Positive electrodes for lithium-ion secondary batteries

Abstract

To provide means for achieving both high cycle durability and high energy density in a lithium ion secondary battery.SOLUTION: In a positive electrode for a lithium ion secondary battery having a positive electrode active material layer including a positive electrode active material, when the crystallite diameter calculated from the Williamson-Hall method is X (nm) and the average particle diameter (D50) calculated from the laser diffraction method is Y (nm), the positive electrode active material satisfies Y / X≤70, and the density of the positive electrode active material layer is 3.40 g / cm3 or more and less than 3.80 g / cm3.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a positive electrode for a lithium ion secondary battery. [Background technology]

[0002] In recent years, the widespread use of various electric vehicles is expected to help solve environmental and energy problems. Secondary batteries are being developed as on-board power sources for driving motors and other applications, which are key to the widespread use of these electric vehicles. Non-aqueous electrolyte secondary batteries, such as lithium-ion secondary batteries, are attracting attention for their high energy density and high output.

[0003] Patent Document 1 discloses a technology aimed at improving the utilization efficiency of active materials and achieving high battery capacity without deteriorating the battery life and output characteristics of lithium-ion secondary batteries. Specifically, the technology described in Patent Document 1 controls the volume ratio of the positive electrode active material and the volume ratio of voids in the positive electrode active material layer within predetermined ranges. In this way, the coverage of the positive electrode active material surface with conductive additives and binders is reduced to increase the reactivity of the positive electrode active material, while at the same time ensuring a certain level of voids to maintain the reactivity of the positive electrode active material, thereby enabling the production of a battery with high capacity and long life. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-43257 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the inventors have conducted research and found that even in a lithium-ion secondary battery using the technology described in Patent Document 1, increasing the density of the positive electrode active material layer to improve the energy density may result in a decrease in cycle durability. Thus, even when the technology described in Patent Document 1 is used, there is currently room for improvement.

[0006] Therefore, an object of the present invention is to provide a means for achieving both high cycle durability and high energy density in a lithium ion secondary battery. [Means for solving the problem]

[0007] The present inventors have conducted extensive research to solve the above-mentioned problems. During the research, they discovered that in a positive electrode having a densified positive electrode active material layer, cracks in the positive electrode active material occur or grow during the fabrication of the positive electrode or with repeated charge / discharge cycles, which causes a decrease in cycle durability. Based on this finding, the present inventors discovered that by increasing the proportion of particles consisting of single crystallites among the particles constituting the positive electrode active material, high cycle durability can be achieved even when the positive electrode active material layer is densified, and have completed the present invention.

[0008] That is, according to one aspect of the present invention, there is provided a positive electrode for a lithium ion secondary battery having a positive electrode active material layer containing a positive electrode active material, wherein the positive electrode active material satisfies Y / X≦70, where X (nm) is a crystallite diameter calculated by the Williamson-Hall method and Y (nm) is an average particle diameter (D50) calculated by a laser diffraction method, and the density of the positive electrode active material layer is 3.40 g / cm 3 More than 3.80g / cm 3 The present invention provides a positive electrode for a lithium ion secondary battery, wherein the positive electrode is less than 1000 kJ / cm. [Effects of the Invention]

[0009] According to the present invention, it is possible to achieve both high cycle durability and high energy density in a lithium ion secondary battery. [Brief explanation of the drawings]

[0010] [Figure 1] 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. [Figure 2] FIG. 1 is a cross-sectional view schematically illustrating a bipolar secondary battery according to another embodiment of the present invention. [Figure 3] 1 is a graph showing the relationship between the density of the positive electrode active material layer and the capacity retention rate of the batteries fabricated in Examples 1 to 4 and Comparative Examples 1 to 11. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0011] One aspect of the present invention is a positive electrode for a lithium ion secondary battery having a positive electrode active material layer containing a positive electrode active material, wherein the positive electrode active material satisfies Y / X≦70, where X (nm) is a crystallite diameter calculated by the Williamson-Hall method and Y (nm) is an average particle diameter (D50) calculated by a laser diffraction method, and the density of the positive electrode active material layer is 3.40 g / cm 3 More than 3.80g / cm 3 The positive electrode for a lithium ion secondary battery is less than

[0012] The positive electrode for a lithium ion secondary battery according to the present embodiment can achieve both high cycle durability and high energy density in a lithium ion secondary battery. The present inventors speculate that the mechanism by which such effects are achieved is as follows.

[0013] In lithium-ion secondary batteries using common positive electrode active materials such as lithium-transition metal composite oxides, increasing the density of the positive electrode active material layer beyond a certain level reduces cycle durability. According to the inventors' research, in positive electrodes for lithium-ion secondary batteries using common positive electrode active materials, increasing the density of the positive electrode active material layer leads to cracking of the positive electrode active material particles during the fabrication of the positive electrode, which is the cause of the decrease in cycle durability of the battery. Furthermore, repeated charging and discharging of the battery caused the particle cracks to become larger, which also adversely affected cycle durability.

[0014] In contrast, the positive electrode according to the present embodiment uses a positive electrode active material with a high proportion of particles consisting of a single crystallite (single-crystal particles). At typical positive electrode active material layer densities, single-crystal particles have larger crystallites than polycrystalline particles, making it difficult to ensure sufficient contact area between the positive electrode active material particles. This tends to result in lower cycle durability compared to batteries using polycrystalline positive electrode active materials. However, single-crystal particles lack grain boundaries, making them less susceptible to cracking even when densified. As a result, deterioration in cycle durability due to cracking of the positive electrode active material particles is less likely. Additionally, densification facilitates ensuring sufficient contact area between particles, thereby increasing the area of ​​reaction on the surface of the positive electrode active material and allowing the reaction to occur more uniformly. This improves the cycle durability of the battery. In this way, a battery with excellent cycle durability can be obtained while achieving high energy density through densification of the positive electrode active material layer.

