Method for evaluating lithium-ion battery positive electrode active material, and lithium-ion battery positive electrode active material

Abstract

A method for evaluating a lithium-ion battery positive electrode active material, the method comprising the steps of: measuring XRD diffraction patterns of a group of lithium-ion battery positive electrode active materials which are each represented by the compositional formula LiaNi(1-b-c-d)CobMncMdO2 (where, M is at least one selected from among Mg, Al, Ca, Ta, 0.98≤a≤1.10, 0.13≤b≤0.22, 0.02≤c≤0.32, and 0≤d≤0.007); calculating the crystallite sizes of the (003) plane from the XRD diffraction patterns of the positive electrode active materials using the Scherrer equation; evaluating cycle characteristics of each battery produced using, as an electrode material for the battery, a positive electrode active material different in crystallite size of the (003) plane among the positive electrode active materials, and deriving a correlation between the obtained cycle characteristic evaluation results and the crystallite sizes of the positive electrode active materials; and measuring the crystallite size of a positive electrode active material for evaluation corresponding to the crystallite sizes of the group of positive electrode active materials on the basis of the correlation, and evaluating cycle characteristics of the positive electrode active material for evaluation from the crystallite size of the positive electrode active material for evaluation.

Description

Evaluation method for positive electrode active material for lithium ion battery and positive electrode active material for lithium ion battery

[0001] The present invention relates to a method for evaluating a positive electrode active material for a lithium ion battery and to a positive electrode active material for a lithium ion battery.

[0002] In recent years, with the rapid expansion of small electronic devices such as mobile phones and laptops, the demand for non-aqueous electrolyte secondary batteries as rechargeable power sources has been growing rapidly. As the positive electrode active material for non-aqueous electrolyte secondary batteries, lithium-cobalt composite oxides such as lithium cobalt oxide (LiCoO), lithium-nickel composite oxides such as lithium nickel oxide (LiNiO), and lithium-manganese composite oxides such as lithium manganese oxide (LiMnO) are widely used, and research and development of positive electrode active materials for lithium-ion batteries using these materials is being actively conducted.

[0003] Patent Document 1 describes a composition of the following formula: Li a Ni b Co c Mn d The document discloses a positive electrode active material for lithium ion batteries, which comprises positive electrode active material particles represented by the formula (wherein 1.00≦a≦1.02, 0.8≦b≦0.9, and b+c+d=1) and a coating layer containing lithium niobate that partially coats the surfaces of the positive electrode active material particles, and in which the ratio of Nb to the total amount of Ni, Co, and Mn is 0.4 to 1.2 mol %. It also states that such a configuration can provide a positive electrode active material for lithium ion batteries that has good battery characteristics.

[0004] JP 2019-139862 A

[0005] In the development of positive electrode active materials for lithium-ion batteries, it is important to evaluate the cycle characteristics of the battery. However, evaluating battery characteristics requires fabricating precise cell electrodes and repeatedly charging and discharging them for several days using a charge-discharge tester, which takes a long time to obtain the results.

[0006] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a method for evaluating a positive electrode active material for a lithium ion battery, which allows for efficient evaluation of cycle characteristics without fabricating a battery, and a positive electrode active material for a lithium ion battery.

[0007] The present invention, which was completed based on the above findings, is defined as follows: (1) Each of the compounds has the composition formula: Li a Ni (1-b-c-d) Co b Mn c M d O2 (wherein M is one or more elements selected from Mg, Al, Ca, and Ta, and 0.98≦a≦1.10, 0.13≦b≦0.22, 0.02≦c≦0.32, and 0≦d≦0.007); calculating the crystallite size of the (003) plane from the XRD diffraction pattern of the positive electrode active material using Scherrer's equation; evaluating cycle characteristics of batteries fabricated using, as electrode materials for the batteries, positive electrode active materials having different crystallite sizes of the (003) plane from among the positive electrode active materials, and deriving a correlation between the obtained cycle characteristic evaluation results and the crystallite size of the positive electrode active material; (2) A method for evaluating a positive electrode active material for a lithium ion battery, comprising: a step of measuring the crystallite size of a positive electrode active material for evaluation corresponding to the crystallite size of the group of positive electrode active materials based on the correlation, and evaluating the cycle characteristics of the positive electrode active material for evaluation from the crystallite size of the positive electrode active material for evaluation. a Ni (1-b-c-d) Co b Mn c M dO2 (wherein M is one or more elements selected from Mg, Al, and Ca, and 1.03≦a≦1.10, 0.18≦b≦0.22, 0.28≦c≦0.32, and 0≦d≦0.007). (3) The method for evaluating a positive electrode active material for a lithium ion battery according to (1), wherein the positive electrode active material for evaluation has a crystallite size of 580 to 840 Å, and the cycle characteristics of the positive electrode active material for evaluation are evaluated as good. (4) The method for evaluating a positive electrode active material for a lithium ion battery according to (2), wherein the positive electrode active material has a composition formula: Li a Ni (1-b-c-d) Co b Mn c M d (5) The method for evaluating a positive electrode active material for a lithium ion battery according to (1), wherein the positive electrode active material for evaluation is represented by the formula: LiO2 (wherein M is Ta, and 0.98≦a≦1.09, 0.13≦b≦0.16, 0.02≦c≦0.05, and 0≦d≦0.007). (6) The method for evaluating a positive electrode active material for a lithium ion battery according to (4), wherein the cycle characteristics of the positive electrode active material for evaluation are evaluated as good if the crystallite size of the positive electrode active material for evaluation is 400 to 650 Å. (7) The method for evaluating a positive electrode active material for a lithium ion battery according to (5), wherein the crystallite size of the positive electrode active material for evaluation is 400 to 650 Å. a Ni (1-b-c-d) Co b Mn c M d O2 (wherein M is one or more elements selected from Mg, Al, and Ca, and 1.03≦a≦1.10, 0.18≦b≦0.22, 0.28≦c≦0.32, and 0≦d≦0.007), wherein an XRD diffraction pattern of the positive electrode active material is measured, and the crystallite size of the (003) plane is calculated from the XRD diffraction pattern of the positive electrode active material using Scherrer's equation, and the crystallite size is 580 to 840 Å. (7) Composition formula: Li a Ni (1-b-c-d) Co b Mn c M dO2 (wherein M is Ta, and 0.98≦a≦1.09, 0.13≦b≦0.16, 0.02≦c≦0.05, and 0≦d≦0.007), wherein an XRD diffraction pattern of the positive electrode active material is measured, and a crystallite size of a (003) plane is calculated from the XRD diffraction pattern of the positive electrode active material using Scherrer's equation, and the crystallite size is 400 to 650 Å.

