Positive electrode active material for non-aqueous electrolyte secondary battery, method for producing the positive electrode active material, and non-aqueous electrolyte secondary battery

Abstract

To provide a positive electrode active material that has the same charge / discharge capacity as a non-aqueous electrolyte secondary battery using a non-aqueous electrolyte, even when used as the positive electrode of an all-solid-state battery, and a method for manufacturing the same. [Solution] A positive electrode active material for a non-aqueous electrolyte secondary battery, comprising particles of a lithium transition metal composite oxide and a coating layer containing a lithium niobium compound covering the surface of the particles. The particles contain Li, Ni, Co, and element M, with a molar ratio of Li:Ni:Co:M=t:(1-xy):x:y (where M is an element such as Mn, 0.95≦t≦1.20, 0.15)

Description

[Technical Field]

[0001] This disclosure relates to a positive electrode active material for a non-aqueous electrolyte all-solid-state secondary battery, a method for producing the positive electrode active material, and a non-aqueous electrolyte secondary battery using the positive electrode active material as a positive electrode material. [Background technology]

[0002] In recent years, with the spread of electric vehicles, there has been a strong demand for the development of small, lightweight rechargeable batteries with high energy density. One such rechargeable battery is the lithium-ion battery, which is a non-aqueous electrolyte rechargeable battery.

[0003] In typical non-aqueous electrolyte secondary batteries, lithium transition metal composite oxides such as LiCoO2, LiNiO2, and LiMn2O4 are used as the positive electrode active material, lithium metal, lithium alloys, metal oxides, and carbon are used as the negative electrode active material, and a non-aqueous electrolyte is used which is prepared by dissolving Li salts such as LiClO4 and LiPF6 as supporting salts in organic solvents such as ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate.

[0004] Among the components of non-aqueous electrolyte secondary batteries, the non-aqueous electrolyte, in particular, is a limiting factor in battery performance such as fast charging, safety, and lifespan due to its chemical properties, including heat resistance and potential window. Therefore, research and development are currently actively underway on all-solid-state batteries, which improve battery performance by using a solid electrolyte instead of a non-aqueous electrolyte.

[0005] Japanese Patent Publication No. 2014-056661 states that sulfide solid electrolytes exhibit high lithium ion conductivity during charging and discharging, and that using sulfide solid electrolytes as solid electrolytes can increase the energy density of all-solid-state batteries. However, as disclosed in "LiNbO3-coated LiCoO2as cathodematerial for all solid-state lithium secondary batteries" by Narumi Ohta et al., when a sulfide solid electrolyte and a positive electrode active material made of oxide come into contact, a reaction occurs at the interface between the solid electrolyte and the positive electrode active material during charging and discharging, generating a high-resistance phase that hinders the operation of the all-solid-state battery. In this document, it is proposed to provide a coating layer made of lithium niobate (LiNbO3) on the surface of the positive electrode active material by spraying a mixture of anhydrous ethanol in which lithium metal is dissolved and pentaethoxyniobium onto the surface of the positive electrode active material in order to suppress the generation of the high-resistance phase.

[0006] On the other hand, in conventional non-aqueous electrolyte secondary batteries using non-aqueous electrolytes, in order to increase their energy density, LiNiO2 and LiNi, which have high charge and discharge capacities, are used. 0.80 Co 0.15 Al 0.05 O2, LiLiLi 0.6 Co 0.2 Mn 0.2 It is considered preferable to use a positive electrode active material with a high Ni content, such as O2. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2014-056661 [Non-patent literature]

[0008] [Non-Patent Document 1] Narumi Ohta et al., “LiNbO3-coated LiCoO2 as cathode material for all solid-state lithium secondary batteries”, Electrochemistry Communications, Volume 9, Issue 7, July 2007, Pages 1486-1490 [Summary of the Invention] [Problem to be Solved by the Invention]

[0009] Studies have been conducted on the applicability of the above-mentioned cathode active material with high Ni content to all-solid-state batteries, and it has been found that in all-solid-state batteries, the energy density obtained from these cathode active materials is lower than the energy density expected from conventional non-aqueous electrolyte secondary batteries using non-aqueous electrolyte.

