BET specific surface area" to the "geometric specific surface area calculated from the average particle size" calculated above (= BET specific surface area/geometric specific surface area).

[Y/X]
(Crystallite diameter X)
The crystallite diameter X of the positive electrode active material was calculated using the Williamson-Hall method (Williamson-Hall method, Hall, W.II., J.Inst.Met., 75, 1127 (1950); Idem, Proc.Phys.Soc., A62, 741 (1949)). For the measurement, an X-ray diffraction (XRD) device (manufactured by Rigaku) using CuKα radiation was used. According to the Williamson-Hall method, the following relationship holds between the θ of the diffraction peak obtained by X-ray diffraction measurement of the positive electrode active material and its integrated value β.

βcosθ/λ=2η(sinθ/λ)+(1/ε)
Here, ε corresponds to the crystallite diameter, λ corresponds to the wavelength of CuKα radiation, and η corresponds to the strain of the positive electrode active material. Taking sin θ as the X axis and β cos θ as the Y axis, the θ of the diffraction peak of the obtained positive electrode active material and its integral width β are plotted on the X axis and Y axis, respectively, and an approximate straight line is drawn using the least squares method. The crystallite size ε is determined from the intersection with the Y axis.

(Average particle diameter (D50))
The average particle size (D50) of the positive electrode active material was measured by laser diffraction. D50 represents the particle size when the cumulative value of the particle size distribution on a volume basis is 50%.

[Crystal structure, peak intensity ratio (003)/(104)]
Powder X-ray diffraction measurements to calculate the crystal structure and peak intensity ratio (I(003)/I(104)) of the positive electrode active material were performed using an X-ray diffractometer (manufactured by Rigaku) using CuKα radiation, and analysis was performed using Fundamental Parameters. Analysis was performed using the analysis software Topas Version 3 using X-ray diffraction patterns obtained from a diffraction angle 2θ range of 15 to 120°.

[Tap density]
The positive electrode active material was placed in a 10 mL glass measuring cylinder, and the powder packing density after tapping 200 times was measured, and this was taken as the tap density of the positive electrode active material.

<Preparation of positive electrode>
[Example 1]
A solid content consisting of 97 parts by mass (97.98% by mass) of the above single crystal NMC composite oxide as the positive electrode active material, 1 part by mass (1.01% by mass) of carbon nanotubes (abbreviation: CNT, fiber diameter: 10 nm, fiber length: 20 μm, aspect ratio: 2000) as a conductive additive, and 1 part by mass (1.01% by mass) of PVDF-HFP (manufactured by Arkema, Kyner Flex 2501, proportion of the number of hexafluoropropylene-derived structural units: 6.9 mol%) as a binder was prepared. N-methyl-2-pyrrolidone (NMP) as a solvent was added to this solid content and mixed to obtain a positive electrode slurry having a solid content concentration of 70% by mass. Using a doctor blade and a coater (manufactured by Tester Sangyo Co., Ltd.) with a gap adjusted so that the basis weight was 2.5 g / cm ³ , the positive electrode slurry was applied to an aluminum foil having a thickness of 20 μm, and then the coating film was dried for 1 hour on a hot plate adjusted to 80 ° C. The dried coating film was pressed using a small desktop roll press (manufactured by Tester Sangyo Co., Ltd.) to a porosity of 20%. Thereafter, this was placed in a vacuum dryer and dried at 130 ° C. for 8 hours in a vacuum state to obtain the positive electrode of this example.

[Example 2]
The positive electrode of this example was obtained in the same manner as in Example 1, except that the blending amount of PVDF-HFP as a binder was set to 2 parts by mass (2.00% by mass).

[Example 3]
The positive electrode of this example was obtained in the same manner as in Example 1, except that the blending amount of PVDF-HFP as a binder was set to 3 parts by mass (2.97% by mass).

[Comparative Example 1]
The positive electrode of this comparative example was obtained in the same manner as in Example 1, except that the blending amount of PVDF-HFP as a binder was set to 0.3 parts by mass (0.31% by mass).

[Comparative Example 2]
The positive electrode of this comparative example was obtained in the same manner as in Example 1, except that the blending amount of PVDF-HFP as a binder was set to 5 parts by mass (4.85% by mass).

[Comparative Example 3]
A positive electrode of this comparative example was obtained in the same manner as in Example 1, except that 97 parts by mass (97.98% by mass) of the polycrystalline NMC composite oxide was used as the positive electrode active material.