[0015] The above-mentioned embodiments of the present invention will be described below with reference to the drawings. However, the technical scope of the present invention should be defined based on the claims and is not limited to the following embodiments. Note that the dimensional ratios in the drawings are exaggerated for the convenience of explanation and may differ from the actual ratios. 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, etc. are performed at room temperature (20 to 25°C) and a relative humidity of 40 to 50% RH.

[0016] FIG. 1 is a cross-sectional view that schematically illustrates a flat (stacked) non-bipolar (internal parallel connection) secondary battery (hereinafter also simply referred to as a "stacked secondary battery") according to one embodiment of the present invention.

[0017] 1, the stacked secondary battery 10a of this embodiment has a structure in which a substantially rectangular power generating element 21, where charge and discharge reactions actually proceed, is sealed inside a laminate film 29. Here, the power generating element 21 has a configuration in which 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 are laminated. Specifically, the positive electrode, the electrolyte layer, and the negative electrode are laminated 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.

[0018] As a result, the positive electrode, electrolyte layer, and negative electrode constitute one cell layer 19. Therefore, the stacked secondary battery 10a shown in FIG. 1 can be said to have a configuration in which a plurality of cell layers 19 are stacked and electrically connected in parallel. Although the positive electrode active material layer 13 is disposed on only one side of each of the outermost positive electrode current collectors located on both outermost layers of the power generating element 21, 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 electrode and negative electrode 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.

[0019] A positive electrode current collector 25 and a negative electrode current collector 27, which are electrically connected to the 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 ends of the laminate film 29 and led out of the laminate film 29. 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 the electrodes by ultrasonic welding, resistance welding, or the like, via a positive electrode terminal lead and a negative electrode terminal lead (not shown), respectively, as necessary.

[0020] Fig. 2 is a cross-sectional view schematically illustrating a bipolar secondary battery according to another embodiment of the present invention. Bipolar secondary battery 10b shown in Fig. 2 has a structure in which a substantially rectangular power generating element 21, where charge and discharge reactions actually occur, is sealed inside a laminate film 29, which is a battery exterior. In this specification, a bipolar lithium-ion secondary battery may also be simply referred to as a "bipolar secondary battery," and an electrode for a bipolar lithium-ion secondary battery may also be simply referred to as a "bipolar electrode."

[0021] As shown in FIG. 2 , the power generating element 21 of the bipolar secondary battery 10b of this embodiment has a plurality of 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 such 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 therebetween. That is, 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.

[0022] Adjacent positive electrode active material layers 13, electrolyte layers 17, and negative electrode active material layers 15 constitute a single cell layer 19. Therefore, it can be said that the bipolar secondary battery 10b has a configuration in which the cell layers 19 are stacked. In addition, a seal portion (insulating layer) 31 is disposed on the outer periphery of the cell layer 19. This prevents a liquid junction due to leakage of the electrolyte solution 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 the positive electrode active material layer 13 formed on only one surface. The negative electrode side outermost current collector 11b, which is the outermost layer of the power-generating element 21, has the negative electrode active material layer 15 formed on only one surface.

[0023] 2, a positive electrode current collector (positive electrode tab) 25 is disposed adjacent to the outermost current collector 11a on the positive electrode side, and extends from the laminate film 29, which is the battery outer casing. On the other hand, a negative electrode current collector (negative electrode tab) 27 is disposed adjacent to the outermost current collector 11b on the negative electrode side, and similarly extends from the laminate film 29.

[0024] The number of times that cell layers 19 are stacked is adjusted according to the desired voltage. In addition, in bipolar secondary battery 10b, the number of times that cell layers 19 are stacked may be reduced as long as sufficient output can be ensured even if the thickness of the battery is made as thin as possible. In bipolar secondary battery 10b as well, it is preferable to use a structure in which power generating element 21 is vacuum-encapsulated in laminate film 29, which is the battery exterior, and positive electrode current collector 25 and negative electrode current collector 27 are exposed to the outside of laminate film 29, in order to protect against external impacts and environmental deterioration during use.

[0025] 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. The positive electrode active material layer is formed on the surface of an optional current collector.

[0026] [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.

[0027] Specifically, examples of metals include aluminum, nickel, iron, stainless steel, titanium, and copper. In addition, a clad material of nickel and aluminum, a clad material of copper and aluminum, and the like may also be used. Furthermore, a foil in which a metal surface is coated with aluminum may also be used. Among these, aluminum, stainless steel, copper, and nickel are preferred from the viewpoints of electronic conductivity, battery operating potential, and the like.

[0028] The latter conductive resin may be a resin in which a conductive filler is added to a non-conductive polymer material.

[0029] Examples of non-conductive polymer 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).

[0030] The conductive filler can be any material that is conductive. Examples of materials with excellent conductivity, potential resistance, or lithium ion blocking properties include metals and conductive carbon. While there are no particular limitations on the metal, it is preferable to use 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. Furthermore, there are no particular limitations on the conductive carbon. Preferably, the conductive carbon contains at least one 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.

[0031] The amount of conductive filler added is not particularly limited as long as it is an amount that can impart sufficient conductivity to the current collector, and is generally about 5 to 80 mass % relative to the total mass of the current collector (100 mass %).

