Positive electrode active material for batteries, positive electrode, battery, and manufacturing method of positive electrode active material

Abstract

To provide a positive electrode active material for batteries capable of suppressing the capacity loss in the battery when being used in a battery.SOLUTION: The positive electrode active material for batteries comprises a positive electrode active material in which multiple primary particles are aggregated to form secondary particles. At least some of the primary particles have depressions on their surfaces. Defining that the primary particles in which the ratio d / D of the depth of the depression d to the particle size D of the primary particles is 0.010≤d / D≤0.890 as primary particles E, the number ratio of particle E to all primary particles is 1.0% or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a cathode active material for a battery, a cathode, a battery, and a method for manufacturing the cathode active material. [Background technology]

[0002] Conventionally, a positive electrode active material in which a plurality of primary particles are aggregated to form secondary particles has been used as a positive electrode active material for batteries having excellent resistance characteristics. For example, Patent Document 1 discloses a positive electrode active material having the chemical formula LiNi x Co y Mn z A large grain aggregate ternary cathode material comprising a nickel cobalt manganese lithium compound, O2, where x+y+z≦1, 0.2≦x≦0.8, 0.1≦y≦0.3, and having an average particle size D50 of 1 μm≦D50≦2.5 μm, is disclosed. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2023-36570 Summary of the Invention [Problem to be solved by the invention]

[0004] Conventional positive electrode active materials have secondary particles formed by aggregation of highly circular primary particles. When this positive electrode active material is charged and discharged, the primary particles expand and contract. During the contraction, the primary particles may become isolated, resulting in the presence of particles that cannot maintain the state of the aggregated secondary particles. The presence of particles that cannot maintain the state of the secondary particles may result in a decrease in capacity in batteries containing the positive electrode active material.

[0005] The present disclosure has been made in view of the above circumstances, and aims to provide a positive electrode active material for a battery that suppresses a decrease in capacity of the battery when used in the battery, a positive electrode including the positive electrode active material, and a battery. [Means for solving the problem]

[0006] Means for solving the above problems include the following aspects. <1> A positive electrode active material in which a plurality of primary particles are aggregated to form secondary particles, At least some of the primary particles have depressions on their surfaces, When the primary particles having a ratio d / D of the depth d of the depression to the particle diameter D of the primary particle are defined as particles E, the proportion of the number of particles E to all primary particles is 1.0% or more. Positive electrode active material for batteries. <2> The positive electrode active material contains elements of Li, Ni, Co, Mn, and O, and the composition of the elements is Li x Ni a Co b Mn c O y is expressed as <1> The positive electrode active material according to claim 1. (In the above composition, 0.1≦x≦1.5, 0.5≦a≦1.0, 0≦b≦0.3, 0≦c≦0.3, a+b+c=1.0, and 1.5≦y≦2.1.) <3> <1> or <2> A positive electrode comprising the positive electrode active material according to claim 1. <4> <3> A battery comprising the positive electrode described in claim 1. <5> a compression step of compressing particles of the positive electrode active material; a crushing step of subjecting the particles after the compression step to a crushing treatment; The method for producing a positive electrode active material for a battery, comprising: [Effects of the Invention]

[0007] According to the present disclosure, there are provided a positive electrode active material for a battery that, when used in a battery, suppresses a decrease in capacity of the battery, a positive electrode including the positive electrode active material, and a battery. [Brief explanation of the drawings]