[0010] The present disclosure aims to provide a cathode active material for non-aqueous electrolyte secondary batteries (hereinafter also referred to as "cathode active material") and a method for producing the same, which exhibits high charge-discharge capacity when a cathode active material with high Ni content is used for cathodes of either non-aqueous electrolyte secondary batteries using electrolyte or all-solid-state batteries using solid electrolyte, and particularly has charge-discharge capacity equivalent to that exhibited by non-aqueous electrolyte secondary batteries using non-aqueous electrolyte even when used for cathodes of all-solid-state batteries. Furthermore, the present disclosure also aims to provide a non-aqueous electrolyte secondary battery that exhibits high charge-discharge capacity by using such a cathode active material for the cathode. Hereinafter, "non-aqueous electrolyte" refers to the combination of "non-aqueous electrolyte" and "solid electrolyte", and "non-aqueous electrolyte secondary battery" refers to the combination of "non-aqueous electrolyte secondary battery using non-aqueous electrolyte" and "all-solid-state battery". [Means for Solving the Problem]

[0011] The cathode active material for non-aqueous electrolyte secondary batteries according to one aspect of the present disclosure comprises lithium transition metal composite oxide particles, and a coating layer covering at least part of the surface of the particles, Particles of the lithium transition metal composite oxide contain Li, Ni, Co, and an element M in a molar ratio of Li:Ni:Co:M = t:(1-x-y):x:y, where M is at least one element selected from the group consisting of Mg, Al, Ca, Si, Mn, Ti, V, Fe, Cu, Cr, Zn, Zr, Nb, Mo, and W, 0.95 ≦ t ≦ 1.20, 0.15 < x ≦ 0.4, 0 ≦ y ≦ 0.4, and 0.33 ≦ (1-x-y) < 0.8, the coating layer contains a lithium niobium compound, a reflectance difference obtained by subtracting the reflectance at 400 nm from the reflectance at 800 nm of the diffuse reflection ultraviolet-visible spectrum of the positive electrode active material (reflectance difference = (reflectance at 800 nm) - (reflectance at 400 nm)) is greater than 0, and a change in reflectance in the range from 200 nm to 300 nm of the diffuse reflection ultraviolet-visible spectrum of the positive electrode active material (change in reflectance = [(reflectance at 300 nm) - (reflectance at 200 nm)] / 100 nm) is less than 0% / nm, characterized in that.

[0012] It is preferable that the reflectance difference is 0.2% or more and the change in reflectance is -0.005% / nm or less, and it is more preferable that the reflectance difference is 0.5% or more and the change in reflectance is -0.010% / nm or less.

[0013] The element M is preferably at least one element selected from Mn and Al, and more preferably Mn.

[0014] The positive electrode active material for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure can be suitably applied particularly to a positive electrode of an all-solid-state battery in which a solid electrolyte is used as the non-aqueous electrolyte.

[0015] A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure is characterized by mixing particles of the lithium transition metal composite oxide with a liquid coating agent containing a lithium compound and a niobium compound, drying the mixture, and performing heat treatment to obtain a positive electrode active material for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure, which has particles of the lithium transition metal composite oxide and a coating layer covering at least a portion of the surface of the particles.

[0016] The heat treatment is preferably carried out in an oxygen-containing atmosphere at a temperature of 200°C to 500°C for 1 hour to 5 hours.

[0017] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure comprises a positive electrode, a negative electrode, and a solid electrolyte (all-solid-state battery), or comprises a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte (non-aqueous electrolyte secondary battery using a non-aqueous electrolyte), characterized in that the positive electrode active material for non-aqueous electrolyte secondary batteries of the present disclosure is used as the positive electrode active material used in the positive electrode. [Effects of the Invention]