[0032] The current collector may have a single layer structure made of a single material, or may have a laminate structure made of an appropriate combination of layers made of these materials. From the viewpoint of reducing the weight of the current collector, it is preferable that the current collector includes at least a conductive resin layer made of a resin having electrical conductivity. Furthermore, from the viewpoint of blocking the movement of lithium ions between the cell layers, a metal layer may be provided on a part of the current collector. Furthermore, if the positive electrode active material layer and the negative electrode active material layer described later are electrically conductive and can perform a current collecting function, it is not necessary to use a current collector as a separate member from these electrode active material layers. In such a configuration, the positive electrode active material layer described later constitutes the negative electrode, and the negative electrode active material layer described later constitutes the positive electrode.

[0033] [Cathode active material layer] The positive electrode active material layer contains a positive electrode active material and may also contain a binder, a conductive additive, an electrolytic solution (liquid electrolyte), and the like.

[0034] (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.

[0035] Examples of positive electrode active materials belonging to the space group R3m include lithium-transition metal composite oxides such as LiCoO2, LiNiO2, LiMnO2, Li(Ni-Mn-Co)O2, and Li(Ni-Co-Al)O2, 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. A composite oxide containing lithium and nickel is more preferred, and Li(Ni-Mn-Co)O2 and a portion of the transition metal is substituted with another element (hereinafter simply referred to as "NMC composite oxide") or Li(Ni-Co-Al)O2 and a portion of the transition metal is substituted with another element (hereinafter simply referred to as "NCA composite oxide") is even more preferred, with NMC composite oxide being 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 oxides and NCA composite oxides have a layered crystal structure in which lithium atomic layers and transition metal atomic layers are stacked alternately with oxygen atomic layers interposed between them. They contain one Li atom per transition metal M atom, and the amount of Li that can be extracted is twice that of spinel-type lithium manganese oxides, meaning that the supply capacity is doubled, resulting in high capacity.

[0036] As described above, the NMC composite oxide and the NCA composite oxide also include composite oxides in which some of the transition metal elements are substituted by other metal elements. Examples of the other elements in that case include Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, Cr, Fe, B, Ga, In, Si, Mo, Y, Sn, V, Cu, Ag, Zn, etc. Preferably, they are Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, Cr. More preferably, they are Ti, Zr, P, Al, Mg, Cr. From the viewpoint of improving cycle characteristics, even more preferably, they are Ti, Zr, Al, Mg, Cr. However, the other metal elements that can substitute the transition metal elements of the NCA composite oxide are those other than Al.

[0037] Since the NMC composite oxide has a high theoretical discharge capacity, preferably, it has a composition represented by the general formula (1): Li a Ni b Mn c Co d M x O2 (wherein, in the formula, a, b, c, d, 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, 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, in the general formula (1), it is preferable that 0.4 ≦ b ≦ 0.92. The composition of each element can be measured, for example, by inductively coupled plasma (ICP) optical emission spectrometry.

[0038] In general, nickel (Ni), cobalt (Co), and manganese (Mn) are known to contribute to capacity and output characteristics by improving the purity and electronic conductivity of the material. Ti and other elements partially substitute for transition metals in the crystal lattice. From the viewpoint of cycle characteristics, some of the transition metal elements may be substituted with 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.

[0039] In a more preferred embodiment, in the general formula (1), b, c, and d are in the range of 0.44≦b≦0.92, 0.05≦c≦0.31, and 0.03≦d≦0.26, which is preferable from the viewpoint of improving the balance between capacity and life characteristics. 0.5 Mn 0.3 Co 0.2 O2 is LiCoO2, LiMn2O4, LiNi, which are widely used in consumer batteries. 1 / 3 Mn 1 / 3 Co 1 / 3 Compared to O2, etc., it has a larger capacity per unit mass and can improve energy density, which has the advantage of allowing the creation of compact, high-capacity batteries, and is also preferable from the perspective of driving range. 0.8 Co 0.1 Al 0.1 O2 and LiNi 0.8 Mn 0.1 Co 0.1 O2, LiNi 0.88 Mn 0.06 Co 0.06 O2, LiNi 0.86 Mn 0.08 Co 0.06 O2 is more favorable. On the other hand, LiNi 0.5 Mn 0.3 Co 0.2 O2 is LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 It has excellent lifespan characteristics comparable to O2.

[0040] In another preferred embodiment, the lithium-nickel-manganese-cobalt composite oxide having a layered structure has a nickel content of 80 atomic % or more but less than 90 atomic % relative to the total amount of metal atoms other than lithium. For example, in general formula (1), b satisfies 0.8≦b<0.90. A nickel content of 80 atomic % or more is preferable because it provides a better balance between capacity and life characteristics. Furthermore, a nickel content of less than 90 atomic % can reduce gas generation associated with the electrode reaction. Therefore, it is preferable because it is less likely to cause a decrease in cycle durability due to gas generation. In addition, since increasing the density of the positive electrode active material layer tends to make it more difficult for gas to be released outside the system, the positive electrode of this embodiment can achieve the above-mentioned effect more significantly. In this case, it is more preferable that c and d in general formula (1) satisfy 0.05≦c≦0.31 and 0.03≦d≦0.26, respectively, from the viewpoint of improving the balance between capacity and life characteristics.

[0041] The positive electrode for a lithium ion secondary battery according to this embodiment is characterized in that 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 laser diffraction. Here, Y / X is an index of the number of crystal grains constituting the positive electrode active material particles, and the smaller this value, the fewer crystal grains constituting the particles. In other words, the smaller this value, the higher the number ratio of particles consisting of a single crystallite (single crystal) to the particles constituting the positive electrode active material. The crystallite diameter X (nm) and the average particle diameter (D50) Y can be measured by the methods described in the Examples section below. When Y / X exceeds 70, the number ratio of polycrystalline particles in the particles constituting the positive electrode active material increases, and the density of the positive electrode active material layer becomes 3.40 g / cm. 3If the ratio is more than this, the cycle durability will decrease. Y / X is more preferably 50 or less, and even more preferably 40 or less. On the other hand, the lower limit of Y / X is 1 or more. When the positive electrode active material is a mixture of two or more positive electrode active materials, the crystallite diameter X (nm) and average particle diameter (D50) of each positive electrode active material constituting the mixture are first measured by the above-mentioned method, and the obtained values ​​are added up in the mixing ratio to calculate the crystallite diameter X (nm) and average particle diameter (D50) Y of the mixture. Then, Y / X is calculated from these calculated values.

