JP2023141825A - Positive electrode for lithium ion secondary battery - Google Patents

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

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

In recent years, various electric vehicles are expected to become popular in order to solve environmental and energy problems. Rechargeable batteries are being actively developed as in-vehicle power supplies such as motor drive power supplies that are key to the spread of electric vehicles. As secondary batteries, nonaqueous electrolyte secondary batteries such as lithium ion secondary batteries are attracting attention because they are expected to have high energy density and high output.

Here, Patent Document 1 discloses a technology that aims to improve the utilization efficiency of active materials and realize high battery capacity without deteriorating the battery life and output characteristics of lithium ion secondary batteries. There is. Specifically, the technique described in Patent Document 1 controls the volume ratio of the positive electrode active material and the volume ratio of the voids in the positive electrode active material layer, respectively, within predetermined ranges. In this way, the coating of the surface of the cathode active material with the conductive additive and binder is reduced, increasing the reactivity of the cathode active material, and at the same time maintaining the reactivity of the cathode active material by securing a certain amount of voids. A battery with high capacity and long life can be obtained.

JP 2015-43257 Publication

However, the present inventors conducted a study and found that even in a lithium ion secondary battery using the technology described in Patent Document 1, if the density of the positive electrode active material layer is increased in order to improve the energy density, the cycle It has been found that durability may be reduced. As described above, even when the technology described in Patent Document 1 is used, there is currently room for improvement.

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.

The present inventors conducted extensive studies to solve the above problems. In the process, in a positive electrode with a high-density positive electrode active material layer, cracks may occur in the positive electrode active material or cracks may become larger during the fabrication of the positive electrode or with repeated charging and discharging. It was found that this was the cause of the decrease in durability. Based on this knowledge, the present inventors have determined that the density of the positive electrode active material layer can be increased by increasing the number ratio of particles consisting of a single crystallite among the particles constituting the positive electrode active material. The present inventors have discovered that high cycle durability can be obtained even when using the same method, and have completed the present invention.

That is, according to one embodiment 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 has a crystallite diameter calculated from the Williamson-Hall method. is X (nm) and the average particle diameter (D50) calculated from laser diffraction method is Y (nm), Y/X≦70 is satisfied, and the density of the positive electrode active material layer is 3.40 g/ A positive electrode for a lithium ion secondary battery is provided that has a density of at least ^{3 cm 3} and less than 3.80 g/cm ³ .

According to the present invention, it is possible to achieve both high cycle durability and high energy density in a lithium ion secondary battery.

1 is a cross-sectional view schematically showing a stacked (flat) non-bipolar (internal parallel connection type) secondary battery, which is an embodiment of the present invention. FIG. 3 is a cross-sectional view schematically showing a bipolar secondary battery, which is another embodiment of the present invention. FIG. 3 is a diagram showing the relationship between the density of the positive electrode active material layer and the capacity retention rate of the batteries produced in Examples 1 to 4 and Comparative Examples 1 to 11.

One form 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 has a crystallite diameter of X (nm) calculated by the Williamson-Hall method. ), when the average particle diameter (D50) calculated by laser diffraction method is Y (nm), Y/X≦70 is satisfied, and the density of the positive electrode active material layer is 3.40 g/cm ³ or more 3 It is a positive electrode for a lithium ion secondary battery, which is less than .80 g/cm ³ .

According to the positive electrode for a lithium ion secondary battery according to the present embodiment, it is possible to 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 an effect is produced is as follows.

In a lithium ion secondary battery using a common positive electrode active material such as a lithium-transition metal composite oxide, if the density of the positive electrode active material layer is increased beyond a certain level, cycle durability will decrease. Here, according to the studies of the present inventors, in a positive electrode for a lithium ion secondary battery using a general positive electrode active material, when the positive electrode active material layer is made to have a high density, the positive electrode active material particles are It was found that cracking occurred, which caused a decrease in the cycle durability of the battery. It was also found that repeated charging and discharging of the battery caused cracks in the particles to become larger, which also had an adverse effect on cycle durability.

On the other hand, in the positive electrode according to this embodiment, a positive electrode active material having a high number of particles consisting of a single crystallite (single crystal particles) is used. At a typical density of a positive electrode active material layer, since single crystal particles have larger crystallites than polycrystalline particles, it is difficult to ensure a contact area between the positive electrode active material particles. Therefore, the cycle durability of the battery tends to be lower than that when a polycrystalline positive electrode active material is used. However, single-crystal grains do not have grain boundaries, so even if the density is increased, cracks are less likely to occur. As a result, cycle durability is less likely to deteriorate due to cracking of the positive electrode active material particles. In addition, increasing the density makes it easier to secure a contact area between particles, so the area involved in the reaction becomes larger on the surface of the positive electrode active material, and the reaction can occur more uniformly. Therefore, the cycle durability of the battery can be improved. In this way, it is possible to obtain a battery that achieves high energy density by increasing the density of the positive electrode active material layer and also has excellent cycle durability.

Hereinafter, the embodiments of the present invention described above will be described with reference to the drawings, but the technical scope of the present invention should be determined based on the claims and is not limited only to the following embodiments. Note that the dimensional ratios in the drawings are exaggerated for convenience of explanation and may differ from the actual ratios. In this specification, the range "X to Y" means "more than or equal to X and less than or equal to Y." Further, unless otherwise specified, operations and measurements of physical properties, etc. are performed at room temperature (20 to 25°C) and relative humidity of 40 to 50% RH.

FIG. 1 schematically shows a flat (stacked) non-bipolar (internal parallel connection type) secondary battery (hereinafter also simply referred to as a "stacked secondary battery"), which is an embodiment of the present invention. FIG.

As shown in FIG. 1, the stacked secondary battery 10a of this embodiment has a structure in which a substantially rectangular power generation element 21 in which a charge/discharge reaction actually proceeds is sealed inside a laminate film 29. Here, the power generation element 21 includes 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 consisting of a separator containing an electrolytic solution, and an electrolyte layer 17 on both sides of a negative electrode current collector 12. It has a structure in which a negative electrode on which a negative electrode active material layer 15 is arranged is laminated. Specifically, one positive electrode active material layer 13 and an adjacent negative electrode active material layer 15 face each other with an electrolyte layer 17 in between, and the positive electrode, electrolyte layer, and negative electrode are stacked in this order. .