[0018] In one embodiment of the present disclosure, even with a positive electrode active material that has a high Ni content, the formation of a high-resistance phase due to reactions at the interface between the non-aqueous electrolyte, particularly a sulfide solid electrolyte, and the positive electrode active material is sufficiently suppressed. Therefore, the positive electrode active material of one embodiment of the present disclosure achieves high charge and discharge capacity when applied to the positive electrode of either a non-aqueous electrolyte secondary battery using a non-aqueous electrolyte or an all-solid-state battery using a solid electrolyte. In particular, when the positive electrode active material of one embodiment of the present disclosure is applied to the positive electrode of an all-solid-state battery, it is possible to achieve a charge and discharge capacity equivalent to that of a non-aqueous electrolyte secondary battery using a non-aqueous electrolyte. Thus, by applying the positive electrode active material of one embodiment of the present disclosure, it is possible to provide a non-aqueous electrolyte secondary battery that exhibits high charge and discharge capacity in both non-aqueous electrolyte secondary batteries using a non-aqueous electrolyte and all-solid-state batteries. [Brief explanation of the drawing]

[0019] [Figure 1]Figure 1 is a schematic process diagram showing a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to one embodiment of the present invention. [Figure 2] Figure 2 is a schematic cross-sectional view showing the configuration of the test battery (all-solid-state battery) used to evaluate the battery characteristics. [Modes for carrying out the invention]

[0020] The inventors of this disclosure have conducted diligent studies to solve the above-mentioned problems and have found that by coating at least a portion of the particles of a lithium transition metal composite oxide with a coating layer sufficiently containing a lithium niobium compound with high ion conductivity, it is possible to provide a positive electrode active material for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery that exhibits a high charge / discharge capacity, and in particular, a positive electrode active material for an all-solid-state battery and an all-solid-state battery that have a charge / discharge capacity equivalent to that of a lithium-ion secondary battery using a non-aqueous electrolyte. Based on this finding, the inventors have completed this disclosure. A preferred embodiment of this disclosure will be described below.

[0021] However, this disclosure is not limited to the following embodiments, and various modifications and improvements can be made to the following embodiments without departing from the scope of this disclosure.

[0022] (1) Positive electrode active material for nonaqueous electrolyte secondary batteries A positive electrode active material for a non-aqueous electrolyte secondary battery according to one embodiment of this disclosure will be described.

[0023] The positive electrode active material in this example comprises particles of a lithium transition metal composite oxide and a coating layer that covers at least a portion of the surface of the particles.

[0024] <Particles of lithium transition metal composite oxides> The lithium transition metal composite oxide particles constituting the positive electrode active material in this example contain lithium (Li), nickel (Ni), cobalt (Co), and element M in a molar ratio of Li:Ni:Co:M=t:(1-xy):x:y.

[0025] Furthermore, the element M is at least one element selected from magnesium (Mg), aluminum (Al), calcium (Ca), silicon (Si), manganese (Mn), titanium (Ti), vanadium (V), iron (Fe), copper (Cu), chromium (Cr), zinc (Zn), zirconium (Zr), niobium (Nb), molybdenum (Mo), and tungsten (W).

[0026] Furthermore, t, x, and y, which indicate the molar ratio, satisfy 0.95≦t≦1.20, 0.15<x≦0.4, 0≦y≦0.4, and 0.33≦(1-x-y)<0.8.

[0027] The value of t, which indicates the content of lithium constituting the lithium-transition metal composite oxide, is not less than 0.95 and not more than 1.20. It is preferable that the value of t is not less than 0.97 and not more than 1.10.

[0028] By setting the value of t indicating the lithium content to 0.95 or more, the internal resistance of an all-solid-state battery can be suppressed, and the output characteristics thereof can be improved. Further, by setting the value of t to 1.20 or less, the initial discharge capacity of the all-solid-state battery can be maintained at a high level. That is, by setting the value of t within the above range, the output characteristics and capacity characteristics of an all-solid-state battery using a positive electrode active material containing particles of the lithium-transition metal composite oxide can be improved.

[0029] Nickel in the particles of the lithium-transition metal composite oxide is an element that contributes to increasing the capacity of a non-aqueous electrolyte secondary battery. The value of (1-x-y), which indicates the nickel content, is not less than 0.33 and less than 0.8. It is preferable that the value of (1-x-y) is not less than 0.5 and less than 0.8.