[0042] The crystallite diameter X of the positive electrode active material is not particularly limited, but is preferably 100 nm or more from the viewpoint of further improving cycle durability.

[0043] The average particle diameter (D50)Y of the positive electrode active material is not particularly limited, but from the viewpoint of achieving high output, it is preferably 1 to 10 μm, more preferably 1.5 to 6 μm, and even more preferably 2 to 5 μm.

[0044] As a method for controlling the Y / X ratio to a value of 70 or less or for controlling the X ratio to a value of, for example, 100 nm or more, reference can be made to the methods disclosed 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).

[0045] In the positive electrode for a lithium ion secondary battery according to this embodiment, the roughness factor of the positive electrode active material is preferably 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" to the "geometric specific surface area calculated from the average particle diameter" of the positive electrode active material (=BET specific surface area / geometric specific surface area) by the method described in the Examples section below. When the roughness factor of the positive electrode active material is 3 or less, the surface smoothness of the particles of the positive electrode active material is sufficiently high and there are few grain boundaries, making cracks less likely to occur even when the density of the positive electrode active material layer is increased. Furthermore, the reaction can proceed more uniformly on the surface of the positive electrode active material. As a result, cycle durability can be further improved. The roughness factor value is preferably 2.7 or less, more preferably 2.4 or less. Meanwhile, the lower limit of the roughness factor is 1 or more. When the positive electrode active material is a mixture of two or more materials, the BET specific surface area and the geometric specific surface area of ​​each positive electrode active material constituting the mixture are measured by the above-mentioned method, and the BET specific surface area and the geometric specific surface area of ​​the mixture are calculated by adding the obtained values ​​by the mixing ratio.The roughness factor is then calculated from these calculated values ​​by the same method as above.

[0046] 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 appropriately referred to. For example, as an example of a method for producing an NMC composite oxide, first, (1) a precursor (Ni x Co y Mn z )(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 time. The precipitate is separated by filtration and then dried at a temperature of about 80°C for 8 to 15 hours to obtain a precursor. Next, (2) the obtained precursor is calcined at a temperature of about 600 to 800°C for 3 to 7 hours to obtain Ni x Co y Mnz O2 is obtained. (3) Then, (4) a lithium salt (e.g., lithium carbonate) is added to the obtained oxide in a 1-1.2 stoichiometric ratio, followed by pulverization and mixing in a ball mill. (4) Then, (5) the powder is calcined in air or an oxygen atmosphere at a temperature of approximately 600-800°C for approximately 3-5 hours, and then further calcined at a temperature of approximately 750-1000°C for approximately 2-12 hours. (5) Finally, (6) the calcined powder is washed with water to remove residual lithium salt, and then dried at approximately 80°C for approximately 10-15 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 500-1000°C for 3-5 hours. The roughness factor of the NMC composite oxide 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). 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 carrying out the re-firing (6) for a long time and at a high temperature, the surface after grinding becomes smoother and the roughness factor can be reduced.

[0047] 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. Furthermore, 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 of the positive electrode active material 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. The peak intensity ratio is an index of the crystallinity of the positive electrode active material, and a higher peak intensity ratio indicates higher crystallinity. A peak intensity ratio within the above range reduces defects within the crystal, thereby suppressing a decrease in battery charge / discharge capacity and durability. The peak intensity ratio can be controlled by the raw materials, composition, firing conditions, and the like. The crystal structure and peak intensity ratio of the positive electrode active material can be determined by the measurement methods described in the Examples below.

[0048] In the positive electrode for a lithium ion secondary battery according to this embodiment, the tap density of the positive electrode active material is 2 g / cm 3 Preferably, it is 1.9 g / cm or less. 3 More preferably, it is 1.8 g / cm or less. 3 The lower limit of the tap density is not particularly limited, but is preferably 1.2 g / cm. 3 That's all. The smaller the tap density value, the smaller the 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 contact area between particles of the positive electrode active material can be made larger. As a result, the electrode reaction proceeds more efficiently, and cycle durability can be further improved.

[0049] 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 mass %, and more preferably in the range of 80 to 98 mass %, relative to 100 mass % of the total solid content of the positive electrode active material layer.

[0050] (Conductive additive) The conductive additive functions to form 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 can be reduced and the output characteristics at high rates can be improved.

[0051] Examples of the conductive aid include particulate carbon materials such as acetylene black, carbon black, channel black, thermal black, and Ketjen Black (registered trademark), and 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 aid may be used alone, or two or more types may be used in combination.

[0052] The content of the conductive additives that can be contained in the positive electrode active material layer (the total amount when two or more types are contained) is not particularly limited, but is preferably 0.5 to 10 mass %, and more preferably 1 to 5 mass %, relative to 100 mass % of the total solid content of the positive electrode active material layer. When the content of the conductive additive is 0.5 mass % or more, aggregation of the conductive additives is suppressed, thereby forming an electron conduction path well, thereby further improving the output characteristics. Furthermore, it is possible to further improve the energy density of the lithium ion secondary battery. Furthermore, when the content is 10 mass % or less, there is enough conductive additive to form an electron conduction path, thereby further improving the output characteristics.