[0008] [Figure 1]FIG. 1A is a schematic diagram showing a secondary particle in a positive electrode active material according to an embodiment of the present disclosure, and FIG. 1B is a schematic diagram showing a state after charging and discharging the secondary particle of FIG. [Figure 2] FIG. 2 is a schematic cross-sectional view showing a primary particle having a depression (i.e., particle E) contained in a positive electrode active material according to an embodiment of the present disclosure. [Figure 3] 1 is a schematic diagram illustrating a method for producing a positive electrode active material according to an embodiment of the present disclosure. FIG. [Figure 4] FIG. 1A is a schematic diagram showing the secondary particles in a conventional positive electrode active material, and FIG. 1B is a schematic diagram showing the secondary particles of FIG. 1A after charging and discharging. [Figure 5] FIG. 1 is a schematic cross-sectional view showing the structure of a battery according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0009] <Positive electrode active material for batteries> A positive electrode active material for a battery according to an embodiment of the present disclosure (hereinafter also simply referred to as "the positive electrode active material of the present disclosure") is a positive electrode active material in which a plurality of primary particles are aggregated to form secondary particles. In the positive electrode active material of the present disclosure, at least some of the primary particles have depressions on their surfaces. When the primary particles having depressions on their surfaces have a ratio d / D of the depression depth d to the primary particle diameter D in the range of 0.010≦d / D≦0.890, which is designated as particles E, the proportion of particles E to all primary particles in the positive electrode active material of the present disclosure is 1.0% or more.

[0010] Here, we first describe a conventional cathode active material having only primary particles with a d / D ratio in the range of 0.010 or greater, i.e., a cathode active material having only highly circular primary particles without particles E having a predetermined depression. As shown in FIG. 4(A), secondary particles 21B are formed by agglomeration of highly circular primary particles 19, resulting in many points of contact between the primary particles 19 and a small contact area. When secondary particles 21B of this cathode active material are charged and discharged, the primary particles 19 expand and contract. As shown in FIG. 4(B), the contraction can result in isolated primary particles 19, resulting in particles that cannot maintain the state of agglomerated secondary particles 21B. As a result, a decrease in capacity can occur in batteries containing this cathode active material.

[0011] In contrast, the positive electrode active material of the present disclosure contains primary particles (particles E) with a ratio d / D of 0.010≦d / D≦0.890, with a proportion of 1.0% or more of the total number of primary particles. That is, the positive electrode active material contains 1.0% or more primary particles (particles E) with depressions whose depth d is 0.010 or more relative to the particle diameter D of the primary particles. Therefore, as shown in FIG. 1(A), for example, the aggregated secondary particles 21A include primary particles 11A with predetermined depressions (i.e., corresponding to particles E) and primary particles 121A and 122A without predetermined depressions. Primary particles 121A without predetermined depressions fit into the depressions of primary particles 11A, increasing the number of contact points for surface contact and increasing the contact area. Furthermore, if the depressions of primary particles 11A have sharp edges, primary particles 122A without predetermined depressions may bite into these sharp edges. Even if secondary particles 21A of this positive electrode active material are charged and discharged, causing expansion and contraction of primary particles 11A, 121A, and 122A, isolation of primary particles 11A, 121A, and 122A is suppressed as shown in FIG. 1(B). In other words, a state is created in which many particles remain in the form of aggregated secondary particles 21B. As a result, a decrease in capacity of a battery containing the positive electrode active material is suppressed.

[0012] As a result, the positive electrode active material of the present disclosure suppresses a decrease in the capacity of the battery.

[0013] Next, the positive electrode active material of the present disclosure will be described in more detail.

[0014] Number ratio of particles E In the positive electrode active material of the present disclosure, at least some of the primary particles have depressions on their surfaces. When primary particles E are those having a ratio d / D of the depression depth d to the primary particle diameter D in the range of 0.010≦d / D≦0.890, the number ratio of particles E to all primary particles is 1.0% or more. When the number ratio of particles E is 1.0% or more, the primary particles are prevented from becoming isolated, and the state of aggregated secondary particles is maintained even after charging and discharging, thereby preventing a decrease in capacity in a battery containing the positive electrode active material. The ratio of the number of particles E to all primary particles is more preferably 1.4% or more from the viewpoint of suppressing a capacity decrease in a battery containing the positive electrode active material. The upper limit of the ratio of the number of particles E to all primary particles is not particularly limited, but may be, for example, 80.0% or less, or 70.0% or less.

[0015] The particle E contained in the positive electrode active material of the present disclosure has a ratio d / D of the depth d to the particle diameter D in the range of 0.010≦d / D≦0.890. The upper limit of the ratio d / D is not particularly limited, but may be, for example, 0.800 or less, or 0.785 or less.