[0008] According to the present invention, it is possible to provide a method for evaluating a positive electrode active material for a lithium ion battery, which enables efficient evaluation of cycle characteristics without fabricating a battery, and a positive electrode active material for a lithium ion battery.

[0009] 1 is an example of an XRD diffraction pattern of a positive electrode active material for a lithium ion battery. 2 is a graph in which the crystallite size of the positive electrode active materials of Test Examples 1A to 11A is plotted on the X axis and the cycle characteristics are plotted on the Y axis. 3 is a graph in which the crystallite size of the positive electrode active materials of Test Examples 1B to 10B is plotted on the X axis and the cycle characteristics are plotted on the Y axis.

[0010] Next, the embodiments for carrying out the present invention will be described in detail. It should be understood that the present invention is not limited to the following embodiments, and that appropriate design changes, improvements, etc. may be made based on the ordinary knowledge of those skilled in the art without departing from the spirit of the present invention.

[0011] (Positive electrode active material for lithium ion battery) The positive electrode active material for lithium ion battery according to the embodiment of the present invention has the composition formula: Li a Ni (1-b-c-d) Co b Mn c M dO2 (wherein M is one or more elements selected from Mg, Al, Ca, and Ta, and 0.98≦a≦1.10, 0.13≦b≦0.22, 0.02≦c≦0.32, and 0≦d≦0.007). In the composition formula of this positive electrode active material, the value of a, which indicates the lithium composition, satisfies 0.98≦a≦1.10. Because the value of a, which indicates the lithium composition, is 0.98 or more, it is possible to suppress the reduction of nickel due to lithium deficiency. Furthermore, because the value of a, which indicates the lithium composition, is 1.10 or less, it is possible to suppress residual alkaline components, such as lithium carbonate and lithium hydroxide, present on the surface of the positive electrode active material particles, which may become resistance components when the material is made into a battery.

[0012] In the positive electrode active material for lithium ion batteries according to the embodiment of the present invention, the nickel composition in the composition formula is controlled to 1-b-c-d (0.453≦1-b-c-d≦0.85), and since the nickel composition is 0.453 or more, good battery capacity can be obtained for lithium ion batteries. Furthermore, since the nickel composition is 0.85 or less, the crystal structure is stable and the expansion and contraction behavior of the crystal lattice due to lithium insertion and extraction during charge and discharge can be reduced, thereby improving cycle characteristics.

[0013] In the positive electrode active material for a lithium ion battery according to an embodiment of the present invention, the sum of b representing the Co composition, c representing the Mn composition, and d representing one or more elements selected from Mg, Al, Ca, and Ta in the composition formula satisfies 0.15≦b+c+d≦0.547, thereby improving cycle characteristics and reducing the expansion and contraction behavior of the crystal lattice due to the insertion and extraction of lithium during charge and discharge. If the sum of b representing the Co composition, c representing the Mn composition, and d representing one or more elements selected from Mg, Al, Ca, and Ta exceeds 0.547, the amount of Co, Mn, Mg, Al, Ca, and Ta added may be too large, resulting in a significant decrease in initial discharge capacity or may be disadvantageous in terms of cost.

[0014] In the positive electrode active material for a lithium ion battery according to an embodiment of the present invention, d, which represents one or more elements selected from Mg, Al, Ca, and Ta, satisfies 0≦d≦0.007. Since d, which represents one or more elements selected from Mg, Al, Ca, and Ta, is 0.007 or less, it is possible to prevent a significant decrease in initial discharge capacity due to excessive addition of Mg, Al, Ca, and Ta.

[0015] The positive electrode active material for a lithium ion battery according to an embodiment of the present invention has the composition formula: Li a Ni (1-b-c-d) Co b Mn c M d O2 (wherein M is one or more elements selected from Mg, Al, and Ca, and 1.03≦a≦1.10, 0.18≦b≦0.22, 0.28≦c≦0.32, and 0≦d≦0.007). Furthermore, although details will be described later, when the XRD diffraction pattern of the positive electrode active material represented by this composition formula is measured and the crystallite size of the (003) plane is calculated from the XRD diffraction pattern of the positive electrode active material using the Scherrer equation, the crystallite size is preferably 580 to 840 Å.

[0016] The positive electrode active material for a lithium ion battery according to an embodiment of the present invention has the composition formula: Li a Ni (1-b-c-d) Co b Mn c M d O2 (wherein M is Ta, and 0.98≦a≦1.09, 0.13≦b≦0.16, 0.02≦c≦0.05, and 0≦d≦0.007). Furthermore, although details will be described later, when the XRD diffraction pattern of the positive electrode active material represented by this composition formula is measured and the crystallite size of the (003) plane is calculated from the XRD diffraction pattern of the positive electrode active material using the Scherrer equation, the crystallite size is preferably 400 to 650 Å.