[0053] The optional binder used in the positive electrode active material layer is not particularly limited, but examples thereof include the following materials: Thermoplastic polymers such as polybutylene terephthalate, polyethylene terephthalate, polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements), polyethylene, polypropylene, polymethylpentene, polybutene, polyethernitrile, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer and its hydrogenated products, styrene-isoprene-styrene block copolymer and its hydrogenated products, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene Examples of the fluororesin include ethylene-chlorotrifluoroethylene copolymer (ECTFE) and polyvinyl fluoride (PVF), vinylidene fluoride-based fluororubbers such as vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-TFE-based fluororubber), vinylidene fluoride-pentafluoropropylene-based fluororubber (VDF-PFP-based fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-PFP-TFE-based fluororubber), vinylidene fluoride-perfluoromethylvinyl ether-tetrafluoroethylene-based fluororubber (VDF-PFMVE-TFE-based fluororubber), and vinylidene fluoride-chlorotrifluoroethylene-based fluororubber (VDF-CTFE-based fluororubber), and epoxy resins. Among these, polyimide, styrene-butadiene rubber, carboxymethyl cellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide are more preferable.

[0054] The content of the binder that can be contained in the positive electrode active material layer (the total amount when two or more types are contained) is not particularly limited, but is preferably 0.5 to 10 mass % and more preferably 1 to 5 mass % relative to the total solid content of the positive electrode active material layer.

[0055] The positive electrode (positive electrode active material layer) can be formed by, but not limited to, a method of applying (coating) a normal slurry.

[0056] In the positive electrode for a lithium ion secondary battery of this embodiment, the density of the positive electrode active material layer is 3.40 g / cm 3 More than 3.80g / cm 3 The density of the positive electrode active material layer is less than 3.40 g / cm 3 If the density of the positive electrode active material layer is less than 3.80 g / cm, it is difficult to ensure sufficient contact area between particles in the single crystal, and therefore high cycle durability cannot be obtained. In addition, a battery with sufficient energy density cannot be obtained. On the other hand, if the density of the positive electrode active material layer is less than 3.80 g / cm, 3 It is difficult to fabricate a positive electrode having a density of 3.40 to 3.70 g / cm3 from the viewpoint of achieving a better balance between cycle durability and energy density. 3 It is preferable that the density is 3.40 to 3.68 g / cm 3 More preferably, it is 3.45 to 3.65 g / cm 3 It is even more preferable that the density is 3.50 to 3.60 g / cm 3 In this specification, the density of the positive electrode active material layer is measured by the following method.

[0057] The density of the positive electrode active material layer is calculated according to the following formula: Cathode active material layer density (g / cm 3 ) = mass of solid material (g) ÷ volume of positive electrode active material layer (cm 3 ).

[0058] The mass of the solid material is calculated by adding up only the mass of the solid material among the masses of the materials in the positive electrode active material layer, and the volume of the positive electrode active material layer is calculated from the thickness and application area of ​​the positive electrode active material layer.

[0059] The density of the positive electrode active material layer can be adjusted, for example, by adjusting the proportion of the positive electrode active material in the solid material constituting the positive electrode active material layer, the coating amount of the positive electrode slurry containing the positive electrode active material, or by adjusting the porosity through pressing conditions or the like.

[0060] The thickness of the positive electrode active material layer is not particularly limited, and conventionally known knowledge about batteries can be referred to as appropriate. For example, the thickness of the positive electrode active material layer is usually about 1 to 1000 μm. In the positive electrode of this embodiment, the thickness of the positive electrode active material layer is preferably 10 to 200 μm, more preferably 20 to 100 μm, and even more preferably 20 to 80 μm. Generally, 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. In the positive electrode of this embodiment, the thickness of the positive electrode active material layer can be appropriately adjusted to achieve a desired density of the positive electrode active material layer.

[0061] The porosity of the positive electrode active material layer is not particularly limited, but is preferably 15% or more and less than 25%, more preferably 17 to 24%, even more preferably 18 to 24%, even more preferably 19 to 24%, and particularly preferably 19 to 23%. When the porosity of the positive electrode active material layer is 15% or more, the effects of the present invention can be more pronounced. In addition, sufficient contact between the electron-conductive materials (e.g., conductive additive, positive electrode active material) in the positive electrode active material layer can be maintained, preventing an increase in electron transfer resistance. Furthermore, since sufficient electrolyte exists between the positive electrode active material particles, an increase in lithium ion transfer resistance can be prevented. As a result, the output characteristics of the lithium ion secondary battery can be further improved. On the other hand, when the porosity is less than 25%, the effect of improving cycle durability by using a positive electrode active material having a predetermined Y / X value can be more pronounced. Furthermore, the battery has excellent energy density.

[0062] By applying the positive electrode for a lithium ion secondary battery according to this embodiment to a lithium ion secondary battery, a lithium ion secondary battery excellent in both cycle durability and energy density can be obtained. Therefore, according to another embodiment of the present invention, there is provided a lithium ion secondary battery including a power generating element having the positive electrode for a lithium ion secondary battery according to the embodiment of the present invention described above. Components of the lithium ion secondary battery other than the positive electrode will be briefly described below.

[0063] [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.

[0064] Examples of the negative electrode active material include carbon materials such as graphite, soft carbon, and hard carbon, and lithium-transition metal composite oxides (e.g., Li4Ti5O 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 (for example, lithium-tin alloy, lithium-silicon alloy, lithium-aluminum alloy, lithium-aluminum-manganese alloy, etc.). In some cases, two or more types of 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.

[0065] Average particle diameter of the negative electrode active material (D 50 ) 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.