[0016] The depressions of particle E will be explained using FIG. 2. FIG. 2 is a schematic cross-sectional view showing a primary particle 110 included in the positive electrode active material of the present disclosure, the primary particle 110 having depressions 113 on its surface. The "depression" refers to a portion having a shape that is hollowed out more inward of the particle than line X when a line X is drawn connecting edge 111 and edge 112 in the cross section. The "depth d of the depression" refers to the distance from line X to point Y on the outline of depression 113 that is the farthest in the vertical direction. The "particle diameter D of the primary particle" refers to the maximum length of the outline of primary particle 110 having depression 113, that is, the linear distance of the longest part between two points on the outline of primary particle 110.

[0017] 2 shows primary particle 110 having only one large depression, depression 113, but the present invention is not limited to this. The primary particles contained in the positive electrode active material of the present disclosure may have multiple depressions.

[0018] (Calculation method for the number ratio of particle E) The ratio of the number of particles E to the total number of primary particles in the positive electrode active material is calculated by the following method. First, the method for measuring the depth d of the depressions and the particle size D of the primary particles will be described. First, an image of the cross section of the positive electrode active material layer is obtained using a scanning electron microscope (SEM, 5000 to 20000 magnifications). In the obtained cross-sectional image, primary particles in which the entire particle outline can be confirmed are used as the measurement target. For depressions on the surface of the primary particle to be measured, a line X connecting the edges (in FIG. 2, a line X connecting edge 111 and edge 112) is drawn. The depth d is determined by measuring the distance from line X to the point on the outline of the depression farthest in the vertical direction (point Y in FIG. 2). Note that if the primary particle to be measured has multiple depressions on its surface, the longest depth of the multiple depressions is taken as the depth d of the depression of the primary particle. The particle size D of the primary particle is determined by measuring the maximum length of the outline of the primary particle to be measured, that is, the linear distance between two points on the outline of the primary particle. Then, the ratio d / D is calculated from the depth d of the depressions and the particle diameter D of the primary particles that are measured.

[0019] This ratio d / D is calculated for 1,000 primary particles. The number of particles E whose ratio d / D falls within the range of 0.010≦d / D≦0.890 is calculated to calculate the ratio of the number of particles E to all primary particles (i.e., 1,000 primary particles).

[0020] ·composition The positive electrode active material of the present disclosure is composed of a plurality of primary particles agglomerated to form secondary particles. The positive electrode active material preferably contains, for example, Li, Ni, Co, Mn, and O, and the composition of these elements is Li. x Ni a Cob Mn c O y It is preferable that the composition be expressed by the following formula: (In the above composition, 0.1≦x≦1.5, 0.5≦a≦1.0, 0≦b≦0.3, 0≦c≦0.3, a+b+c=1.0, and 1.5≦y≦2.1.)

[0021] In the above composition, the Li ratio x is 0.1 or more and 1.5 or less, preferably 0.3 or more and 1.4 or less, and more preferably 0.5 or more and 1.2 or less. The ratio a of Ni is 0.5 or more and 1.0 or less, preferably 0.6 or more and 0.9 or less, and more preferably 0.7 or more and 0.8 or less. The ratio b of Co is 0 or more and 0.3 or less, preferably 0 or more and 0.2 or less, and more preferably 0.1 or more and 0.2 or less. The ratio c of Mn is 0 or more and 0.3 or less, preferably 0 or more and 0.2 or less, and more preferably 0.1 or more and 0.2 or less. The total ratio of Ni, Co, Mn, and M (a+b+c+d) is 1.0. The ratio y of O is 1.5 or more and 2.1 or less, preferably 1.7 or more and 2.1 or less, and more preferably 1.9 or more and 2.0 or less.

[0022] The positive electrode active material of the present disclosure may further contain an additive element. When the positive electrode active material further contains an additive element, the composition of each element is x1 Ni a1 Co b1 Mn c1 M d1 O y1 It is preferable that the composition be expressed by the following formula: (In the above composition, 0.1≦x1≦1.5, 0.5≦a1≦1.0, 0≦b1≦0.3, 0≦c1≦0.3, 0≦d1≦0.1, a1+b1+c1+d1=1.0, and 1.5≦y1≦2.1.)