[Comparative Example 4]
The positive electrode of this comparative example was obtained in the same manner as in Comparative Example 3, except that the blending amount of PVDF-HFP as a binder was set to 2 parts by mass (2.00% by mass).

[Comparative Example 5]
The positive electrode of this comparative example was obtained in the same manner as in Comparative Example 3, except that the blending amount of PVDF-HFP as a binder was set to 3 parts by mass (2.97% by mass).

[Comparative Example 6]
The positive electrode of this comparative example was obtained in the same manner as in Comparative Example 3, except that the blending amount of PVDF-HFP as a binder was set to 5 parts by mass (4.85% by mass).

[Comparative Example 7]
A positive electrode of this comparative example was obtained in the same manner as in Example 1, except that 1 part by mass (1.01% by mass) of PVDF (Kureha KF Polymer W#7200, manufactured by Kureha) was used as the binder.

[Comparative Example 8]
The positive electrode of this comparative example was obtained in the same manner as in Comparative Example 7, except that the blending amount of PVDF as a binder was set to 2 parts by mass (2.00% by mass).

[Comparative Example 9]
The positive electrode of this comparative example was obtained in the same manner as in Comparative Example 7, except that the blending amount of PVDF as a binder was set to 3 parts by mass (2.97% by mass).

[Comparative Example 10]
The positive electrode of this comparative example was obtained in the same manner as in Comparative Example 7, except that the blending amount of PVDF as a binder was set to 5 parts by mass (4.85% by mass).

[Comparative Example 11]
A positive electrode of this comparative example was obtained in the same manner as in Example 1, except that 3 parts by mass (2.97 mass%) of PVDF-HFP (Kyner Flex 2851, manufactured by Arkema, proportion of the number of constituent units derived from hexafluoropropylene: 2.4 mol%) was used as the binder.

[Comparative Example 12]
The positive electrode of this comparative example was obtained in the same manner as in Comparative Example 3, except that 1 part by mass (1.01% by mass) of PVDF (Kureha KF Polymer W#7200, manufactured by Kureha) was used as the binder.

<Preparation of negative electrode>
A solid content consisting of 95.5% by mass of graphite (average particle size: 20 μm) as the negative electrode active material, 0.5% by mass of acetylene black as a conductive additive, and 4% by mass of PVDF as a binder was prepared. N-methyl-2-pyrrolidone (NMP) was added to this solid content and mixed to obtain a negative electrode slurry with a solid content concentration of 70%. Using a doctor blade and a coater (manufactured by Tester Sangyo Co., Ltd.) with the gap adjusted so that the basis weight was 2.5 g/cm ³ , the negative electrode slurry was applied to a copper foil with a thickness of 20 μm, and then the coating film was dried for 1 hour on a hot plate adjusted to 80 ° C. The dried coating film was pressed using a small desktop roll press (manufactured by Tester Sangyo Co., Ltd.) to achieve a porosity of 20%. Thereafter, this was placed in a vacuum dryer and dried at 130 ° C. for 8 hours under vacuum to obtain a negative electrode.

<Fabrication of Lithium-ion Secondary Battery>
The resulting positive electrode was cut to 12 cm ² and the negative electrode to 13 cm ² . For the positive electrode, aluminum foil with an Al terminal was laminated on the current collector foil side of the cut positive electrode. For the negative electrode, copper foil with a Ni terminal was laminated on the current collector foil side of the cut negative electrode. Separators (manufactured by Celgard, manufactured by PP) were inserted on the electrode active material layer sides of the positive and negative electrodes to form a laminate. This laminate was sandwiched between heat-sealed aluminum laminate films and sealed on three sides. 280 μL of electrolyte obtained by dissolving 2 mol/L of Li(FSO ₂ ) ₂ N (lithium bis(fluorosulfonyl)imide; LiFSI) in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (volume ratio 3:7 (EC:EMC)) was injected from the remaining side, and the container was vacuum-sealed at a vacuum degree of 99.7% using a vacuum sealer (manufactured by TOSPACK). In this manner, a pouch-type lithium ion battery was produced in which the positive electrode and the negative electrode were stacked so as to face each other with the separator interposed therebetween.