Thereby, the positive electrode, the electrolyte layer, and the negative electrode constitute one single cell layer 19. Therefore, it can be said that the stacked secondary battery 10a shown in FIG. 1 has a configuration in which a plurality of cell layers 19 are stacked and electrically connected in parallel. Note that the positive electrode active material layer 13 is disposed on only one side of the outermost positive electrode current collectors located on both outermost layers of the power generation element 21, but active material layers may be provided on both sides. . That is, instead of using a current collector exclusively for the outermost layer with an active material layer provided on only one side, a current collector with active material layers on both sides may be used as it is as the outermost layer current collector. In addition, by reversing the arrangement of the positive electrode and negative electrode from FIG. 1, the outermost negative electrode current collector is located on both outermost layers of the power generation element 21, and A negative electrode active material layer may be arranged on both sides.

A positive current collector plate 25 and a negative current collector plate 27 that are electrically connected to each electrode (positive electrode and negative electrode) are attached to the positive electrode current collector 11' and the negative electrode current collector 12, respectively, and are sandwiched between the ends of the laminate film 29. It has a structure in which it is led out to the outside of the laminate film 29 in such a manner that it is exposed. The positive electrode current collector plate 25 and the negative electrode current collector plate 27 are connected to the positive electrode current collector 11' and the negative electrode current collector 12 of each electrode via a positive electrode terminal lead and a negative electrode terminal lead (not shown), respectively, as necessary. It may be attached by ultrasonic welding, resistance welding, or the like.

FIG. 2 is a cross-sectional view schematically showing a bipolar secondary battery according to another embodiment of the present invention. The bipolar secondary battery 10b shown in FIG. 2 has a structure in which a substantially rectangular power generation element 21, in which a charging/discharging reaction actually proceeds, is sealed inside a laminate film 29, which is a battery exterior body. Note that, 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 be simply referred to as a "bipolar electrode."

As shown in FIG. 2, the power generation element 21 of the bipolar secondary battery 10b of this embodiment has a positive electrode active material layer 13 electrically coupled to one surface of the current collector 11. It has a plurality of bipolar electrodes 23 each having an electrically coupled negative electrode active material layer 15 formed on its opposite surface. Each bipolar electrode 23 is stacked with the electrolyte layer 17 in between to form the power generation element 21 . At this time, the positive electrode active material layer 13 of one bipolar electrode 23 and the negative electrode active material layer 15 of another bipolar electrode 23 adjacent to the one bipolar electrode 23 face each other with the electrolyte layer 17 interposed therebetween. Bipolar electrodes 23 and electrolyte layers 17 are alternately stacked. 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 another bipolar electrode 23 adjacent to the one bipolar electrode 23. ing.

Adjacent positive electrode active material layer 13 , electrolyte layer 17 , and negative electrode active material layer 15 constitute one cell layer 19 . Therefore, it can be said that the bipolar secondary battery 10b has a structure in which the single cell layers 19 are stacked. Furthermore, a seal portion (insulating layer) 31 is arranged on the outer periphery of the cell layer 19 . This prevents liquid junctions due to electrolyte leakage from the electrolyte layer 17, and prevents adjacent current collectors 11 from coming into contact with each other within the battery, and slight irregularities at the ends of the cell layers 19 in the power generation element 21. This prevents short circuits from occurring due to Note that the positive electrode active material layer 13 is formed only on one side of the outermost layer current collector 11a on the positive electrode side located at the outermost layer of the power generation element 21. Further, the negative electrode active material layer 15 is formed only on one side of the negative electrode side outermost layer current collector 11b located at the outermost layer of the power generation element 21.

Furthermore, in the bipolar secondary battery 10b shown in FIG. 2, a positive electrode current collector plate (positive electrode tab) 25 is arranged adjacent to the outermost layer current collector 11a on the positive electrode side, and this is extended to form a battery exterior body. It is derived from the laminate film 29. On the other hand, a negative electrode current collector plate (negative electrode tab) 27 is arranged adjacent to the outermost layer current collector 11b on the negative electrode side, and is similarly extended and led out from the laminate film 29.

Note that the number of times the cell layer 19 is stacked is adjusted depending on the desired voltage. Furthermore, in the bipolar secondary battery 10b, the number of times the unit cell layers 19 are stacked may be reduced if sufficient output can be ensured even if the thickness of the battery is made as thin as possible. In the bipolar secondary battery 10b, in order to prevent external impact and environmental deterioration during use, the power generating element 21 is sealed under reduced pressure in a laminate film 29, which is the battery exterior, and the positive electrode current collector plate 25 and the negative electrode collector It is preferable to have a structure in which the electric plate 27 is taken out from the laminate film 29.

The main constituent members of the positive electrode for a lithium ion secondary battery according to this embodiment will be explained 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.

[Current collector]
The current collector has a function of mediating the movement of electrons from the positive electrode active material layer and the negative electrode active material layer, which will be described later. There is no particular restriction on the material constituting the current collector. As the constituent material of the current collector, for example, metal or conductive resin may be used.

Specifically, examples of the metal include aluminum, nickel, iron, stainless steel, titanium, and copper. In addition to these, a cladding material of nickel and aluminum, a cladding material of copper and aluminum, etc. may be used. Alternatively, it may be a foil whose metal surface is coated with aluminum. Among these, aluminum, stainless steel, copper, and nickel are preferred from the viewpoint of electronic conductivity, battery operating potential, and the like.

Further, examples of the latter conductive resin include resins in which a conductive filler is added to a non-conductive polymer material.

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), or polystyrene (PS).