[0030] That is, the positive electrode active material of the present example is characterized in that the content of nickel constituting the particles of the lithium-transition metal-containing composite oxide is high. Within the above range, the higher the value of (1-x-y), the lower the voltage required for charging, resulting in a higher battery capacity. By setting the value of (1-x-y) to less than 0.8, charging and discharging at high voltage can be suitably performed.

[0031] Cobalt is an element that contributes to reducing the irreversible capacity, which is the difference between the charge / discharge capacity and the discharge capacity, in non-aqueous electrolyte secondary batteries. The value of x, which indicates the cobalt content, is greater than 0.15 and less than or equal to 0.4. Preferably, the value of x is between 0.2 and 0.33.

[0032] By increasing the value of x to more than 0.15, it is possible to reduce the irreversible capacity of the all-solid-state battery. Conversely, by setting the value of x to 0.4 or less, it is possible to obtain a high battery capacity.

[0033] The lithium transition metal composite oxide particles contain lithium, nickel, and cobalt, in addition to an additive element M. The type and content of element M are appropriately selected depending on the application and required performance of the all-solid-state battery to which the positive electrode active material of this example is applied. Manganese or aluminum can be suitably used as element M. Element M is particularly preferably manganese.

[0034] Since some elements M do not contribute to the redox reaction, the value of y, which indicates the content of element M, should be 0.4 or less. Preferably, the value of y should be 0.2 or less, and more preferably 0.1 or less. Since the particles of lithium transition metal-containing composite oxide do not need to contain element M, the lower limit of y is 0.

[0035] Furthermore, the lithium transition metal composite oxide particles may include secondary particles formed by the aggregation of multiple primary particles, or they may include single primary particles. They may also be a mixture of single primary particles and secondary particles.

[0036] <Coating layer> The coating layer contains a lithium niobium compound. The lithium niobium compound is a compound containing lithium atoms and niobium atoms, and preferably contains lithium niobate. Lithium niobate is a composite oxide of niobium and lithium, and examples of lithium niobate include Li3NbO4, LiNbO3, LiNb3O8, and Li8Nb2O9. These lithium niobates fully exhibit the function of suppressing the formation of a high-resistance phase without hindering the conductivity of lithium ions in the coating layer. It is more preferable that the lithium niobium compound contains at least one of these. Lithium niobate also includes mixed forms such as Li3NbO4, LiNbO3, LiNb3O8, and Li8Nb2O9. Furthermore, from the viewpoint of more appropriately exhibiting the function of the lithium niobium compound, it is effective to have a balanced ratio of lithium to niobium in the coating layer. For example, if there is an excess of niobium in the ratio of lithium to niobium, the lithium conductivity of the coating layer may deteriorate. Therefore, among these lithium niobate compounds, LiNbO3 is more preferred.

[0037] Furthermore, amorphous Nb2O5 / LiNbO3 can also be used as the lithium niobium compound. When the lithium niobium compound is amorphous, lithium ion conductivity is improved, and the positive electrode resistance in non-aqueous electrolyte secondary batteries can be further reduced. The crystallinity of the lithium niobium compound can be confirmed by X-ray diffraction (XRD) measurement.

[0038] The coating layer covers at least a portion of the surface of the lithium transition metal composite oxide particles. The lithium niobium compound constituting the coating layer has the function of suppressing the reaction between the lithium transition metal composite oxide and the sulfide solid electrolyte. Therefore, by arranging the coating layer, it is possible to suppress the increase in interfacial resistance between the positive electrode active material and the non-aqueous electrolyte (solid electrolyte) in non-aqueous electrolyte secondary batteries, especially all-solid-state batteries.

[0039] There is no need for a clear boundary between the coating layer and the lithium transition metal composite oxide particles. Therefore, the coating layer is defined as a region or area on the surface side of the particles constituting the positive electrode active material in this example, where the niobium concentration is higher than that of the central region consisting of the lithium transition metal composite oxide covered by the coating layer. The coating layer, or the niobium constituting the coating layer, may be partially in solid solution with the lithium transition metal composite oxide.