[0017] The positive electrode active material for a lithium ion battery according to the embodiment of the present invention may be in the form of secondary particles formed by agglomeration of a plurality of primary particles, with some primary particles not agglomerated as secondary particles. The shapes of the primary particles constituting the secondary particles and the primary particles present alone are not particularly limited, and may be various shapes, such as substantially spherical, substantially elliptical, substantially plate-like, or substantially needle-like. The form in which the plurality of primary particles are agglomerated is also not particularly limited, and may be various forms, such as agglomeration in random directions or agglomeration approximately uniformly radially from the center to form substantially spherical or substantially elliptical secondary particles.

[0018] The positive electrode active material for a lithium ion battery according to an embodiment of the present invention may have a 50% cumulative volume particle size D50 of 3.0 to 11.0 μm. Here, the 50% cumulative volume particle size D50 is the volume particle size at 50% accumulation in a volume-based cumulative particle size distribution curve. If the 50% cumulative volume particle size D50 of the positive electrode active material for a lithium ion battery is less than 3.0 μm, the tap density decreases, resulting in a decrease in energy density per volume. If the 50% cumulative volume particle size D50 of the positive electrode active material for a lithium ion battery exceeds 11.0 μm, the number of coarse particles increases, resulting in poor applicability when the slurried positive electrode active material is applied to a current collecting band. The 50% cumulative volume particle size D50 of the positive electrode active material for a lithium ion battery is more preferably 7.0 to 10.0 μm. The 50% cumulative volume particle size D50 was measured by first dispersing 100 mg of a positive electrode active material sample (powder) using a Microtrac laser diffraction particle size distribution analyzer "MT3300EXII" at a 50% flow rate under 40 W ultrasonic irradiation for 60 seconds, and then measuring the particle size distribution to obtain a volume-based cumulative particle size distribution curve. Next, in the obtained cumulative particle size distribution curve, the volume particle size at 50% accumulation can be used as the 50% cumulative volume particle size D50 of the positive electrode active material powder. The aqueous solvent used in the measurement was passed through a 0.02 μm filter, with a solvent refractive index of 1.333, particle permeability conditions of 1.81, and shape of asphericity. The measurement range was 0.021 to 2000 μm, and the measurement time was 30 seconds.

[0019] The positive electrode active material for a lithium ion battery according to an embodiment of the present invention preferably has a tap density of 2.0 to 2.6 g / cc. A positive electrode active material having a tap density of 2.0 g / cc or higher can form a battery with a high energy density per volume. The tap density of the positive electrode active material is more preferably 2.1 to 2.6 g / cc, and even more preferably 2.3 to 2.6 g / cc. The tap density of the positive electrode active material is measured, for example, by placing 5 g of the positive electrode active material (powder) into a 10 cc graduated cylinder, placing it in a powder density measuring instrument "KYT-4000K" manufactured by Seishin Enterprise Co., Ltd., and tapping 1,500 times with a stroke length of 55 mm, followed by reading the graduations on the graduated cylinder. Next, the "sample amount (5 g) / graduated cylinder graduation reading (cc)" is calculated, and this is taken as the tap density (g / cc).

[0020] The positive electrode active material for a lithium ion battery according to an embodiment of the present invention has a BET specific surface area of ​​0.20 to 0.80 m 2 / g. The BET specific surface area is preferably 0.20 m 2 When the BET specific surface area is 0.80 m / g or more, the contact area of ​​the positive electrode active material is increased, and the Li ion conductivity is improved. Therefore, it is possible to manufacture a high-capacity lithium ion battery. 2 If the BET specific surface area is more than 0.3 to 0.70 m / g, the precipitation reaction of lithium ions from the residual alkali in the positive electrode active material is accelerated during repeated charge and discharge. The precipitated lithium compounds become the internal resistance of the battery, reducing the charge and discharge capacity. 2 / g is more preferable. The BET specific surface area can be measured by the following method. That is, first, 1.0 g of the positive electrode active material (powder) is weighed into a glass cell, set in a degassing device, and the glass cell is filled with nitrogen gas. After that, the cell is heat-treated in a nitrogen gas atmosphere at 40°C for 20 minutes to degas the sample. Thereafter, the glass cell containing the degassed sample (powder) is set in a specific surface area measuring device "Monosorb Model MS-21" manufactured by Quantachrome, and the specific surface area X is measured by the BET method (single-point method) while flowing a He:70 at%-N2:30 at% mixed gas as the adsorption gas.

[0021] (Method for evaluating positive electrode active materials for lithium ion batteries) Next, a method for evaluating positive electrode active materials for lithium ion batteries according to an embodiment of the present invention will be described in detail below. a Ni (1-b-c-d) Co b Mn c M d The XRD diffraction patterns of a group of positive electrode active materials for lithium ion batteries represented by the formula (II) are measured. The XRD diffraction patterns can be measured, for example, using the following XRD diffractometer under the following conditions: XRD diffractometer: SmartLab (manufactured by Rigaku Corporation); Radiation source: CuKα (λ=1.5406 Å); A sample (positive electrode active material) is applied to a glass sample holder (2 cm × 1.5 cm, 0.3 mm deep); Detector: D / tex; Measurement range: 2θ=10° to 80°; Scan axis: 2θ / θ; Scan speed: 1 degree min -1 Step width: 0.01 degree Slit width: IS (DS) 1 / 4°, RS1 10 mm, RS2 10 mm An example of the XRD diffraction pattern obtained here is shown in FIG.