The conductive filler can be used without particular limitation as long as it is a substance that has conductivity. For example, metals, conductive carbon, and the like are examples of materials with excellent conductivity, potential resistance, or lithium ion blocking properties. The metal is not particularly limited, but includes at least one metal selected from the group consisting of Ni, Ti, Al, Cu, Pt, Fe, Cr, Sn, Zn, In, and Sb, or containing these metals. Preferably, it contains an alloy or metal oxide. Furthermore, there are no particular limitations on the conductive carbon. Preferably, it is selected from the group consisting of acetylene black, Vulcan (registered trademark), Black Pearl (registered trademark), carbon nanofiber, Ketjenblack (registered trademark), carbon nanotube, carbon nanohorn, carbon nanoballoon, and fullerene. It contains at least one kind.

The amount of conductive filler added is not particularly limited as long as it can impart sufficient conductivity to the current collector, and is generally 5 to 80% by mass based on 100% by mass of the total mass of the current collector. That's about it.

Note that the current collector may have a single-layer structure made of a single material, or may have a laminated structure in which layers made of these materials are appropriately combined. From the viewpoint of reducing the weight of the current collector, it is preferable to include at least a conductive resin layer made of a resin having conductivity. Further, 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, which will be described later, have conductivity by themselves and can exhibit a current collection function, it is possible to use a current collector as a member separate from these electrode active material layers. It is not necessary. In such a form, the positive electrode active material layer described later directly constitutes the negative electrode, and the negative electrode active material layer described later directly constitutes the positive electrode.

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

(Cathode active material)
The positive electrode active material has a function of releasing ions such as lithium ions during charging and occluding ions such as lithium ions during discharging. In the positive electrode for a lithium ion secondary battery according to the present embodiment, the type of positive electrode active material is not particularly limited, but it is preferably made of a space group of R3m because it has a higher capacity. The positive electrode active material belonging to the space group R3m has a layered structure (layered rock salt type structure) in which lithium atomic layers and transition metal atomic layers are stacked alternately. Therefore, by using such a positive electrode active material, the battery capacity of a lithium ion secondary battery can be improved.

Examples of positive electrode active materials belonging to space group R3m include LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , Li(Ni-Mn-Co)O ₂ , Li(Ni-Co-Al)O ₂ and these transition metals. Examples include lithium-transition metal composite oxides in which a portion of the transition metal is substituted with other elements. In some cases, two or more types of positive electrode active materials may be used together. More preferably, a composite oxide containing lithium and nickel is used, and even more preferably a composite oxide containing Li(Ni-Mn-Co)O ₂ and a part of these transition metals substituted with other elements (hereinafter referred to as (also simply referred to as "NMC composite oxide") or Li(Ni-Co-Al)O ₂ and those in which some of these transition metals are replaced with other elements (hereinafter also simply referred to as "NCA composite oxide") ) is used, and NMC composite oxide is particularly preferably used. 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 stacked alternately through oxygen atomic layers, and one Li atom per atom of transition metal M. The amount of Li that can be contained and taken out is twice that of spinel-based lithium manganese oxide, that is, the supply capacity is doubled, and it can have a high capacity.

As described above, the NMC composite oxide and the NCA composite oxide also include composite oxides in which a part of the transition metal element is replaced with another metal element. 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 Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, Cr, more preferably Ti, Zr, P, Al, Mg, Cr, and from the viewpoint of improving cycle characteristics, Ti, Zr, Al, Mg, and Cr are more preferred. However, other metal elements that can replace the transition metal elements in the NCA composite oxide are other than Al.

Since the NMC composite oxide has a high theoretical discharge capacity, it is preferably formed using the general formula (1): Li _a Ni _b Mn _c Co _d M _x O ₂ (wherein a, b, c, d, x satisfies 0.9≦a≦1.2, 0<b<1, 0<c≦0.5, 0<d≦0.5, 0≦x≦0.3, b+c+d+x=1.M is It has a composition represented by 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. represents. From the viewpoint of cycle characteristics, in general formula (1), it is preferable that 0.4≦b≦0.92. Note that the composition of each element can be measured by, for example, plasma (ICP) emission spectrometry.

Generally, nickel (Ni), cobalt (Co), and manganese (Mn) are known to contribute to capacity and output characteristics from the viewpoint of improving material purity and electron conductivity. Ti and the like partially substitute 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 crystal structure is stabilized by the solid solution of at least one member selected from the group consisting of Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, and Cr. It is thought that even if the process is repeated, a decrease in battery capacity can be prevented and excellent cycle characteristics can be achieved.

As a more preferred embodiment, in general formula (1), b, c and d are 0.44≦b≦0.92, 0.05≦c≦0.31, 0.03≦d≦0.26 This is preferable from the viewpoint of improving the balance between capacity and life characteristics. For example, LiNi _0.5 Mn _0.3 Co _0.2 O ₂ is similar to LiCoO ₂ , LiMn ₂ O ₄ , LiNi _1/3 Mn _1/3 Co _1/3 O _2, etc., which have a proven track record in general consumer batteries. Compared to this, it has a larger capacity per unit mass and can improve energy density, making it possible to produce a compact and high-capacity battery, which is also preferable from the viewpoint of cruising distance. In terms of larger capacity, LiNi _0.8 Co _0.1 Al _0.1 O ₂ , LiNi _0.8 Mn _0.1 Co _0.1 O ₂ , LiNi _0.88 Mn _0.06 Co _{0. 06} O ₂ , LiNi _0.86 Mn _0.08 Co _0.06 O ₂ are more preferred. On the other hand, LiNi _0.5 Mn _0.3 Co _0.2 O ₂ has excellent life characteristics comparable to LiNi _1/3 Mn _1/3 Co _1/3 O ₂ .