[Measurement of DC Resistance (DCR)]
Under conditions of 25 ° C., 1C = 220 mAh / g, the first and second charge and discharge were 0.1C, voltage range 2.5 to 4.6V, charge CC-CV mode, discharge CC mode (rest time between charge and discharge 8 hours) was performed. At SOC 50% (voltage 3.9V), discharge was performed at 0.1C, 0.5C, and 1C, and the direct current resistance (DCR; Direct Current Resistance) value (Ω cm) was calculated from the current value (1 sec, 10 sec, 30 sec) flowing at that time. Then, the DCR value in Comparative Example 12 was set to X ₀ , the DCR value in each example was set to X ₁ , and the reduction rate (%) of X ₁ based on X ₀ was calculated using the formula: {(X ₀ -X ₁ ) / X ₀ } × 100.

The results are shown in Table 1 below.

The results in Table 1 show that the positive electrode for a lithium ion secondary battery of the present invention significantly reduces direct current resistance (DCR) compared to Comparative Example 12. Therefore, by applying this positive electrode to a lithium ion secondary battery, it is possible to improve the output characteristics of the lithium ion secondary battery.

On the other hand, in Comparative Example 1, the binder content was so low that the structure of the positive electrode active material layer could not be maintained, resulting in electrode cracking. In Comparative Example 2, the binder content was so high that DC resistance actually increased.

In the examples using polycrystalline NMC composite oxide in Comparative Examples 3 to 5, no reduction in DC resistance was observed, or the reduction was slight, compared to Comparative Example 12.

In the examples using PVDF (Comparative Examples 7 to 9) and the example using PVDF-HFP with a low HFP ratio (Comparative Example 11), a reduction in DC resistance was observed compared to Comparative Example 12, but the extent of the reduction was not sufficient.

10a stacked secondary battery,
10b bipolar secondary battery;
11 current collector,
11a: outermost current collector on the positive electrode side;
11b: outermost current collector on the negative electrode side;
11′ positive electrode current collector 12 negative electrode current collector 13 positive electrode active material layer,
15 negative electrode active material layer,
17 electrolyte layer,
19 cell layer,
21 power generating element,
23 bipolar electrodes,
25 Positive electrode current collector (positive electrode tab),
27 negative electrode current collector plate (negative electrode tab),
29 Laminating film,
31 Sealing part.

Claims (6)

A positive electrode for a lithium ion secondary battery having a positive electrode active material layer including a positive electrode active material, a binder, and a conductive additive,
the positive electrode active material has a roughness factor of 3 or less, which is defined as the ratio of a BET specific surface area to a geometric specific surface area calculated from an average particle size;
the binder is a vinylidene fluoride-hexafluoropropylene copolymer, and in this case, the ratio of the number of structural units derived from hexafluoropropylene to the total number of structural units of the copolymer is 5 mol% or more;
the content of the binder is greater than 0.31% by mass and less than 4.85% by mass with respect to the total solid content of the positive electrode active material layer ,
The positive electrode active material satisfies Y/X≦70, where X (nm) is the crystallite diameter calculated by the Williamson-Hall method and Y (nm) is the average particle diameter (D50) calculated by a laser diffraction method . A positive electrode for a lithium ion secondary battery having a positive electrode active material layer including a positive electrode active material, a binder, and a conductive additive,
the positive electrode active material has a roughness factor of 3 or less, which is defined as the ratio of a BET specific surface area to a geometric specific surface area calculated from an average particle size;
the binder is a vinylidene fluoride-hexafluoropropylene copolymer, and in this case, the ratio of the number of structural units derived from hexafluoropropylene to the total number of structural units of the copolymer is 5 mol% or more;
the content of the binder is greater than 0.31% by mass and less than 4.85% by mass with respect to the total solid content of the positive electrode active material layer,
the positive electrode active material is composed of a space group of R3m, and has a peak intensity ratio (003)/(104) of a diffraction peak of a (003) plane to a diffraction peak of a (104) plane obtained by X-ray diffraction measurement of 1.35 or more. 3. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the tap density of the positive electrode active material is 2 g/cm ^<3> or less. 4. The positive electrode for a lithium ion secondary battery according to claim 1 , wherein the positive electrode active material is a lithium-nickel-manganese-cobalt composite oxide having a layered structure. The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 4 , wherein the conductive additive is a carbon nanotube. A lithium ion secondary battery comprising a power generating element having the positive electrode for a lithium ion secondary battery according to any one of claims 1 to 5 .