[0040] In this example, the characteristics of the coating layer, specifically the coating state of the coating layer over the lithium transition metal composite oxide particles, are evaluated and determined by the diffuse reflectance ultraviolet-visible spectrum of the positive electrode active material obtained by diffuse reflectance ultraviolet-visible spectrophotometry. In other words, for the positive electrode active material in this example, the difference in reflectance obtained by subtracting the reflectance at 400 nm from the reflectance at 800 nm in the diffuse reflectance ultraviolet-visible spectrum of the positive electrode active material is greater than 0, and the change in reflectance in the range from 200 nm to 300 nm in the diffuse reflectance ultraviolet-visible spectrum of the positive electrode active material is less than 0% / nm. Therefore, the coating layer only needs to cover part or all of the surface of the lithium transition metal composite oxide particles, even if there is no clear boundary between the coating layer and the lithium transition metal composite oxide particles; however, in any case, the diffuse reflectance ultraviolet-visible spectrum of the positive electrode active material must have these characteristics.

[0041] The difference in reflectance can be expressed by the formula: Difference in reflectance = (Reflectance at 800 nm) - (Reflectance at 400 nm). Furthermore, the change in reflectance can be expressed by the formula: Change in reflectance = [(Reflectance at 300 nm) - (Reflectance at 200 nm)] / 100 nm.

[0042] Since electromagnetic waves of the wavelengths handled by diffuse reflectance ultraviolet-visible absorbance spectrophotometry do not penetrate into the interior of lithium transition metal composite oxide particles, information about the coating layer and the surface of the lithium transition metal composite oxide particles can be selectively obtained.

[0043] The lithium niobium compound contained in the coating layer exhibits a monotonically decreasing change in reflectance in the diffuse reflectance ultraviolet-visible spectrum from 200 nm to 300 nm, and a tendency for monotonically increasing reflectance from 400 nm to 800 nm. Therefore, the coating state of the lithium niobium compound coating layer can be evaluated and determined by the difference in reflectance obtained by subtracting the reflectance at 400 nm from the reflectance at 800 nm in the diffuse reflectance ultraviolet-visible spectrum, and by the change in reflectance in the diffuse reflectance ultraviolet-visible spectrum from 200 nm to 300 nm.

[0044] In other words, if the difference in reflectance obtained by subtracting the reflectance at 400 nm from the reflectance at 800 nm in the diffuse reflectance ultraviolet-visible spectrum of the positive electrode active material is greater than 0, and the change in reflectance in the range of 200 nm to 300 nm in the diffuse reflectance ultraviolet-visible spectrum is less than 0% / nm, it means that niobium is present in the coating layer as a lithium niobium compound and has not diffused to the lithium transition metal composite oxide particles. Therefore, by forming the coating layer such that the diffuse reflectance ultraviolet-visible spectrum of the positive electrode active material has these characteristics, a sufficient amount of niobium present in the coating layer as a lithium niobium compound is ensured. Thus, in the positive electrode active material of this example, a coating layer sufficient and uniform to suppress side reactions between the positive electrode active material and the solid electrolyte is formed.

[0045] Preferably, the difference in reflectance obtained by subtracting the reflectance at 400 nm from the reflectance at 800 nm in the diffuse reflectance ultraviolet-visible spectrum of the positive electrode active material is 0.3% or more, and the change in reflectance in the range from 200 nm to 300 nm in the diffuse reflectance ultraviolet-visible spectrum is -0.005% / nm or less. More preferably, the difference in reflectance obtained by subtracting the reflectance at 400 nm from the reflectance at 800 nm in the diffuse reflectance ultraviolet-visible spectrum of the positive electrode active material is 0.5% or more, and the change in reflectance in the range from 200 nm to 300 nm in the diffuse reflectance ultraviolet-visible spectrum is -0.01% / nm or less. This is because the diffuse reflectance ultraviolet-visible spectrum of lithium niobium compounds shows a monotonically increasing reflectance from 400 nm to 800 nm and a monotonically decreasing reflectance from 200 nm to 300 nm. This trend becomes more pronounced as the surface coverage increases, resulting in a large difference between the reflectance at 800 nm and the reflectance at 400 nm, and thus a smaller change in reflectance in the diffuse reflectance ultraviolet-visible spectrum between wavelengths of 200 nm and 300 nm.