[0022] Next, the crystallite size of the (003) plane is calculated from the measured XRD diffraction patterns of the group of lithium ion battery positive electrode active materials using the Scherrer equation. Specifically, the crystallite size of the (003) plane is calculated from the half-width of the diffraction peak of the (003) plane in the XRD diffraction pattern of the lithium ion battery positive electrode active material using the Scherrer equation represented by the following formula (1): s = Kλ / B cos θ (1)

[0023] In the above formula (1), s is the crystallite size of the (003) plane, λ is the wavelength of the X-ray, B is the half-width of the diffraction peak of the (003) plane, θ is the diffraction angle (rad), and K is the Scherrer constant. In this embodiment, K is set to 0.9.

[0024] Next, the cycle characteristics of each battery fabricated using the positive electrode active materials with different (003) crystallite sizes as the electrode material of the battery are evaluated. Specifically, first, multiple positive electrode active materials with different (003) crystallite sizes calculated using the Scherrer formula are selected. The number of (003) crystallite sizes of the selected positive electrode active materials is not particularly limited, and for example, 5 or more are preferably selected, 7 or more are more preferably selected, 10 or more are even more preferably selected, and 15 or more are even more preferably selected. Next, for each of the multiple selected positive electrode active materials with different (003) crystallite sizes, the selected positive electrode active materials are used as the electrode material to fabricate a battery. The batteries fabricated here are not particularly limited as long as they use the selected positive electrode active materials as the positive electrode material. The batteries are fabricated, for example, as follows, and the discharge capacity and cycle characteristics (capacity retention rate) of the batteries can be evaluated. First, the positive electrode active material, the conductive material acetylene black, and the binder polyvinylidene fluoride were weighed in a ratio of 90:5:5, the binder polyvinylidene fluoride was dissolved in an organic solvent (N-methylpyrrolidone), and the positive electrode active material and the conductive material were mixed to form a slurry, which was then applied to aluminum foil, dried, and press-molded to form a positive electrode. Next, as a battery structure, a 2032-type coin cell for evaluation was prepared using Li as the counter electrode cathode, and the discharge capacity (0.05C) and charge / discharge cycle (capacity retention rate) were measured under the following conditions using an electrolyte solution of 1M-LiPF6 dissolved in EC-DMC (3:7). Discharge capacity (0.05C) Temperature: 25°C, charge: 4.30V, 0.05C, 20 hours, discharge: 3.0V, 0.05C Charge / discharge cycle (capacity retention rate) Temperature: 55°C, charge: 4.30V, 1C, 2.5 hours, discharge: 3.0V, 1C Ratio of the discharge capacity at the 20th cycle to the discharge capacity at the 1st cycle. Such evaluation of cycle characteristics was performed for each battery produced using, as the electrode material of the battery, positive electrode active materials each having a different crystallite size in the (003) plane.

[0025] Next, a correlation between the obtained cycle characteristic evaluation results and the crystallite size of the positive electrode active material is derived. Specifically, a graph is created with the crystallite size on the X axis and the cycle characteristic on the Y axis. In this graph, cycle characteristics above a predetermined value, such as 95.0% or more or 96.0% or more, are determined to be good. Then, from this graph, a range of crystallite sizes corresponding to cycle characteristics above the predetermined value is derived. The resulting relationship (i.e., if the crystallite size of the positive electrode active material is within a certain range, the obtained cycle characteristic will be above a predetermined value) represents the correlation between the obtained cycle characteristic evaluation results and the crystallite size of the positive electrode active material.

[0026] In particular, it can be seen that good cycle characteristics are obtained when the crystallite size of the (003) plane is within a predetermined size. This is because if the crystallite size of the (003) plane is large, the expansion and contraction associated with the absorption and release of lithium increases, resulting in greater cycle deterioration due to structural distortion. Also, if the crystallite size of the (003) plane is too small, the stability of the crystal itself decreases, and structural deterioration associated with charge and discharge easily progresses, resulting in greater cycle deterioration. Thus, for a lithium-ion battery positive electrode active material having a predetermined composition, if the crystallite size of the (003) plane is within a predetermined range, it can be determined that the lithium-ion battery positive electrode active material has good cycle characteristics. For example, a material having the composition formula: Li a Ni (1-b-c-d) Co b Mn c M d For a positive electrode active material for a lithium ion battery represented by the formula: LiO2 (wherein M is one or more selected from Mg, Al, and Ca, and 1.03≦a≦1.10, 0.18≦b≦0.22, 0.28≦c≦0.32, and 0≦d≦0.007), if the crystallite size of the (003) plane is 580 to 840 Å, it can be determined that the material has good cycle characteristics. a Ni (1-b-c-d) Co b Mn c M dA positive electrode active material for a lithium ion battery represented by the formula O2 (wherein M is Ta, and 0.98≦a≦1.09, 0.13≦b≦0.16, 0.02≦c≦0.05, and 0≦d≦0.007) can be determined to have good cycle characteristics if the crystallite size of the (003) plane is 400 to 650 Å.

[0027] Next, based on the correlation, the crystallite size of a test positive electrode active material corresponding to the crystallite size of the group of test positive electrode active materials is measured, and the cycle characteristics of the test positive electrode active material are evaluated from the crystallite size of the test positive electrode active material. Specifically, first, a test positive electrode active material is prepared. As described above, the test positive electrode active material has the composition formula: Li a Ni (1-b-c-d) Co b Mn c M d O2 (wherein M is one or more elements selected from Mg, Al, Ca, and Ta, and 0.98≦a≦1.10, 0.13≦b≦0.22, 0.02≦c≦0.32, and 0≦d≦0.007). Next, the crystallite size of a positive electrode active material for evaluation corresponding to the crystallite size of the group of positive electrode active materials is measured. More specifically, the crystallite size of the positive electrode active material for evaluation is measured by the same measurement method as used to measure the crystallite size of the group of positive electrode active materials. Next, based on the correlation between the cycle performance evaluation results and the crystallite size of the positive electrode active material, the cycle performance of the positive electrode active material for evaluation is evaluated from the crystallite size of the positive electrode active material for evaluation. For example, if there is a correlation between the above cycle characteristic evaluation results and the crystallite size of the positive electrode active material, such that a positive electrode active material for a lithium ion battery having a predetermined composition can be determined to have good cycle characteristics if the crystallite size of the (003) plane is 400 to 650 Å, then it can be determined that the positive electrode active material for evaluation has good cycle characteristics if the crystallite size of the positive electrode active material for evaluation is within the range of 400 to 650 Å.