[0023] The ratio d1 of M is 0 or more and 0.1 or less, preferably 0.01 or more and 0.09 or less, and more preferably 0.03 or more and 0.07 or less. Examples of the additional element (M in the above composition) include at least one element selected from the group consisting of B, Nb, W, Sr, Pr, La, Ba, Mg, Al, Zr, Sc, Ti, Y, Hf, and Sn.

[0024] <Method of manufacturing positive electrode active material> Next, a method for producing a positive electrode active material for a battery according to an embodiment of the present disclosure (hereinafter also simply referred to as "the method for producing a positive electrode active material according to the present disclosure") will be described. The above-described positive electrode active material of the present disclosure (i.e., a positive electrode active material in which a plurality of primary particles aggregate to form secondary particles, at least some of the primary particles have depressions on their surfaces, and the ratio of primary particles (particles E) in which the ratio d / D of the depression depth d to the primary particle particle diameter D is 0.010≦d / D≦0.890 is 1.0% or more of all the primary particles) can be produced, for example, by the production method for a positive electrode active material of the present disclosure.

[0025] The method for producing a positive electrode active material according to the present disclosure includes the following steps. (1) A compression process in which particles of the positive electrode active material are compressed. (2) A crushing process in which the particles after the compression process are crushed. Each step will be specifically described below.

[0026] (0) Step of preparing particles of positive electrode active material First, the process for preparing particles (primary particles) of the positive electrode active material will be described. The primary particles of the positive electrode active material can be prepared, for example, through the following process.

[0027] First, a solution containing a Ni-containing raw material (e.g., a sulfate such as NiSO4), a Co-containing raw material (e.g., a sulfate such as CoSO4), and a Mn-containing raw material (e.g., a sulfate such as MnSO4) is prepared (raw material dissolution). Next, the solution is added to an alkaline solution to precipitate hydroxides (crystallization). In this crystallization, for example, the alkaline solution containing the precipitated hydroxides is adjusted to a constant pH (e.g., pH 10-12) while the solution and NH3 are added dropwise, thereby precipitating the transition metal hydroxides. Next, the precipitate is collected from the alkaline solution. Examples of methods for collecting the precipitate particles include filtration and washing with water. Next, the precipitate is mixed with a Li-containing raw material (e.g., Li2CO3 and LiOH) to obtain a mixture (Li raw material addition). Next, the mixture is calcined. For example, the mixture can be calcined in a calcination furnace (e.g., a muffle furnace). Calcination conditions can be, for example, a temperature of 800°C to 1100°C in an oxygen atmosphere for 5 to 20 hours. In order to make the mixture have a predetermined particle size, the mixture after firing may be crushed, for example, by crushing with a crusher (for example, a jet mill).

[0028] (1) A compression process in which particles of the positive electrode active material are compressed. In the compression step, the prepared particles (primary particles) of the positive electrode active material are subjected to a compression treatment. For example, as shown in FIG. 3, by subjecting primary particles of a positive electrode active material ("1. Active material particles" in FIG. 3) to a compression treatment, adjacent primary particles compress each other, forming depressions on the surfaces of at least some of the primary particles. That is, primary particles (i.e., particles E) 11A having depressions with a ratio d / D in a predetermined range are formed. This results in a positive electrode active material containing primary particles (particles E) 11A having the predetermined depressions and primary particles 12A without the predetermined depressions ("2. Compression" in FIG. 3).

[0029] An example of a method for carrying out the compression treatment is to press a powder containing primary particles of the positive electrode active material using a press.

[0030] (2) A crushing process in which the particles after the compression process are crushed. In the crushing step, the compressed particles (primary particles) are subjected to a crushing treatment. For example, as shown in FIG. 3, by subjecting compressed primary particles of a positive electrode active material ("2. Compression" in FIG. 3) to a crushing process, a positive electrode active material is obtained in which primary particles (particles E) 11A having predetermined depressions and primary particles 12A not having predetermined depressions exist separately without agglomeration ("3. Crushing" in FIG. 3).