[0046] The diffuse reflectance ultraviolet-visible spectrum of the positive electrode active material can be obtained using an ultraviolet-visible near-infrared spectrophotometer.

[0047] In this example, the positive electrode active material incorporates lithium into the coating layer, forming a lithium niobium compound. Compared to a coating layer formed solely of niobium compounds, this improves lithium ion conductivity. Therefore, the coating layer is less likely to hinder the intercalation reaction of lithium into the lithium transition metal composite oxide particles. This reduces the internal resistance of non-aqueous electrolyte secondary batteries, particularly all-solid-state batteries, and suppresses the decrease in their discharge capacity.

[0048] <Volume-average particle size MV> The volume-average particle diameter MV of lithium transition metal composite oxide particles constituting the positive electrode active material is, for example, 5 µm or more and 30 µm or less, preferably 5 µm or more and 20 µm or less. When the volume-average particle diameter MV of lithium transition metal composite oxide particles is less than 5 µm, an increase in the specific surface area of the positive electrode active material allows a non-aqueous electrolyte secondary battery to obtain high output, but the packing density of the positive electrode decreases, which reduces the charge-discharge capacity per volume, and the dispersibility of the conductive agent and the positive electrode active material may deteriorate when preparing an electrode paste. In addition, since the voltage applied to individual positive electrode active material particles in the electrode becomes non-uniform, particles subjected to high voltage deteriorate with repeated charge and discharge, which may reduce the charge-discharge capacity of the non-aqueous electrolyte secondary battery. Conversely, when the volume-average particle diameter MV of lithium transition metal composite oxide particles exceeds 30 µm, the specific surface area of the positive electrode active material decreases, and the interface with the electrolytic solution reduces, which decreases the number of paths through which lithium ions enter and exit, so the resistance of the positive electrode increases and the output characteristics of the battery may degrade. Note that the volume-average particle diameter MV is a value measured by laser diffraction scattering method.

[0049] <Broadening of particle size distribution> [(d90 - d10) / volume-average particle diameter MV], which is an indicator showing the broadening of the particle size distribution of lithium transition metal composite oxide particles, is not particularly limited, but from the viewpoint of achieving high packing properties, it is preferably 0.70 or more, and more preferably 0.70 or more and 1.2 or less. Here, d10 means the particle diameter at which the cumulative volume, when accumulating the number of particles for each particle diameter from the smaller particle diameter side, accounts for 10% of the total volume of all particles, and d90 similarly means the particle diameter at which the cumulative volume accounts for 90% of the total volume of all particles. In addition, d10 and d90 can be obtained from the cumulative volume value measured with a laser diffraction scattering particle size analyzer, similarly to the average particle diameter.

[0050] The BET specific surface area of lithium transition metal composite oxide particles is 0.3 m 2 / g or more and 1.8 m 2 / g or less, preferably 0.4 m 2 / g or more and 1.0 m 2It is more preferable that the BET specific surface area is less than or equal to / g. When the BET specific surface area is within the above range, the contact area with non-aqueous electrolytes, especially solid electrolytes, can be made sufficient, thereby reducing the positive electrode resistance and obtaining sufficient output characteristics. The BET specific surface area is measured by the BET method using nitrogen gas adsorption.

[0051] <Average particle size of primary particles> When the lithium transition metal composite oxide particles include secondary particles composed of multiple primary particles, the average particle size of the primary particles is preferably, for example, 0.2 μm or more and 1.0 μm or less, and more preferably 0.3 μm or more and 0.7 μm or less. When the average particle size of the primary particles is within the above range, higher output characteristics, battery capacity, and even higher cycle characteristics can be obtained when used as the positive electrode of a non-aqueous electrolyte secondary battery. If the average particle size of the primary particles is less than 0.2 μm, the firing may be insufficient, and sufficient battery performance may not be obtained. Also, if the average particle size of the primary particles exceeds 1.0 μm, high output characteristics and high cycle characteristics may not be obtained.