[0066] The content of the negative electrode active material in the negative electrode active material layer is, for example, 60% by mass or more and less than 100% by mass, preferably 80% by mass or more and 99.5% by mass or less, more preferably more than 95% by mass and 99.0% by mass or less, and even more preferably 97% by mass or more and 98.5% by mass or less, relative to 100% by mass of the total solid content. If the content of the negative electrode active material is within the above range, both battery capacity and output characteristics can be achieved.

[0067] Furthermore, the negative electrode active material layer may further contain, as necessary, a conductive additive, a binder, and the like, similar to those described above for the positive electrode active material layer.

[0068] The thickness of the negative electrode active material layer is not particularly limited, and conventionally known knowledge about batteries can be referred to as appropriate. For example, the thickness of the negative electrode active material layer is usually about 1 to 1000 μm, preferably 10 to 800 μm, more preferably 15 to 600 μm, and even more preferably 20 to 200 μm. The thicker the negative electrode active material layer, the more negative electrode active material can be retained to achieve sufficient capacity (energy density). On the other hand, the thinner the negative electrode active material layer, the more the discharge rate characteristics can be improved.

[0069] The negative electrode (negative electrode active material layer) can be formed by a method of applying (coating) a normal slurry.

[0070] [Electrolyte layer] The electrolyte layer preferably has a configuration in which a separator is impregnated with an electrolytic solution (liquid electrolyte).

[0071] (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.

[0072] 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 γ-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).

[0073] Examples of lithium salts include Li(FSO2)2N (lithium bis(fluorosulfonyl)imide; LiFSI), Li(C2F5SO2)2N, LiPF6, LiBF4, LiClO4, LiAsF6, and LiCF3SO3.

[0074] The concentration of the lithium salt in the electrolyte is preferably 0.1 mol / L or more and less than 3.0 mol / L, and even more preferably 0.8 to 2.2 mol / L. When the concentration of the lithium salt is 0.1 mol / L or more, the lithium ion concentration on the lithium ion absorption side becomes sufficiently high, thereby improving the rate characteristics of the battery and further improving the cycle durability. On the other hand, when the concentration is less than 3.0 mol / L, the lithium ion concentration gradient in the thickness direction of the electrode can be appropriately controlled even in a high-density electrode, thereby improving the rate characteristics of the battery and further improving the cycle durability. In one embodiment of the present invention, the concentration of the lithium salt in the electrolyte is preferably a concentration such that the lithium ion concentration is less than 50% of the saturated lithium ion concentration of the electrolyte at 25°C, from the viewpoint of ensuring lithium ion transportability in a high-density electrode.

[0075] The electrolyte may further contain additives other than the above-mentioned components. 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 the additive 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. The amount of additive used in the electrolyte solution can be adjusted as appropriate.

[0076] (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.

[0077] The separator may be in the form of, for example, a porous sheet separator made of polymer or fiber that absorbs and retains the electrolyte solution, or a nonwoven fabric separator.

[0078] As a separator made of a porous sheet made of a polymer or fiber, for example, a microporous material (microporous membrane) can be used. Specific forms of the porous sheet made of a polymer or fiber include microporous (microporous membrane) separators made of polyolefins such as polyethylene (PE) and polypropylene (PP), laminates of multiple layers of these (for example, 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.

[0079] The thickness of the microporous (microporous membrane) separator cannot be uniquely defined because it varies depending on the intended use. For example, in applications such as motor-driven secondary batteries for electric vehicles (EVs), hybrid electric vehicles (HEVs), and fuel cell vehicles (FCVs), the thickness is preferably 4 to 60 μm for a single layer or multiple layers. The micropore diameter of the microporous (microporous membrane) separator is preferably a maximum of 1 μm or less (usually a pore diameter of about several tens of nanometers). The porosity of the microporous (microporous membrane) separator is not particularly limited, but is, for example, 30 to 70%.

[0080] As the nonwoven fabric separator, conventional materials such as cotton, rayon, acetate, nylon, polyester; polyolefins such as PP and PE; polyimide, aramid, etc. may be used alone or in combination. The bulk density of the nonwoven fabric is not particularly limited as long as sufficient battery characteristics can be obtained with the impregnated polymer gel electrolyte. The thickness of the nonwoven fabric separator may be the same as that of the electrolyte layer, and is preferably 5 to 200 μm, and particularly preferably 10 to 100 μm. The porosity of the nonwoven fabric separator is also not particularly limited, but is, for example, 30 to 70%.

[0081] Furthermore, the separator is preferably a separator 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. The separator with a heat-resistant insulating layer is highly heat-resistant, with a melting point or thermal softening point of 150°C or higher, preferably 200°C or higher. The presence of the 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 the 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.

[0082] [Positive and negative current collector plates] 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.

[0083] [Positive and negative leads] Although not shown, the current collector 11 and the current collector plates (25, 27) may be electrically connected via a positive electrode lead and a negative electrode lead. The 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 the product (e.g., automobile parts, particularly electronic devices).

[0084] [Sealing part] The sealing portion (insulating layer) 31 is a component specific to bipolar secondary batteries (series-stacked batteries) and functions to prevent leakage of the electrolyte solution from the electrolyte layer. The sealing portion 31 also functions to prevent contact between current collectors and short-circuiting at the ends of the cell layers. The sealing portion may be made of any material that has insulating properties, sealing properties against the loss of the solid electrolyte and against moisture penetration from the outside, 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). Other suitable materials include isocyanate-based adhesives, acrylic resin-based adhesives, and cyanoacrylate-based adhesives, as well as hot-melt adhesives (urethane resin, polyamide resin, polyolefin resin). 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 copolymerized with ethylene, propylene, and butene, with amorphous polypropylene resin as the main component.