[0028] As described above, according to the method for evaluating a positive electrode active material for a lithium-ion battery according to an embodiment of the present invention, once a correlation between the results of the cycle performance evaluation and the crystallite size of the positive electrode active material is derived, the crystallite size of the lithium-ion battery positive electrode active material to be evaluated can be measured and applied to the correlation to evaluate the battery's characteristics, such as cycle performance. Therefore, there is no need to fabricate a precise cell electrode and repeatedly charge and discharge the cell electrode for several days using a charge-discharge tester to obtain data, eliminating the problem of the long time it takes to determine the results. Thus, according to the method for evaluating a positive electrode active material for a lithium-ion battery according to an embodiment of the present invention, it is possible to efficiently evaluate cycle performance without fabricating a battery.

[0029] (Positive electrode for lithium-ion battery and lithium-ion battery) A positive electrode for lithium-ion battery according to an embodiment of the present invention can be fabricated using a positive electrode active material for lithium-ion battery that has been determined to have good cycle characteristics as described above. The positive electrode for lithium-ion battery according to an embodiment of the present invention has a structure in which a positive electrode mixture prepared by mixing the positive electrode active material for lithium-ion battery, a conductive additive, and a binder is provided on one or both sides of a current collector. Furthermore, the lithium-ion battery according to an embodiment of the present invention includes a positive electrode for lithium-ion battery having such a configuration and a known negative electrode for lithium-ion battery.

[0030] Examples of conductive additives include metal-based conductive additives (aluminum, stainless steel (SUS), silver, gold, copper, titanium, etc.), carbon-based conductive additives (graphite and carbon black (acetylene black, ketjen black, furnace black, channel black, thermal lamp black), etc.), and mixtures thereof. These conductive additives may be used alone or in combination of two or more. They may also be used as alloys or metal oxides. Among these, from the viewpoint of electrical stability, more preferred are aluminum, stainless steel, silver, gold, copper, titanium, carbon-based conductive additives, and mixtures thereof, even more preferred are silver, gold, aluminum, stainless steel, and carbon-based conductive additives, and particularly preferred are carbon-based conductive additives. Furthermore, these conductive additives may be particulate ceramic materials or resin materials coated with a conductive material (preferably a metal one of the above-mentioned conductive additives) by plating or the like. The shape (form) of the conductive additive is not limited to a particulate form, and may be a form other than a particulate form, such as carbon nanofibers or carbon nanotubes, which are so-called filler-based conductive additives in practical use.

[0031] Examples of binders include substances commonly used in positive electrode mixtures for lithium ion batteries, but copolymers having a structure derived from vinylidene fluoride, polyvinylidene fluoride (PVDF), copolymers or homopolymers having a structure derived from tetrafluoroethylene (TEF), and copolymers or homopolymers having a structure derived from hexafluoropropylene (HFP) are preferred. Specific examples include PVDF-HFP, PVDF-HFP-TEF, PVDF-TEF, and TEF-HFP.

[0032] The positive electrode mixture is prepared by mixing a positive electrode active material for a lithium ion battery, a conductive additive, and a binder in a solvent to form a positive electrode mixture slurry, which is then applied to one or both sides of a current collector and, after drying or the like, provided on the current collector to form a positive electrode active material layer.

[0033] As the solvent for the positive electrode mixture slurry, known organic solvents such as hydrocarbon organic solvents, amide compounds, lactam compounds, urea compounds, organic sulfur compounds, and cyclic organic phosphorus compounds can be used alone or as a mixed solvent. Examples of hydrocarbon organic solvents that can be used include saturated hydrocarbons, unsaturated hydrocarbons, and aromatic hydrocarbons. Examples of saturated hydrocarbons include hexane, pentane, 2-ethylhexane, heptane, decane, and cyclohexane. Examples of unsaturated hydrocarbons include hexene, heptene, and cyclohexene. Examples of aromatic hydrocarbons include toluene, xylene, decalin, and 1,2,3,4-tetrahydronaphthalene. Of these, toluene and xylene are particularly preferred.

[0034] Materials constituting the current collector include metal materials such as copper, aluminum, titanium, stainless steel, nickel, and alloys thereof, as well as baked carbon, conductive polymer materials, conductive glass, etc. Among these, aluminum is more preferable from the viewpoints of weight reduction, corrosion resistance, and high conductivity. Furthermore, the current collector is preferably a resin current collector made of a conductive polymer material. The shape of the current collector is not particularly limited, and may be a sheet-like current collector made of the above material, or a sediment layer made of fine particles composed of the above material. The thickness of the current collector is not particularly limited, but is preferably 1 to 30 μm. Examples of conductive polymer materials that can be used to constitute the resin current collector include conductive polymers and resins to which a conductive material has been added as needed.

[0035] From the viewpoint of battery performance, the thickness of the positive electrode for a lithium ion battery is preferably 10 to 100 μm, and more preferably 20 to 50 μm.

[0036] Lithium-ion batteries using a lithium-ion battery positive electrode are produced by combining a counter electrode with a negative electrode, placing them together with a separator in a cell container, injecting an electrolyte, and sealing the cell container. Alternatively, a bipolar electrode can be produced by forming a positive electrode on one side of a current collector and a negative electrode on the other, stacking the bipolar electrode with a separator, placing it in a cell container, injecting an electrolyte, and sealing the cell container.