[0031] The crushing treatment may be carried out, for example, by crushing using a crusher (such as a jet mill).

[0032] In this way, the above-mentioned positive electrode active material of the present disclosure (i.e., a positive electrode active material in which a plurality of primary particles aggregate to form secondary particles, at least some of the primary particles have depressions on their surfaces, and the ratio of primary particles (particles E) in number, where d / D is the ratio of the depth d of the depression to the particle diameter D of the primary particles, satisfies 0.010≦d / D≦0.890, to all the primary particles is 1.0% or more) is obtained.

[0033] The positive electrode active material obtained through the compression and crushing processes is used in a positive electrode (more specifically, a positive electrode active material layer) of a battery. That is, as shown in "4. Positive Electrode Active Material Layer" in FIG. 3 , the secondary particles 2A of the positive electrode active material in the positive electrode active material layer include primary particles 11A having a predetermined depression (i.e., corresponding to particle E) and primary particles 121A and 122A not having the predetermined depression. Then, when forming the positive electrode active material layer, a press process is again performed. This press process causes the primary particles 121A not having the predetermined depression to fit into the depressions of the primary particles 11A, increasing the number of points of surface contact and increasing the contact area. Furthermore, if the depressions of the primary particles 11A have sharp edges, the primary particles 122A not having the predetermined depression may bite into these sharp edges. Therefore, in the cathode active material obtained by the method for producing a cathode active material according to the present disclosure, isolation of primary particles is suppressed even after charging and discharging, and a large number of particles remain in the form of aggregated secondary particles, thereby suppressing a decrease in capacity in a battery containing the cathode active material.

[0034] <Positive electrode and battery> A positive electrode according to an embodiment of the present disclosure includes the positive electrode active material of the present disclosure described above. Batteries according to embodiments of the present disclosure include positive electrodes according to embodiments of the present disclosure. The battery includes, for example, a negative electrode, a positive electrode, a separator, and an electrolyte. The battery according to the embodiment of the present disclosure is preferably used, for example, in a liquid battery having a liquid electrolyte. In particular, a liquid battery having a non-aqueous electrolyte is preferable. Furthermore, the battery may be a bipolar battery having a positive electrode active material layer and a negative electrode active material layer on both sides of a current collector that functions as a positive electrode current collector and a negative electrode current collector.

[0035] Here, the configuration of a battery according to an embodiment of the present disclosure will be described using a bipolar secondary battery as an example. In FIG. 1, "top" refers to the upper side of the figure, and "bottom" refers to the lower side of the figure. FIG. 1 is a schematic cross-sectional view illustrating the structure of a secondary battery, showing one energy storage module 11 of the secondary battery. The secondary battery is provided with a laminate in which multiple such energy storage modules 11 and conductive plates (not shown) are alternately arranged.

[0036] The energy storage module 11 is a flat-plate-shaped cell as a whole, and the energy storage module 11 in this embodiment is a bipolar lithium-ion secondary battery. The energy storage module 11 includes an electrode stack formed by stacking a plurality of bipolar electrodes 12, and a plurality of sealing bodies 20 provided on each bipolar electrode 12. The plurality of bipolar electrodes 12 are stacked in the thickness direction (thickness direction in the flat plate shape), and a sealing body 20 is disposed on each bipolar electrode 12.

[0037] The bipolar electrode 12 includes a current collector foil 13, a positive electrode active material layer 14 provided on the lower surface of the current collector foil 13, a negative electrode active material layer 15 provided on the upper surface of the current collector foil 13, and a separator 16. The current collector foil 13 is a foil-like conductive member having a rectangular shape in a plan view, and is a laminated foil in which multiple dissimilar metal foils are laminated.