[0085] [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 to these. A laminate film is desirable from the viewpoint of achieving 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 preferable for the exterior because it allows for easy adjustment of the collective pressure applied to the power-generating element from the outside.

[0086] The lithium ion secondary battery according to the present embodiment can achieve both excellent cycle durability and high energy density, and is therefore suitable for use as a power source for driving EVs and HEVs.

[0087] [Battery pack] 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.

[0088] A small, detachable assembled battery can be formed by connecting multiple batteries in series or in parallel. Furthermore, a large-capacity, high-output assembled battery (such as a battery module or battery pack) can be formed by further connecting multiple such small, detachable assembled batteries in series or in parallel, suitable for use as a vehicle drive power source or auxiliary power source, which require high volumetric energy density and high volumetric power density. The number of batteries to be connected to form a battery assembly and the number of stacked small assembled batteries to form a large-capacity assembled battery can be determined based on the battery capacity and output of the vehicle (electric vehicle) in which the battery will be installed.

[0089] [vehicle] A battery or a battery pack formed by combining a plurality of such batteries can be mounted on a vehicle. The present invention makes it possible to construct a long-life battery with excellent long-term reliability. Therefore, by incorporating such a battery, a plug-in hybrid electric vehicle with a long EV driving range or an electric vehicle with a long driving range per charge can be constructed. For example, a battery or a battery pack formed by combining a plurality of such batteries can be used in a hybrid vehicle, a fuel cell vehicle, or an electric vehicle (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) to produce a vehicle with a long life and high reliability. However, the application is not limited to automobiles, and the battery pack 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. [Example]

[0090] The present invention will be described in more detail below using examples and comparative examples, but the present invention is not limited to the following examples.

[0091] <Cathode active material> The following two types of commercially available positive electrode active materials were prepared.

[0092] Single crystal NMC composite oxide 1 (single crystal 1) (Easpring, ME88SC, LiNi 0.86 Mn 0.08 Co 0.06 O2, 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 / cm 3 ).

[0093] Polycrystalline NMC composite oxide 1 (polycrystalline 1) (manufactured by Ecopro, NMC811, LiNi 0.8 Mn 0.1 Co 0.1O2, roughness factor: 16.5, crystallite diameter (X): 80 nm, average particle diameter (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 / cm 3 ).

[0094] The physical properties of each positive electrode active material were measured by the following methods.

[0095] [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)). An X-ray diffraction (XRD) device (manufactured by Rigaku Corporation) using CuKα radiation was used for the measurement. 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 integral value β:

[0096] βcosθ / λ=2η(sinθ / λ)+(1 / ε) Here, ε is the crystallite diameter, λ is the wavelength of CuKα radiation, and η is the distortion of the positive electrode active material. With sinθ on the X axis and βcosθ on the Y axis, the θ of the diffraction peak of the positive electrode active material and its integral width β are plotted on the X and Y axes, and an approximate straight line is drawn using the least squares method. The crystallite diameter ε(X) is calculated from the point where it intersects with the Y axis.

[0097] (Average particle diameter (D50)Y) The average particle diameter (D50) Y of the positive electrode active material was measured by laser diffraction, where D50 represents the particle diameter when the cumulative value of the particle size distribution on a volume basis is 50%.

[0098] [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 outline 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.

[0099] Next, from the value of the average particle diameter calculated above, the volume [m 3 ] and particle surface area [m 2 ] was calculated, and the specific gravity of the active material (here, 4.78 [g / cm 3 The mass [g] of the spherical particles was calculated by multiplying the surface area of ​​the particles calculated above by the mass of the particles also calculated above. The geometric specific surface area [m 2 / g] was calculated.

[0100] On the other hand, in accordance with the "Method for measuring the specific surface area of ​​powders (solids) by gas adsorption" described in JIS Z8830:2013 (ISO 9277:2010), measurements were performed using nitrogen gas as the adsorption gas by the static volume method, and the BET specific surface area [m 2 / g] was calculated. 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).

[0101] [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 (Rigaku Corporation) with CuKα radiation, and analysis was performed using fundamental parameters. X-ray diffraction patterns obtained from a diffraction angle range of 2θ = 15 to 120° were analyzed using the analysis software Topas Version 3.

[0102] [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.

[0103] <Preparation of positive electrode> [Example 1] A positive electrode slurry was obtained by mixing 95 parts by mass of the above single crystal NMC composite oxide 1 (single crystal 1) as the positive electrode active material, 3 parts by mass of acetylene black (AB) as a conductive additive, and 2 parts by mass of polyvinylidene fluoride (PVDF) as a binder with N-methyl-2-pyrrolidone (NMP) as a solvent. PVDF was dispersed in NMP at 7% by mass in advance, and then added and mixed. The solid concentration was also adjusted using NMP. The obtained positive electrode slurry was applied to a substrate with a basis weight (single-sided coating amount) of 20 mg / cm. 2 The coating was applied to a 20 μm thick aluminum foil using a doctor blade and a coater (manufactured by Tester Sangyo Co., Ltd.) with the gap adjusted so that the coating was 90 ° C. for 1 hour. The dried coating was pressed using a small desktop roll press (manufactured by Tester Sangyo Co., Ltd.) so that the final thickness of the positive electrode active material layer was 58.8 μm. Thereafter, this was placed in a vacuum dryer and dried at 130 ° C. for 8 hours under vacuum to obtain the positive electrode of this example.

[0104] [Examples 2 to 4, Comparative Examples 8 to 11] Positive electrodes of Examples 2 to 4 and Comparative Examples 8 to 11 were produced in the same manner as in Example 1, except that the pressing conditions in Example 1 were changed so that the final thickness of the positive electrode active material layer would be the thickness shown in Table 1 below.