[0037] The negative electrode may include a negative electrode active material, a conductive additive, a current collector, etc. As the negative electrode active material, known negative electrode active materials for lithium ion batteries can be used, and examples thereof include carbon-based materials (graphite, non-graphitizable carbon, amorphous carbon, burned resins (e.g., phenolic resins, furan resins, etc., which are burned and carbonized), cokes (e.g., pitch coke, needle coke, petroleum coke, etc.), and carbon fibers), silicon-based materials (silicon, silicon oxide (SiO x ), silicon-carbon composites (carbon particles whose surfaces are coated with silicon and / or silicon carbide, silicon particles or silicon oxide particles whose surfaces are coated with carbon and / or silicon carbide, and silicon carbide, etc.) and silicon alloys (silicon-aluminum alloys, silicon-lithium alloys, silicon-nickel alloys, silicon-iron alloys, silicon-titanium alloys, silicon-manganese alloys, silicon-copper alloys, silicon-tin alloys, etc.), conductive polymers (e.g., polyacetylene and polypyrrole), metals (tin, aluminum, zirconium, titanium, etc.), metal oxides (titanium oxide and lithium-titanium oxide, etc.) and metal alloys (e.g., lithium-tin alloys, lithium-aluminum alloys, lithium-aluminum-manganese alloys, etc.), and mixtures of these with carbon-based materials, etc. Examples of the conductive additive include the same conductive additives as those used in the positive electrode described above.

[0038] The current collector may be the same as the current collector constituting the positive electrode described above, and is preferably copper from the viewpoints of weight reduction, corrosion resistance, and high conductivity. A resin current collector may also be used, and the same current collector as the current collector constituting the positive electrode described above can be suitably used. The thickness of the current collector is not particularly limited, but is preferably 10 to 60 μm.

[0039] Examples of the separator include known separators for lithium ion batteries, such as porous films made of polyethylene or polypropylene, laminated films of porous polyethylene film and porous polypropylene, nonwoven fabrics made of synthetic fibers (polyester fibers, aramid fibers, etc.) or glass fibers, and those having ceramic fine particles such as silica, alumina, or titania attached to the surface thereof.

[0040] The following examples are provided to provide a better understanding of the present invention and its advantages, but the present invention is not limited to these examples.

[0041] Example 1 First, the correlation between the cycle performance evaluation results and the crystallite size of the positive electrode active material was derived by the following method. a Ni (1-b-c-d) Co b Mn c M d A group of lithium-ion battery positive electrode active materials represented by the formula (I) were prepared. The formula (I) is a formula of the positive electrode active materials for lithium-ion batteries represented by the formula (I) in which M is one or more elements selected from Mg, Al, and Ca, and 1.03≦a≦1.10, 0.18≦b≦0.22, 0.28≦c≦0.32, and 0≦d≦0.007. Next, the XRD diffraction patterns of the group of lithium-ion battery positive electrode active materials were measured. The XRD diffraction patterns were measured using the following XRD diffractometer under the following conditions: XRD diffractometer: SmartLab (manufactured by Rigaku Corporation) Radiation source: CuKα (λ=1.5406 Å) A sample (positive electrode active material) was applied to a glass sample holder (2 cm × 1.5 cm, 0.3 mm deep). Detector: D / tex Measurement range: 2θ = 10° to 80° Scan axis: 2θ / θ, Scan speed: 1 degree min -1 Step width: 0.01 degrees Slit width: IS (DS) 1 / 4°, RS1 10 mm, RS2 10 mm

[0042] Next, the crystallite size of the (003) plane was calculated from the half-width of the diffraction peak of the (003) plane in the XRD diffraction pattern of the measured group of positive electrode active materials for lithium ion batteries by the Scherrer equation represented by the following formula (1): s = Kλ / B cos θ (1)

[0043] In the above formula (1), s is the crystallite size of the (003) plane, λ is the wavelength of the X-ray, B is the half-width of the diffraction peak of the (003) plane, θ is the diffraction angle (rad), and K is the Scherrer constant. In this embodiment, K is set to 0.9.

[0044] Next, the cycle characteristics of batteries fabricated using positive electrode active materials with different crystallite sizes in the (003) plane were evaluated. Specifically, 11 positive electrode active materials with different crystallite sizes in the (003) plane calculated using the Scherrer equation were selected. These 11 positive electrode active materials are designated Test Examples 1A to 11A, and their compositions, average particle diameters D50, BET specific surface areas, and tap densities are shown in Table 1.

[0045] (Average particle size D50) The average particle size D50 of the positive electrode active material of Test Examples 1A to 11A was measured as follows. First, 100 mg of positive electrode active material (powder) was dispersed by irradiating it with 40 W ultrasound for 60 seconds at a 50% flow rate using a Microtrac laser diffraction particle size distribution analyzer "MT3300EXII." The particle size distribution was measured, and a volume-based cumulative particle size distribution curve was obtained. In the obtained cumulative particle size distribution curve, the volume particle size at 50% accumulation was taken as the 50% cumulative volume particle size D50 (average particle size D50) of the positive electrode active material powder. Note that the water-soluble solvent used in the measurement was passed through a 0.02 μm filter, the solvent refractive index was 1.333, the particle permeability conditions were transmission, the particle refractive index was 1.81, the shape was aspherical, the measurement range was 0.021 to 2000 μm, and the measurement time was 30 seconds.

[0046] (Tap Density) The tap density of the positive electrode active materials of Test Examples 1A to 11A was measured as follows. First, 5 g of a sample (powder) of the positive electrode active material was placed in a 10 cc graduated cylinder and placed in a powder density measuring instrument "KYT-4000K" manufactured by Seishin Enterprise Co., Ltd. After tapping 1,500 times with a stroke length of 55 mm, the graduations on the graduated cylinder were read. Next, "sample amount (5 g) / graduated cylinder graduation reading (cc)" was calculated, and this was taken as the tap density (g / cc).