[0038] The positive electrode active material layer 14 constitutes the positive electrode of the bipolar electrode 12, and is disposed via an adhesive layer on the lower surface of the current collector foil 13. The positive electrode active material layer 14 contains the positive electrode active material of the present disclosure, and may further contain a conductive additive, a binder, and the like. Examples of binders include fluorine-containing resins such as polyvinylidene fluoride, polytetrafluoroethylene, and fluororubber; thermoplastic resins such as polypropylene and polyethylene; imide resins such as polyimide and polyamideimide; alkoxysilyl group-containing resins; acrylic resins containing monomer units such as acrylic acid and methacrylic acid; styrene-butadiene rubber (SBR); carboxymethyl cellulose; alginates such as sodium alginate and ammonium alginate; water-soluble cellulose ester crosslinks; and starch-acrylic acid graft polymers. These binders can be used alone or in combination. Examples of conductive additives include acetylene black, carbon black, and graphite.

[0039] The negative electrode active material layer 15 constitutes the negative electrode of the bipolar electrode 12 and is disposed on the upper surface of the current collector foil 13 . The negative electrode active material layer 15 may contain a negative electrode active material, a conductive additive, and a binder. The conductive additive and the binder can be considered to be similar to those of the positive electrode active material layer 14. Examples of the negative electrode active material include carbon such as graphite, artificial graphite, highly oriented graphite, mesocarbon microbeads, hard carbon, and soft carbon, metal compounds, elements that can be alloyed with lithium or compounds of such elements, boron-doped carbon, etc. Examples of elements that can be alloyed with lithium include silicon and tin.

[0040] The separator 16 is, for example, a porous sheet or nonwoven fabric containing a polymer that absorbs and retains the electrolyte, and is placed on the upper surface of the negative electrode active material layer 15 .

[0041] A conventionally known method such as roll coating, die coating, dip coating, doctor blade coating, spray coating, or caulking coating can be used to form the positive electrode active material layer 14 and the negative electrode active material layer 15 on the current collector foil 13. Specifically, an active material, a solvent, and, if necessary, a binder and a conductive additive are mixed to prepare a slurry, which is then applied to the upper and lower surfaces of the current collector foil 13 and then dried.

[0042] In the electrode laminate, adjacent bipolar electrodes 12 in the stacking direction are stacked so that one positive electrode active material layer 14 overlaps the other separator 16. The electrode laminate also has a positive electrode terminal electrode 17 at the top end and a negative electrode terminal electrode 18 at the bottom end of the stack of bipolar electrodes 12. The positive electrode terminal electrode 17 has a current collector foil 13 and a positive electrode active material layer 14 provided on the underside of the current collector foil 13. The positive electrode active material layer 14 is stacked on the adjacent bipolar electrode 12, and the current collector foil 13 is stacked on its upper surface. The negative electrode terminal electrode 18 has a current collector foil 13 and a negative electrode active material layer 15 provided on the upper surface of the current collector foil 13, and a separator 16 stacked on its upper surface. The separator 16 is stacked on the adjacent bipolar electrode 12, and the negative electrode active material layer 15 is stacked on its lower surface, and the current collector foil 13 is stacked on its lower surface. The positive terminal electrode 17 and the negative terminal electrode 18 are each formed by stacking a current collecting foil 13 on the adjacent conductive plate.

[0043] The sealing body 20 is a member that is disposed on the outer peripheral edge of the bipolar electrode 12 to seal the bipolar electrode 12. This also seals the space between adjacent bipolar electrodes 12 in the stacking direction. The sealing body 20 has a first sealing member 21, a second sealing member 22, and a spacer 23.

[0044] The first sealing member 21 is a frame-shaped member that is disposed along the outer peripheral end (outer edge) of the bipolar electrode 12. Specifically, the first sealing member 21 is disposed and joined between the upper surface of the current collector foil 13 and the lower surface of the separator 16 at the outer peripheral end of the bipolar electrode 12, thereby disposing the negative electrode active material layer 15 within the frame. A predetermined gap is provided between the inner edge of the first sealing member 21 and the negative electrode active material layer 15, forming a space. Meanwhile, the outer edge of the first sealing member 21 is configured so that the first sealing member 21 protrudes outward from the current collector foil 13.