[0105] [Comparative Examples 1 to 7] Positive electrodes of Comparative Examples 1 to 7 were prepared in the same manner as in Example 1, except that in Example 1, the positive electrode active material was changed to the above-mentioned polycrystalline NMC composite oxide 1 (polycrystalline 1), and the pressing conditions were changed so that the final thickness of the positive electrode active material layer would be the thickness shown in Table 1 below.

[0106] <Fabrication of lithium-ion secondary batteries> The positive electrode and negative electrode (Li metal) fabricated above were placed facing each other, with a glass fiber separator (200 μm thick) between them. The stack of positive electrode, separator, and negative electrode (Li metal) was then placed at the bottom of a coin cell (CR2025). A gasket was attached to maintain insulation between the electrodes, and 150 μm of electrolyte was injected using a pipette. A spring and spacer were then added, and the top of the coin cell was placed on top of it and sealed by crimping to produce a lithium-ion secondary battery (coin cell). The electrolyte used was a solution (Kishida Chemical Co., Ltd.) prepared by dissolving 61 mol / L of LiPF in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio 3:7). All cell fabrication was carried out in a glove box (dew point -60°C) in a dry room.

[0107] [Measurement of cycle durability] At 25°C, the initial and second charge / discharge cycles were performed in CC-CV mode (1C = 230mAh / g) and CC mode (8-hour rest between charge and discharge) at a voltage range of 2.5 to 4.3V. The charge / discharge rate was 0.05C for the initial charge only, and 0.1C for the rest. The cycle test consisted of charge / discharge at 0.33C / 0.33C, with a check-up cycle at 0.1C / 0.1C every 50 cycles. The capacity retention rate [%] was calculated as the ratio of the discharge capacity at the 100th cycle to the discharge capacity at the first cycle.

[0108] The electrode energy density was calculated for the first cycle discharge using the following formula.

[0109]

number

[0110] In the above formula, the positive electrode capacity density (Ah / m 2 ) is the capacity of the positive electrode active material (Ah / g) × the amount of active material carried per unit area of ​​the positive electrode (g / m 2) The average voltage (V) was calculated by calculating the total energy (mWh) from the area of ​​the discharge curve (curve with capacity on the horizontal axis and voltage on the vertical axis) and dividing this by the resulting discharge capacity (mAh). The electrode layer thickness (m) represents the thickness of (positive electrode current collector - positive electrode active material layer - separator - negative electrode active material layer - negative electrode current collector).

[0111] The results are shown in Table 1 below and Fig. 3. Fig. 3 is a graph showing the relationship between the density of the positive electrode active material layer and the capacity retention rate of the batteries using the positive electrodes produced in Examples 1 to 4 and Comparative Examples 1 to 11.

[0112] [Table 1]

[0113] From the results in Table 1 and FIG. 3, it was found that when the positive electrodes of Examples 1 to 4, which are the positive electrodes for lithium ion secondary batteries of the present invention, were used, batteries excellent in both capacity retention rate and electrode energy density were obtained.

[0114] On the other hand, when a polycrystalline positive electrode active material was used, the density of the positive electrode active material layer was 3.40 g / cm as in Comparative Examples 1 to 3. 3 If the density of the positive electrode active material layer is less than 3.40 g / cm, the thickness of the positive electrode active material layer will be too large to achieve a sufficient electrode energy density. 3 Furthermore, even if a single-crystal positive electrode active material is used, the density of the positive electrode active material layer is 3.40 g / cm. 3 It has been confirmed that if the electrode energy density is less than 1000 kJ / cm, a sufficient electrode energy density cannot be achieved. [Explanation of symbols]

[0115] 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 generation elements, 23 Bipolar electrodes, 25 Positive current collector plate (positive tab), 27 negative electrode current collector plate (negative electrode tab), 29 Laminating film, 31 Seal part.

Claims

1. A positive electrode for a lithium ion secondary battery having a positive electrode active material layer containing a positive electrode active material, The positive electrode active material satisfies Y / X≦70, where X (nm) is a crystallite diameter calculated by the Williamson-Hall method and Y (nm) is an average particle diameter (D50) calculated by a laser diffraction method, The density of the positive electrode active material layer is 3.40 g / cm ３ 3.80g / cm or more ３ is less than The positive electrode for a lithium ion secondary battery has a roughness factor of 3 or less, which is defined as the ratio of the BET specific surface area to the geometric specific surface area calculated from the average particle diameter of the positive electrode active material.

2. The density of the positive electrode active material layer is 3.45 g / cm ３ 3.65g / cm or more ３ The positive electrode for a lithium ion secondary battery according to claim 1, wherein:

3. 3. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the positive electrode active material is composed of a space group of R3m, and 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 is 1.35 or more.

4. The tap density of the positive electrode active material is 2 g / cm ３ The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 3, wherein:

5. 5. 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.

6. A positive electrode for a lithium ion secondary battery having a positive electrode active material layer containing a positive electrode active material, The positive electrode active material satisfies Y / X≦70, where X (nm) is a crystallite diameter calculated by the Williamson-Hall method and Y (nm) is an average particle diameter (D50) calculated by a laser diffraction method, the density of the positive electrode active material layer is 3.40 g / cm 3 or more and less than 3.80 g / cm 3 ; the positive electrode active material is a lithium-nickel-manganese-cobalt composite oxide having a layered structure, In the lithium-nickel-manganese-cobalt composite oxide having a layered structure, the content of nickel is in the range of 80 atomic % or more and less than 90 atomic % based on the total amount of metal atoms other than lithium.

7. 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 6.

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