[0047] (BET Specific Surface Area) The BET specific surface area of ​​the positive electrode active materials of Test Examples 1A to 11A was measured as follows. First, 1.0 g of a sample (powder) of the positive electrode active material was weighed into a glass cell, set in a degassing device, and filled with nitrogen gas. The glass cell was then heat-treated in a nitrogen gas atmosphere at 40°C for 20 minutes to degas the sample. Thereafter, the glass cell containing the degassed sample (powder) was set in a specific surface area measuring device "Monosorb Model MS-21" manufactured by Quantachrome, and the specific surface area X was measured by the BET method (single-point method) while flowing a He:70 at%-N:30 at% mixed gas as the adsorption gas.

[0048] Next, for each of the positive electrode active materials of Test Examples 1A to 11A, batteries were fabricated as follows using the positive electrode active material as the electrode material. First, the positive electrode active material, acetylene black as a conductive material, and polyvinylidene fluoride as a binder were weighed out in a ratio of 90:5:5. The polyvinylidene fluoride binder was dissolved in an organic solvent (N-methylpyrrolidone), and the positive electrode active material and the conductive material were mixed to form a slurry. The slurry was then applied to aluminum foil, dried, and press-molded to form a positive electrode. Next, a 2032-type coin cell for evaluation was fabricated as a battery structure, using Li as the counter electrode cathode. The discharge capacity (0.05 C) and charge / discharge cycle (capacity retention rate) were measured under the following conditions using an electrolyte solution of 1M-LiPF6 dissolved in EC-DMC (3:7). Discharge capacity (0.05C): Temperature 25°C, charge: 4.30V, 0.05C, 20h, discharge: 3.0V, 0.05C. Charge / discharge cycle (capacity retention): Temperature 55°C, charge: 4.30V, 1C, 2.5h, discharge: 3.0V, 1C. The ratio of the discharge capacity at the 20th cycle to the discharge capacity at the 1st cycle. Such cycle characteristics were evaluated for each battery produced using the positive electrode active materials of Test Examples 1A to 11A as the electrode material of the battery. The evaluation results are shown in Table 1.

[0049]

[0050] Next, a correlation between the obtained cycle performance evaluation results and the crystallite size of the positive electrode active material was derived. Specifically, based on Table 1, a graph was created in which the crystallite size of the positive electrode active materials of Test Examples 1A to 11A was plotted on the X axis and the cycle performance on the Y axis. This graph is shown in Figure 2. In the graph of Figure 2, positive electrode active materials with cycle performance of 96.0% or higher were determined to have good cycle performance. The graph of Figure 2 shows that when the crystallite size of the (003) plane is within the range of 580 to 840 Å, good cycle performance of 96.0% or higher is obtained. The relationship derived from this (i.e., when the crystallite size of the positive electrode active material is within the range of 580 to 840 Å, the obtained cycle performance is 96.0% or higher) was used as the correlation between the obtained cycle performance evaluation results and the crystallite size of the positive electrode active material.

[0051] Next, as a positive electrode active material for a lithium ion battery for evaluation, a composition formula: Li 1.06 Ni 49.9 Co 19.7 Mn 30.4 Ca 0.013 A lithium-ion battery cathode active material represented by O2 was prepared, and its XRD diffraction pattern was measured under the same conditions as in Test Examples 1A to 11A. The crystallite size of the test cathode active material was then measured using the same method as in Test Examples 1A to 11A. The resulting crystallite size was 824 Å. Based on the correlation between the cycle performance evaluation results and the crystallite size of the cathode active material, the cycle performance of the test cathode active material was evaluated from the crystallite size of the test cathode active material. The resulting crystallite size was in the range of 580 to 840 Å, indicating that the cycle performance of the test cathode active material was 96.0% or higher.

[0052] Example 2 First, the correlation between the cycle performance evaluation results and the crystallite size of the positive electrode active material was derived by the following method. a Ni (1-b-c-d) Co b Mn c M dA group of lithium-ion battery positive electrode active materials represented by the formula (I) was prepared. The group of lithium-ion battery positive electrode active materials was then subjected to XRD measurement under the same conditions as in Example 1.

[0053] Next, from the half-width of the diffraction peak of the (003) plane in the XRD diffraction pattern of the measured group of positive electrode active materials for lithium ion batteries, the crystallite size of the (003) plane was calculated by the Scherrer equation represented by the above formula (1), in the same manner as in Example 1.

[0054] Next, the cycle characteristics of each battery fabricated using positive electrode active materials with different (003) crystallite sizes as the electrode material were evaluated. Specifically, 10 positive electrode active materials with different (003) crystallite sizes calculated using the Scherrer equation were selected. These 10 positive electrode active materials are designated Test Examples 1B to 10B, and their compositions, average particle diameters D50, BET specific surface areas, and tap densities are shown in Table 2. The average particle diameters D50, BET specific surface areas, and tap densities were measured in the same manner as in Example 1.

[0055] Next, for each of the positive electrode active materials of Test Examples 1B to 10B, batteries were fabricated as electrode materials in the same manner as in Example 1, and the discharge capacity (0.05 C) and charge / discharge cycle (capacity retention rate) were measured under the same conditions. Such cycle characteristics were evaluated for each of the batteries fabricated using the positive electrode active materials of Test Examples 1B to 10B as electrode materials. The evaluation results are shown in Table 2.