In another preferred embodiment, in the lithium-nickel-manganese-cobalt composite oxide having a layered structure, the content ratio of nickel is 80 atomic % or more and 90 atomic % or more with respect to the total amount of metal atoms other than lithium. The range is less than atomic percent. For example, in general formula (1), b satisfies 0.8≦b<0.90. It is preferable that the content ratio of nickel is 80 atomic % or more because it provides a better balance between capacity and life characteristics. Moreover, when the content ratio of nickel is less than 90 atomic %, gas generation accompanying electrode reactions can be reduced. Therefore, it is preferable because cycle durability is less likely to deteriorate due to gas generation. In addition, since increasing the density of the positive electrode active material layer tends to make it difficult to release gas to the outside of the system, the positive electrode of this embodiment can more significantly obtain the above effect. At this time, in general formula (1), c and d satisfy 0.05≦c≦0.31, 0.03≦d≦0.26, which improves the balance between capacity and life characteristics. More preferable from this point of view.

In the positive electrode for a lithium ion secondary battery according to this embodiment, the positive electrode active material has a crystallite diameter of X (nm) calculated by the Williamson-Hall method and an average particle diameter (D50) calculated by a laser diffraction method. It is characterized in that it satisfies Y/X≦70 when Y (nm). Here, Y/X is an index of the number of crystal grains constituting the positive electrode active material particles, and the smaller the value, the fewer the crystal grains constituting the particles. That is, it is considered that the smaller the 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 described later. When Y/X exceeds 70, the number ratio of polycrystalline particles in the particles constituting the positive electrode active material increases, and when the density of the positive electrode active material layer is 3.40 g/cm ³ or more, cycle durability decreases. Resulting in. Y/X is more preferably 50 or less, still more preferably 40 or less. On the other hand, the lower limit of Y/X is 1 or more. In addition, when the positive electrode active material consists of a mixture of two or more types, first, the crystallite diameter X (nm) and average particle diameter (D50) of each positive electrode active material constituting the mixture are measured by the above method Then, the crystallite diameter X (nm) and average particle diameter (D50)Y of the mixture are calculated by adding up the obtained values based on the mixing ratio. Then, it is assumed that Y/X is calculated from these calculated values.

Note that the crystallite diameter X of the positive electrode active material is not particularly limited, but from the viewpoint of further improving cycle durability, it is preferably 100 nm or more.

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

As methods for controlling the above Y/X to a value of 70 or less, or controlling the above X to a value of 100 nm or more, patent documents such as Japanese Patent No. 6574222 and Japanese Patent No. 5702289, Solid State Ionics, Reference can be made to methods disclosed in non-patent documents such as Volume 345, February 2020, 115200, 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 roughness factor of the positive electrode active material is preferably 3 or less. Here, the "roughness factor" is a parameter that is an index of the surface smoothness of the positive electrode active material, and is calculated from the "average particle diameter" of the positive electrode active material using the method described in the Examples section below. It is calculated as the value of the ratio of the "BET specific surface area" to the "geometric specific surface area" (=BET specific surface area/geometric specific surface area). 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, so cracks occur even when the density of the positive electrode active material layer is increased. Hateful. 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. Note that the roughness factor value is preferably 2.7 or less, more preferably 2.4 or less. On the other hand, the lower limit of the roughness factor is 1 or more. In addition, when the positive electrode active material consists of a mixture of two or more types, first, the BET specific surface area and geometric specific surface area of each positive electrode active material constituting the mixture are measured by the above method, and the obtained values are The BET specific surface area and geometric specific surface area of the mixture are calculated by adding the mixture ratios. Then, the roughness factor is calculated from these calculated values using the same method as above.

There is no particular restriction on the method of controlling the roughness factor value of the positive electrode active material to 3 or less, and conventionally known knowledge that can control the surface smoothness of the positive electrode active material particles can be appropriately referred to. For example, as an example of a method for manufacturing an NMC composite oxide, first, (1) a precursor ( _Nix Co _y Mn _z )(OH) ₂ is synthesized by a coprecipitation method. Specifically, an aqueous solution containing raw materials such as nickel sulfate, cobalt sulfate, manganese sulfate, sodium hydroxide, and ammonium hydroxide in a desired stoichiometric ratio is stirred for a predetermined period of time. After the precipitate is separated by filtration, it is dried at a temperature of, for example, about 80° C. for about 8 to 15 hours to obtain a precursor. Next, (2) the obtained precursor is fired at a temperature of about 600 to 800° C. for about 3 to 7 hours to obtain Ni _x Co _y Mn _z O ₂ . Then, (3) lithium salt (for example, lithium carbonate) is added to the obtained oxide at a stoichiometric ratio of 1 to 1.2 times, and the mixture is ground and mixed in a ball mill. (4) After baking in the air or in an oxygen atmosphere at a temperature of about 600 to 800°C for about 3 to 5 hours, it is further baked at a temperature of about 750 to 1000°C for about 2 to 12 hours. Finally, (5) the thus calcined powder is washed with water to remove the remaining lithium salt, and then dried at a temperature of about 80°C for about 10 to 15 hours to obtain an NMC composite oxide. can. (6) If necessary, after pulverizing the sample with a ball mill, it may be re-fired at 500 to 1000° C. for 3 to 5 hours in the air or in an oxygen atmosphere. At this time, in order to control the roughness factor value of the NMC composite oxide, (2) firing temperature and time, (3) degree of pulverization, (4) firing time and temperature, and (6) pulverization are required. By adjusting the degree of oxidation, the size of the primary particles can be controlled. Moreover, by performing the re-firing in (6) for a long time and at high temperature, the surface after pulverization becomes smoother and the roughness factor can be reduced.

In the positive electrode for a lithium ion secondary battery according to the present embodiment, in addition to the positive electrode active material consisting of the space group R3m described above, the (104) plane diffraction peak obtained by X-ray diffraction measurement of the positive electrode active material is It is preferable that the peak intensity ratio ((003)/(104)) of the diffraction peak of the (003) plane is 1.35 or more. The peak intensity ratio ((003)/(104)) is more preferably 1.4 or more, still more preferably 1.45 or more, particularly preferably 1.48 or more. 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 degree of crystallinity of the positive electrode active material, and the larger the value of the peak intensity ratio, the higher the crystallinity. When the peak intensity ratio is within the above range, defects in the crystal will be reduced, and a decrease in battery charge/discharge capacity and durability can be suppressed. The peak intensity ratio can be controlled by raw materials, composition, firing conditions, and the like. Note that the crystal structure and peak intensity ratio of the above-mentioned positive electrode active material are determined by the measurement methods described in Examples below, respectively.