[0045] The second seal member 22 is a frame-shaped member that is disposed along the outer peripheral edge (outer edge) of the bipolar electrode 12. Specifically, the second seal member 22 is disposed and joined between the lower surface of the current collector foil 13 and the upper surface of the spacer 23 at the outer peripheral edge of the bipolar electrode 12, and the positive electrode active material layer 14 is disposed within the frame together with the spacer 23. A predetermined gap is provided between the inner edge of the second seal member 22 and the positive electrode active material layer 14, forming a space. Meanwhile, the outer edge of the second seal member 22 is configured so that the second seal member 22 protrudes outward from the current collector foil 13, and the upper surface of the second seal member 22 and the lower surface of the first seal member 21 are joined.

[0046] The spacer 23 is a frame-shaped member that is disposed along the outer peripheral edge of the bipolar electrode 12. Specifically, by combining the spacer 23 with the second seal member 22, the spacer 23 is disposed and bonded between the lower surface of the second seal member 22 and the upper surface of the separator 16 of the adjacent bipolar electrode 12 at the outer peripheral edge of the bipolar electrode 12, and the positive electrode active material layer 14 is disposed within this frame. The inner edge of the spacer 23 is disposed spaced apart from the positive electrode active material layer 14. Meanwhile, the outer edge of the spacer 23 protrudes outward from the separator 16, and its lower surface is bonded to the upper surface of the first seal member 21 of the adjacent sealing body 20.

[0047] Examples of applications of batteries include power sources for hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and battery electric vehicles (BEVs). [Example]

[0048] The present disclosure will be described below based on examples, but the present disclosure is not limited to these examples in any way.

[0049] Example 1 (Synthesis of positive electrode active material) Li x Ni a Co b Mn c O y A positive electrode active material having active material particles with a composition represented by the formula: where x, a, b, c, and y are in the ratios shown in Table 1 was synthesized by the method shown below.

[0050] ·Raw material solution NiSO4, CoSO4, and MnSO4 were dissolved in ion-exchanged water to obtain a raw material solution. The ratio of Ni / Co / Mn was 1 / 1 / 1 (atm %), and the concentration of the aqueous solution was 30 mass %.

[0051] Crystallization A certain amount of NH3 aqueous solution was placed in a reaction vessel, and the atmosphere was replaced with nitrogen while stirring with a stirrer. NaOH was added to the reaction vessel to make the pH alkaline. Next, the raw material solution and NH3 were added dropwise while maintaining a constant pH (pH 10-12) in the reaction vessel, causing the transition metal hydroxide to precipitate.

[0052] Washing, filtering, drying The precipitated transition metal hydroxide was filtered out, and ion-exchanged water was added and stirred with a spoon to disperse the hydroxide, followed by washing with water. The washed solution was then filtered to remove the transition metal hydroxide. The filtered transition metal hydroxide was then dried at 120°C for 16 hours to evaporate the water.

[0053] Lithium raw material blending The dried transition metal hydroxide and Li2CO3 and LiOH as Li raw materials were mixed in a mortar.

[0054] Firing and primary crushing The mixture of the transition metal hydroxide and the Li raw material was calcined in a calcination furnace (muffle furnace) at 700 to 1100°C in an oxygen atmosphere for 10 hours. The calcined mixture was then pulverized in a pulverizer (jet mill) to obtain a powder with a predetermined particle size.

[0055] ·compression 5 g of the powder was compressed into pellets using a mini hydraulic tablet press (Specac Ltd).

[0056] Second crusher The pellets were crushed to a predetermined particle size by crushing them with a crusher (jet mill).

[0057] In this way, the positive electrode active material of Example 1 was obtained. In the obtained positive electrode active material, the number percentage (%) of primary particles (particles E) in which the ratio d / D of the recess depth d to the primary particle particle diameter D was 0.010≦d / D≦0.890, and the average value of the ratio d / D in particles E were measured by the above-mentioned method. The results are shown in Table 1.

[0058] <Examples 2 and 3> A positive electrode active material was obtained in the same manner as in Example 1, except that the pressure during "compression" using a mini hydraulic press for tableting (manufactured by Specac Ltd.) was adjusted. The number proportion (%) of particles E and the average value of the ratio d / D of particles E in the obtained positive electrode active material were the values ​​shown in Table 1.