[0056]

[0057] Next, a correlation between the obtained cycle performance evaluation results and the crystallite size of the positive electrode active material was derived. Specifically, based on Table 2, a graph was created in which the crystallite size of the positive electrode active materials of Test Examples 1B to 10B was plotted on the X axis and the cycle performance on the Y axis. This graph is shown in Figure 3. In the graph of Figure 3, positive electrode active materials with cycle performance of 95.0% or higher were determined to have good cycle performance. The graph of Figure 3 shows that when the crystallite size of the (003) plane is within the range of 400 to 650 Å, good cycle performance of 95.0% or higher is obtained. The derived relationship (i.e., when the crystallite size of the positive electrode active material is within the range of 400 to 650 Å, the obtained cycle performance is 95.0% or higher) was used as the correlation between the obtained cycle performance evaluation results and the crystallite size of the positive electrode active material.

[0058] Next, as a positive electrode active material for a lithium ion battery for evaluation, a composition formula: Li 1.04 Ni 82.1 Co 14.5 Mn 2.8 Ta 0.5 A lithium-ion battery cathode active material represented by O2 was prepared, and its XRD diffraction pattern was measured under the same conditions as in Test Examples 1B to 10B. The crystallite size of the test cathode active material was then measured using the same method as in Test Examples 1B to 10B. The resulting crystallite size was 433 Å. Based on the correlation between the cycle performance evaluation results and the crystallite size of the cathode active material, the cycle performance of the test cathode active material was evaluated from the crystallite size of the test cathode active material. Since the resulting crystallite size was in the range of 400 to 650 Å, it was found that the cycle performance obtained by the test cathode active material was 95.0% or higher.

[0059] According to one embodiment of the present invention, a method for evaluating positive electrode active materials for lithium-ion batteries can be provided, enabling efficient evaluation of cycle characteristics without the need for battery fabrication. This may lead to the widespread use of non-fossil fuels, reducing the use of fossil fuels such as oil and gas, which currently account for the majority of energy generation, and potentially contributing to the suppression of global warming. Furthermore, since the main materials used are environmentally friendly materials such as lithium, carbon, manganese, nickel, and cobalt, and no hazardous substances such as cadmium, lead, or mercury are used, this may potentially reduce the environmental impact. Therefore, one embodiment of the present invention may contribute to the achievement of Goal 7 of the United Nations-led Sustainable Development Goals (SDGs), "Ensure access to affordable, reliable, sustainable, and modern energy for all," Goal 9, "Build resilient infrastructure, promote inclusive and sustainable industrialization, and foster innovation," and Goal 12, "Ensure sustainable consumption and production patterns."

Claims

1. Each has the formula: Li a Ni (1-b-c-d) Co b Mn c M d O2 (wherein M is one or more elements selected from Mg, Al, Ca, and Ta, and 0.98≦a≦1.10, 0.13≦b≦0.22, 0.02≦c≦0.32, and 0≦d≦0.007); calculating the crystallite size of the (003) plane from the XRD diffraction pattern of the positive electrode active material using Scherrer's equation; evaluating cycle characteristics of batteries fabricated using, as electrode materials for the batteries, positive electrode active materials having different crystallite sizes of the (003) plane from among the positive electrode active materials, and deriving a correlation between the obtained cycle characteristic evaluation results and the crystallite size of the positive electrode active material; measuring the crystallite size of a cathode active material for evaluation corresponding to the crystallite sizes of the group of cathode active materials based on the correlation, and evaluating the cycle characteristics of the cathode active material for evaluation from the crystallite size of the cathode active material for evaluation.

2. The positive electrode active material has the composition formula: Li a Ni (1-b-c-d) Co b Mn c M d 2. The method for evaluating a positive electrode active material for a lithium ion battery according to claim 1, wherein the positive electrode active material is represented by the formula: O2 (wherein M is one or more selected from Mg, Al, and Ca, and 1.03≦a≦1.10, 0.18≦b≦0.22, 0.28≦c≦0.32, and 0≦d≦0.007).

3. The method for evaluating a positive electrode active material for a lithium ion battery according to claim 2, wherein the cycle characteristics of the positive electrode active material for evaluation are evaluated as good if the crystallite size of the positive electrode active material for evaluation is 580 to 840 Å.

4. The positive electrode active material has the composition formula: Li a Ni (1-b-c-d) Co b Mn c M d 2. The method for evaluating a positive electrode active material for a lithium ion battery according to claim 1, wherein the positive electrode active material is represented by the formula: O2 (wherein M is Ta, and 0.98≦a≦1.09, 0.13≦b≦0.16, 0.02≦c≦0.05, and 0≦d≦0.007).

5. The method for evaluating a positive electrode active material for a lithium ion battery according to claim 4, wherein the cycle characteristics of the positive electrode active material for evaluation are evaluated as good if the crystallite size of the positive electrode active material for evaluation is 400 to 650 Å.

6. Composition formula: Li a Ni (1-b-c-d) Co b Mn c M d O2 (wherein M is one or more elements selected from Mg, Al, and Ca, and 1.03≦a≦1.10, 0.18≦b≦0.22, 0.28≦c≦0.32, and 0≦d≦0.007), wherein an XRD diffraction pattern of the positive electrode active material is measured, and the crystallite size of the (003) plane is calculated from the XRD diffraction pattern of the positive electrode active material using Scherrer's equation, and the crystallite size is 580 to 840 Å.

7. Composition formula: Li a Ni (1-b-c-d) Co b Mn c M d O2 (wherein M is Ta, and 0.98≦a≦1.09, 0.13≦b≦0.16, 0.02≦c≦0.05, and 0≦d≦0.007), wherein an XRD diffraction pattern of the positive electrode active material is measured, and a crystallite size of a (003) plane is calculated from the XRD diffraction pattern of the positive electrode active material using Scherrer's equation, and the crystallite size is 400 to 650 Å.

Images