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 1.8 g/cm 3 or less. It is more preferable that it is below ^cm3 . The lower limit of the tap density is not particularly limited, but is preferably 1.2 g/cm ³ or more. The smaller the value of the tap density, the smaller the volume of the space between the uneven portions on the particle surface and the volume of the gaps between the particles. When the tap density is within the above range, the contact area between particles of the positive electrode active material can be further increased. As a result, the electrode reaction proceeds more efficiently, making it possible to further improve cycle durability.

The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but is preferably within the range of 60 to 99% by mass, for example, 80 to 99% by mass, based on 100% by mass of the total solid content of the positive electrode active material layer. More preferably, it is within the range of 98% by mass.

(Conductivity aid)
The conductive aid has a function of forming an electron conduction path (conductive path) in the positive electrode active material layer. When such electron conduction paths are 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.

As conductive aids, particulate carbon materials such as acetylene black, carbon black, channel black, thermal black, Ketjen black (registered trademark), carbon nanotubes (single-walled carbon nanotubes and multi-walled carbon nanotubes), and carbon nanofibers are used. , vapor-grown carbon fibers, electrospun carbon fibers, polyacrylonitrile carbon fibers, pitch carbon fibers, and other fibrous carbon materials. One type of conductive aid may be used alone, or two or more types may be used in combination.

The content of the conductive support agent that can be contained in the positive electrode active material layer (if two or more types are included, the total amount) is not particularly limited, but is 0.5% based on 100% by mass of the total solid content of the positive electrode active material layer. It is preferably 5 to 10% by weight, more preferably 1 to 5% by weight. When the content of the conductive additive is 0.5% by mass or more, aggregation of the conductive additives is suppressed, thereby forming a good electron conduction path, thereby further improving output characteristics. Moreover, it becomes possible to further improve the energy density of the lithium ion secondary battery. Furthermore, when the amount is 10% by mass or less, there is sufficient conductive agent to form an electron conduction path, so that the output characteristics can be further improved.

The optional binder used in the positive electrode active material layer is not particularly limited, but examples include the following materials:
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 hydrogenated products thereof , thermoplastic polymers such as styrene-isoprene-styrene block copolymers and their hydrogenated products, tetrafluoroethylene-hexafluoropropylene copolymers (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymers (PFA) , fluororesins such as ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF), 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-perfluoromethyl vinyl ether-tetrafluoroethylene-based fluororubber ( Examples include vinylidene fluoride-based fluororubbers such as VDF-PFMVE-TFE-based fluororubber), 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 preferred.

The content of the binder that can be contained in the positive electrode active material layer (the total amount if two or more types are included) is not particularly limited, but is 0.5 to 10% by mass based on the total solid content of the positive electrode active material layer. It is preferably 1 to 5% by mass, and more preferably 1 to 5% by mass.

The positive electrode (positive electrode active material layer) is not particularly limited, but can be formed by a conventional slurry coating method.

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 or more and less than 3.80 g/cm ³ . If the density of the positive electrode active material layer is less than 3.40 g/cm ³ , high cycle durability cannot be obtained because it is difficult to ensure a contact area between particles in a single crystal. Furthermore, it is not possible to obtain a battery with sufficient energy density. On the other hand, it is difficult to produce a positive electrode in which the density of the positive electrode active material layer is 3.80 g/cm ³ or more. The density of the positive electrode active material layer is preferably 3.40 to 3.70 ^g /cm 3 , and 3.40 to 3.68 g/cm ³ from the viewpoint of achieving a better balance between cycle durability and energy density. It is more preferably 3.45 to 3.65 g/cm ³ , even more preferably 3.50 to 3.60 g/cm ³ . In addition, in this specification, the density of the positive electrode active material layer shall be 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 ).

Note that the solid material mass is calculated by adding up only the mass of the solid material 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 of the positive electrode active material layer and the coating area.

The density of the positive electrode active material layer can be determined, for example, by adjusting the ratio of the positive electrode active material in the solid material constituting the positive electrode active material layer, by adjusting the coating amount of the positive electrode slurry containing the positive electrode active material, or by controlling the pore density by adjusting the pressing conditions, etc. It can be adjusted by adjusting the rate.

The thickness of the positive electrode active material layer is not particularly limited, and conventionally known knowledge regarding batteries can be appropriately referred to. 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 greater the thickness of the positive electrode active material layer, the more the positive electrode active material can be retained to exhibit sufficient capacity (energy density). On the other hand, the smaller the thickness of the positive electrode active material layer, the better the discharge rate characteristics can be. In the positive electrode of this embodiment, the thickness of the positive electrode active material layer can be adjusted as appropriate to achieve a desired density of the positive electrode active material layer.

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%, still more preferably 18 to 24%, and even more preferably It is 19 to 24%, 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 obtained even more markedly. In addition, sufficient contact between the electron conductive materials (conductivity aid, positive electrode active material, etc.) in the positive electrode active material layer can be maintained, and an increase in electron transfer resistance can be prevented. Furthermore, since a sufficient amount of electrolyte exists between the positive electrode active material particles, an increase in lithium ion transfer resistance can be prevented. As a result, it becomes possible to further improve the output characteristics of the lithium ion secondary battery. On the other hand, if it 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 significantly obtained. Additionally, the battery has excellent energy density.

By applying the positive electrode for a lithium ion secondary battery according to the present 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 aspect of the present invention, there is provided a lithium ion secondary battery including a power generation element having the positive electrode for a lithium ion secondary battery according to one aspect of the present invention described above. Components other than the positive electrode of the lithium ion secondary battery will be briefly explained below.

[Negative electrode active material layer]
(Negative electrode active material)
The negative electrode active material has a function of releasing ions such as lithium ions during discharging and storing ions such as lithium ions during charging.