[0059] <Comparative Example 1> A positive electrode active material was obtained in the same manner as in Example 1, except that the "compression" and "second crushing" were not performed after the first crushing. The number ratio (%) of particles E in the obtained positive electrode active material was the value shown in Table 1.

[0060] In addition, the method of obtaining a positive electrode active material by performing "compression" and "second crushing" after the first crushing as in Examples 1 to 3 is referred to as "synthesis method 2" in this example. Furthermore, a method for obtaining a positive electrode active material without performing "compression" and "second crushing" after performing the first crushing, as in Comparative Example 1, is referred to as "synthesis method 1" in this example.

[0061] [Cell preparation] Cells were fabricated using the positive electrode active materials obtained in each of the Examples and Comparative Examples. Cell configuration Wound cylinder Positive electrode composition: Positive electrode active material / acetylene black (conductive material) / polyvinylidene fluoride = 88 / 10 / 2 (mass%) Negative electrode composition: natural graphite / styrene butadiene rubber (SBR) / carboxymethyl cellulose (CMC) Electrolyte composition: electrolyte = LiPF6 (1M), solvent = ethylene carbonate (EC) / dimethyl carbonate (DMC) / ethyl methyl carbonate (EMC) = 3 / 4 / 3 (volume%)

[0062] Electrode preparation A positive electrode and a negative electrode were applied to a current collector using a film applicator with a film thickness adjustment function (All Good Co., Ltd.), and the applied film was dried in a dryer at 80°C for 5 minutes to prepare a cell.

[0063] [Measurement of capacity retention rate after cycling] The battery capacity of the cells obtained in each example and comparative example was measured before and after cycling under the following test conditions. The results of the percentage of battery capacity after cycling (capacity retention rate (%)) when the battery capacity before cycling is set to "100%" are shown in Table 1. It can be said that the closer the capacity retention rate is to 100%, the better the battery characteristics are. Test conditions: 300 cycles of charge and discharge between SOC 0% and 100% at 60°C and 2C rate.

[0064] [Table 1]

[0065] As shown in Table 1, the positive electrode active materials of each Example, in which the number ratio of primary particles (particles E) with a ratio d / D of 0.010≦d / D≦0.890 to all primary particles is 1.0% or more, maintain a high capacity retention rate after cycling compared to the positive electrode active materials of Comparative Examples, in which the number ratio is less than 1.0%. [Explanation of symbols]

[0066] 1A, 11A, 12A, 19, 110, 121A, 122A primary particles, 2A, 21A, 21B, 22A, 22B secondary particles, 111, 112 edge, 113 depression, 11 energy storage module, 12 bipolar electrode, 13 current collecting foil, 14 positive electrode active material layer, 15 negative electrode active material layer, 16 separator, 20 sealing body, 21 first sealing member, 22 second sealing member, 23 spacer

Claims

1. A positive electrode active material in which a plurality of primary particles are aggregated to form secondary particles, At least some of the primary particles have depressions on their surfaces, When the primary particles having a ratio d / D of the depth d of the depression to the particle diameter D of the primary particle satisfy the condition 0.010≦d / D≦0.890 are defined as particles E, the number ratio of the particles E to all the primary particles is 1.0% or more. Positive electrode active material for batteries.

2. The positive electrode active material contains elements of Li, Ni, Co, Mn, and O, and the composition of the elements is Li ｘ Ni ａ Co ｂ Mn ｃ O ｙ The positive electrode active material according to claim 1 , wherein (In the above composition, 0.1≦x≦1.5, 0.5≦a≦1.0, 0≦b≦0.3, 0≦c≦0.3, a+b+c=1.0, and 1.5≦y≦2.1.)

3. A positive electrode comprising the positive electrode active material according to claim 1 or 2.

4. A battery comprising the positive electrode of claim 3.

5. a compression step of compressing particles of the positive electrode active material; a crushing step of subjecting the particles after the compression step to a crushing treatment; The method for producing a positive electrode active material for a battery, comprising:

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