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

The average particle diameter (D ₅₀ ) of the negative electrode active material is not particularly limited, but from the viewpoint of increasing 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 and less than 100% by mass, preferably 80% by mass or more and 99.5% or less, based on 100% by mass of the total solid content. It is preferably more than 95% by mass and less than 99.0% by mass, and more preferably more than 97% by mass and less than 98.5% by mass. When the content of the negative electrode active material is within the above range, both battery capacity and output characteristics can be achieved.

Further, the negative electrode active material layer further includes the same conductive additive, binder, etc. as described above for the positive electrode active material layer, if necessary.

The thickness of the negative electrode active material layer is not particularly limited, and conventional knowledge regarding batteries may be appropriately referred to. 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 larger the thickness of the negative electrode active material layer, the more the negative electrode active material can be retained to exhibit sufficient capacity (energy density). On the other hand, the smaller the thickness of the negative electrode active material layer, the better the discharge rate characteristics can be.

The negative electrode (negative electrode active material layer) can be formed by a conventional slurry coating method.

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

(electrolyte)
The electrolytic solution functions as a lithium ion carrier. The electrolytic solution has a form in which a lithium salt is dissolved in a non-aqueous solvent.

Non-aqueous solvents include, for example, dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propionate (MP), methyl acetate (MA), and 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, the non-aqueous solvent is preferably a linear carbonate, more preferably diethyl carbonate (DEC), ethylmethyl carbonate (EMC), or dimethyl carbonate ( DMC), more preferably selected from ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC).

Lithium salts include Li( _FSO2 ) _2N (lithium bis( _{fluorosulfonyl} )imide; LiFSI), Li( _C2F5SO2 ) _2N , _LiPF6 , _LiBF4 , _{LiClO4 , LiAsF6} , _LiCF3. Examples include _SO3 .

The concentration of the lithium salt in the electrolytic solution 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 storage side becomes sufficiently high, so that the rate characteristics of the battery can be improved and the cycle durability can be further improved. On the other hand, when it is less than 3.0 mol/L, the concentration gradient of lithium ions in the electrode film thickness direction can be appropriately controlled even in high-density electrodes, so the rate characteristics of the battery are improved and the cycle durability is improved. It can be improved. In one embodiment of the present invention, the concentration of lithium salt in the electrolyte is determined such that the lithium ion concentration is lower than the saturated lithium ion concentration of the electrolyte at 25°C from the viewpoint of ensuring transportability of lithium ions in a high-density electrode. Preferably, the concentration is less than 50%.

The electrolytic solution 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- Divinylethylene carbonate, 1-methyl-1-vinylethylene carbonate, 1-methyl-2-vinylethylene carbonate, 1-ethyl-1-vinylethylene carbonate, 1-ethyl-2-vinylethylene carbonate, vinyl vinylene carbonate, allyl ethylene Carbonate, vinyloxymethylethylene carbonate, allyloxymethylethylene carbonate, acryloxymethylethylene carbonate, methacryloxymethylethylene carbonate, ethynylethylene carbonate, propargylethylene carbonate, ethynyloxymethylethylene carbonate, propargyloxyethylene carbonate, methylene ethylene carbonate, 1 , 1-dimethyl-2-methylene ethylene carbonate and the like. These additives may be used alone or in combination of two or more. Furthermore, the amount of additive used in the electrolytic solution can be adjusted as appropriate.

(Separator)
The separator constituting the electrolyte layer has the function of holding the electrolyte and ensuring lithium ion conductivity between the positive electrode and the negative electrode, and the function of functioning as a partition between the positive electrode and the negative electrode.

Examples of the form of the separator include a porous sheet separator made of a polymer or fiber that absorbs and retains the electrolytic solution, a nonwoven fabric separator, and the like.

As the porous sheet separator made of polymer or fiber, for example, microporous material (microporous membrane) can be used. Specific forms of the porous sheet made of the polymer or fibers include, for example, polyolefins such as polyethylene (PE) and polypropylene (PP); Examples include microporous (microporous membrane) separators made of polyimide, aramid, hydrocarbon resins such as polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), glass fiber, etc.

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

As the nonwoven fabric separator, conventionally known materials such as cotton, rayon, acetate, nylon, polyester; polyolefins such as PP and PE; polyimide and aramid may be used alone or in combination. Further, the bulk density of the nonwoven fabric is not particularly limited as long as sufficient battery characteristics can be obtained by the impregnated polymer gel electrolyte. Furthermore, the thickness of the nonwoven fabric separator may be the same as that of the electrolyte layer, preferably 5 to 200 μm, 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%.

Further, 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 has a melting point or thermal softening point of 150° C. or higher, preferably 200° C. or higher, and has high heat resistance. By having a heat-resistant insulating layer, the internal stress of the separator that increases when the temperature rises is alleviated, so that an effect of suppressing thermal shrinkage can be obtained. As a result, it is possible to prevent short-circuiting between the electrodes of the battery, resulting in a battery configuration in which performance deterioration due to temperature rise is less likely to occur. Moreover, by having a heat-resistant insulating layer, the mechanical strength of the separator with a heat-resistant insulating layer is improved, and membrane rupture of the separator is less likely to occur. Furthermore, due to its heat shrinkage suppressing effect and high mechanical strength, the separator is less likely to curl during the battery manufacturing process.

[Positive current collector plate and negative 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. As the constituent material of the current collector plate, for example, metal materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferable. From the viewpoints of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. Note that the positive electrode current collector plate 25 and the negative electrode current collector plate 27 may use the same material or different materials.

[Positive lead and negative lead]
Further, although not shown, the current collector 11 and the current collecting plates (25, 27) may be electrically connected via a positive electrode lead or a negative electrode lead. As constituent materials for the positive electrode and negative electrode leads, materials used in known lithium ion secondary batteries can be similarly employed. In addition, the parts removed from the exterior body should be covered with heat-resistant insulating material to prevent them from coming into contact with peripheral devices or wiring, causing electrical leakage, and affecting products (e.g., automobile parts, especially electronic equipment, etc.). It is preferable to cover with a shrink tube or the like.

[Seal part]
The seal portion (insulating layer) 31 is a member unique to bipolar secondary batteries (series stacked batteries), and has a function of preventing leakage of electrolyte from the electrolyte layer. Further, the seal portion 31 has a function of preventing contact between current collectors and short circuit at the end of the cell layer. The material constituting the seal part must have insulation properties, sealing properties against falling of the solid electrolyte, sealing properties against permeation of moisture from the outside (sealing properties), and heat resistance under the battery operating temperature. good. For example, acrylic resin, urethane resin, epoxy resin, polyethylene resin, polypropylene resin, polyimide resin, rubber (ethylene-propylene-diene rubber: EPDM), etc. can be used. Further, isocyanate adhesives, acrylic resin adhesives, cyanoacrylate adhesives, etc. may be used, and hot melt adhesives (urethane resins, polyamide resins, polyolefin resins), etc. may also be used. Among these, polyethylene resin and polypropylene resin are preferably used as constituent materials for the sealing part from the viewpoints of corrosion resistance, chemical resistance, ease of production (film formability), economic efficiency, etc. It is more preferable to use a resin obtained by copolymerizing ethylene, propylene, and butene.

[Battery exterior body]
As the battery exterior body, a well-known metal can case can be used, or a bag-shaped case using a laminate film 29 containing aluminum that can cover the power generation element as shown in FIGS. 1 and 2 can be used. It can be done. The laminate film may be, for example, a laminate film having a three-layer structure in which PP, aluminum, and nylon are laminated in this order, but is not limited thereto. A laminate film is desirable from the viewpoint that it has high output and excellent cooling performance, and can be suitably used in batteries for large equipment such as EVs and HEVs. Moreover, since the group pressure applied to the power generation element from the outside can be easily adjusted, the exterior body is more preferably made of aluminate laminate.

The lithium ion secondary battery according to this embodiment can have both excellent cycle durability and high energy density. Therefore, the lithium ion secondary battery according to this embodiment is suitably used as a power source for driving EVs and HEVs.

[Assembled battery]
A battery pack is constructed by connecting a plurality of batteries. Specifically, it is configured by using at least two or more, serially or parallelly, or both. By connecting them in series or parallel, it becomes possible to freely adjust the capacity and voltage.

A plurality of batteries can be connected in series or in parallel to form a small detachable battery pack. By connecting multiple small, removable battery packs in series or in parallel, high-capacity, high-capacity power supplies suitable for vehicle drive power sources and auxiliary power sources that require high volumetric energy density and high volumetric power density can be created. It is also possible to form an assembled battery (battery module, battery pack, etc.) that has an output. How many batteries to connect to make a battery pack, and how many stages of small battery packs to stack to make a large-capacity battery pack depends on the battery capacity of the vehicle (electric vehicle) in which it will be mounted. You can decide according to the output.

[vehicle]
A battery or a battery pack made by combining a plurality of batteries can be mounted on a vehicle. In the present invention, it is possible to configure a long-life battery with excellent long-term reliability, so when such a battery is installed, a plug-in hybrid electric vehicle with a long EV mileage or an electric vehicle with a long mileage per charge can be configured. Batteries or assembled batteries made by combining multiple of these can be used, for example, in the case of automobiles, such as hybrid vehicles, fuel cell vehicles, electric vehicles (all four-wheeled vehicles (passenger cars, commercial vehicles such as trucks, buses, light vehicles, etc.), etc. , two-wheeled vehicles (including motorcycles, and three-wheeled vehicles)) can provide long-life and highly reliable vehicles. However, the application is not limited to automobiles; for example, it can be applied to various power sources for other vehicles, such as trains, and on-board power sources such as uninterruptible power supplies. It is also possible to use it as

EXAMPLES Hereinafter, the present invention will be explained in more detail 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 products were prepared as positive electrode active materials.

Single crystal NMC composite oxide 1 (single crystal 1) (manufactured by Easpring, ME88SC, LiNi _0.86 Mn _0.08 Co _0.06 O ₂ , roughness factor: 2.4, crystallite diameter (X): 110 nm, average Particle diameter (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 ).

Polycrystalline NMC composite oxide 1 (polycrystalline 1) (manufactured by Ecopro, NMC811, LiNi _0.8 Mn _0.1 Co _0.1 O ₂ , roughness factor: 16.5, crystallite diameter (X): 80 nm, average Particle diameter (D50) (Y): 11500 nm, Y/X: 144, crystal structure: space group R3m, peak intensity ratio (003)/(104): 1.29, tap density: 2.68 g/cm ³ ).

Note that the physical property values of each positive electrode active material were measured by the following method.

[Y/X]
(Crystallite diameter X)
The crystallite diameter , 741 (1949)). For the measurement, an X-ray diffraction (XRD) device (manufactured by Rigaku Co., Ltd.) using CuKα radiation was used. According to the Williamson-Hall method, the following relationship holds between θ of a diffraction peak obtained by X-ray diffraction measurement of a positive electrode active material and its integral value β.

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

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

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

Next, from the value of the average particle diameter calculated above, the particle volume [m ³ ] and particle surface area [m ² ] of a true spherical shape with the average particle diameter as the particle diameter are calculated, and the particle volume obtained is The true spherical particle mass [g] was calculated by multiplying by the specific gravity of the substance (here, 4.78 [g/cm ³ ]). Then, the "geometric specific surface area [m ² /g] calculated from the average particle diameter" was calculated by dividing the particle surface area calculated above by the particle mass also calculated above.

On the other hand, according to the "Method for measuring the specific surface area of powder (solid) by gas adsorption" described in JIS Z8830:2013 (ISO 9277:2010), measurements were carried out using nitrogen gas as an adsorbed gas by the static capacitance method. BET specific surface area [m ² /g] was calculated by analysis using the point method. Then, the roughness factor was calculated as the value of the ratio of the "BET specific surface area" to the "geometric specific surface area calculated from the average particle diameter" calculated above (=BET specific surface area/geometric specific surface area).

[Crystal structure, peak intensity ratio (003)/(104)]
For powder X-ray diffraction measurements to calculate the crystal structure and peak intensity ratio (I(003)/I(104)) of the positive electrode active material, an X-ray diffractometer (manufactured by Rigaku Co., Ltd.) using CuKα rays was used. The analysis was conducted using the Fundamental Parameter. Using the X-ray diffraction pattern obtained from the diffraction angle 2θ=15 to 120°, analysis was performed using analysis software Topas Version 3.

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

<Preparation of positive electrode>
[Example 1]
A solid content consisting of 95 parts by mass of the above single crystal NMC composite oxide 1 (single crystal 1) as a positive electrode active material, 3 parts by mass of acetylene black (AB) as a conductive aid, and 2 parts by mass of polyvinylidene fluoride (PVDF) as a binder. On the other hand, a positive electrode slurry was obtained by mixing N-methyl-2-pyrrolidone (NMP) as a solvent. At this time, a solution in which PVDF was dispersed at 7% by mass in NMP was prepared in advance, added and mixed, and the solid content concentration was also adjusted using NMP. The obtained positive electrode slurry was coated onto a 20 μm thick aluminum foil using a doctor blade and a coating machine (manufactured by Tester Sangyo) with a gap adjusted so that the basis weight (coating amount on one side) was 20 mg/cm ² . The coating film was dried on a hot plate adjusted to 90° C. for 1 hour. The dried coating film was pressed using a tabletop small roll press machine (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.

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

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

<Production of lithium ion secondary battery>
The positive electrode prepared above and the Li metal serving as the negative electrode were opposed to each other, and a glass fiber separator (thickness: 200 μm) was placed between them. Next, a laminate of a positive electrode, a separator, and a negative electrode (Li metal) was placed on the bottom side of a coin cell (CR2025). Furthermore, a gasket is attached to maintain insulation between the electrodes, a 150 μm electrolyte is injected with a pipette, a spring and a spacer are stacked, the upper sides of the coin cells are overlapped, and the coin cell is sealed by caulking. A secondary battery (coin cell) was manufactured. The electrolytic solution is an electrolytic solution obtained by dissolving 1 mol/L of LiPF ₆ in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio 3:7) (Kishida Chemical Co., Ltd.). (manufactured by) was used. Furthermore, all cell preparations were performed in a glove box (dew point -60°C) in a dry room.

[Measurement of cycle durability]
Under the condition of 25°C, 1C = 230mAh/g, the first and second charge and discharge are performed in the voltage range of 2.5 to 4.3V in charging CC-CV mode and discharging CC mode (rest time between charging and discharging). 8 hours). The charge/discharge rate was 0.05C only for the first charge, and 0.1C for the rest. In the cycle test, charging/discharging was performed at charge/discharge = 0.33C/0.33C, and a check-up cycle was performed at charge/discharge = 0.1C/0.1C every 50 cycles. Then, the ratio of the discharge capacity at the 100th cycle to the discharge capacity at the 1st cycle was calculated as the capacity retention rate [%].

The electrode energy density was determined using the following formula for the first cycle of discharge.

In the above formula, the positive electrode capacity density (Ah/m ² ) is determined by the capacity of the positive electrode active material (Ah/g)×the amount of active material supported per unit area of the positive electrode (g/m ² ). The average voltage (V) was determined by determining the total energy (mWh) from the area of the discharge curve (horizontal axis capacity, vertical axis voltage curve) and dividing this by the obtained 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).

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

From the results shown in Table 1 and FIG. 3, it can be seen that when the positive electrodes of Examples 1 to 4, which are the positive electrodes for lithium ion secondary batteries of the present invention, are used, a battery excellent in both capacity retention and electrode energy density can be obtained. Understood.

On the other hand, when a polycrystalline positive electrode active material is used, if the density of the positive electrode active material layer is less than 3.40 g/cm ³ as in Comparative Examples 1 to 3, the thickness of the positive electrode active material layer becomes thick. Due to this, sufficient electrode energy density cannot be achieved. On the other hand, it was found that when the density of the positive electrode active material layer was set to 3.40 g/cm ^{3 or} more as in Comparative Examples 4 to 7, the cycle durability decreased. Furthermore, it was confirmed that even if a single-crystal cathode active material was used, sufficient electrode energy density could not be achieved if the density of the cathode active material layer was less than 3.40 g/ ^cm3 . .

10a stacked secondary battery,
10b bipolar secondary battery,
11 current collector,
11a outermost layer current collector on the positive electrode side,
11b outermost layer 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 element,
23 Bipolar electrode,
25 Positive electrode current collector plate (positive electrode tab),
27 Negative electrode current collector plate (negative electrode tab),
29 Laminating film,
31 Seal part.

Claims (8)

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≦, where X (nm) is the crystallite diameter calculated by the Williamson-Hall method, and Y (nm) is the average particle diameter (D50) calculated by the laser diffraction method. meet 70,
A positive electrode for a lithium ion secondary battery, wherein 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 for a lithium ion secondary battery according to claim 1, wherein the density of the positive electrode active material layer is 3.45 g/cm ³ or more and 3.65 g/cm ³ or less. The lithium ion secondary according to claim 1 or 2, wherein the positive electrode active material has a roughness factor value 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. Positive electrode for batteries. The positive electrode active material consists of a space group of R3m, and 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 1.35. The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 3, which is the above. The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the positive electrode active material has a tap density of 2 g/cm ³ or less. 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. In the lithium-nickel-manganese-cobalt composite oxide having a layered structure, the content ratio of nickel is in the range of 80 atomic % or more and less than 90 atomic % in terms of composition ratio with respect to the total amount of metal atoms other than lithium. The positive electrode for a lithium ion secondary battery according to claim 6. A lithium ion secondary battery comprising a power generation element having the positive electrode for a lithium ion secondary battery according to any one of claims 1 to 7.