[0052] The positive electrode active material in this example may contain lithium transition metal composite oxides other than the lithium transition metal composite oxide particles described above, to the extent that they do not impair the effects of this disclosure.

[0053] (2) Method for producing positive electrode active material for non-aqueous electrolyte secondary batteries Next, a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to an example of one embodiment of the present disclosure will be described with reference to the drawings. Figure 1 is a schematic process diagram showing the method for producing the positive electrode active material in this example. In the method for producing the positive electrode active material in this example, lithium transition metal composite oxide particles and a liquid coating agent containing a lithium compound and a niobium compound are mixed, dried, and heat-treated to obtain a positive electrode active material for an all-solid-state lithium-ion secondary battery having lithium transition metal composite oxide particles and a coating layer covering at least a portion of the surface of the particles.

[0054] As shown in Figure 1, the method for producing the positive electrode active material in this example comprises the following three steps.

[0055] <Step 1>: [Lithium transition metal composite oxide synthesis step S1] In the method for producing the positive electrode active material in this example, it is possible to use lithium transition metal composite oxide particles obtained by any manufacturing method. Therefore, in the method for producing the positive electrode active material in this example, the lithium transition metal composite oxide synthesis step S1 is an arbitrary step.

[0056] In the lithium transition metal composite oxide synthesis step S1, particles of nickel composite hydroxide, or particles of nickel composite oxide obtained by oxidizing and roasting nickel composite hydroxide, are used as a precursor. These precursors are mixed with a lithium compound and then calcined to obtain particles of lithium transition metal composite oxide.

[0057] The lithium transition metal composite oxide synthesis step S1 is not particularly limited in its specific means; for example, nickel composite hydroxide particles as a precursor can be obtained by crystallization. Both batch and continuous methods are applicable for producing the nickel composite hydroxide particles. From the viewpoint of cost and obtaining nickel composite hydroxide particles with a broader particle size distribution, it is preferable to apply a continuous method in which nickel composite hydroxide particles overflowing from the reaction vessel are continuously recovered.

[0058] The firing conditions for the mixture obtained by mixing the precursor and the lithium compound are not particularly limited and are set as appropriate so that the lithium transition metal composite oxide can obtain the desired properties. In addition, if slight sintering is observed in the lithium transition metal composite oxide particles after firing, a crushing treatment can be added.

[0059] <2nd process>: [Coating process S2] The coating step S2 forms a coating layer on at least a portion of the surface of the lithium transition metal composite oxide particles. Specifically, for example, the lithium transition metal composite oxide particles obtained in the lithium transition metal composite oxide synthesis step S1 are mixed with a liquid coating agent, dried, and then subjected to heat treatment in an oxygen-containing atmosphere to provide a coating layer on the surface of the lithium transition metal composite oxide particles.

[0060] In coating step S2, first, the specific surface area (m²) of the lithium transition metal composite oxide particles obtained in lithium transition metal composite oxide synthesis step S1 is measured. 2 The amount of niobium supported in the target coating layer is measured (coating preparation step) and a liquid coating agent is prepared according to the total content of metal elements other than Li (Ni, Co, element M).

[0061] More specifically, the difference in reflectance obtained by subtracting the reflectance at 400 nm from the reflectance at 800 nm in the diffuse reflectance ultraviolet-visible spectrum of the final non-aqueous electrolyte secondary battery, and the correlation between the change in reflectance in the range of 200 nm to 300 nm in the diffuse reflectance ultraviolet-visible spectrum, the composition of the lithium transition metal composite oxide particles, and the specific surface area of ​​the lithium transition metal composite oxide particles are obtained in advance through tests and other means.

[0062] The specific surface area of ​​lithium transition metal composite oxide particles can be measured by methods such as the nitrogen adsorption BET method. The amount of niobium supported in the coating layer per 1 g of positive electrode active material is measured by the specific surface area (m²) of the lithium transition metal composite oxide particles. 2 By dividing by ( / g), the surface area of ​​the lithium transition metal composite oxide particles is calculated as 1 m². 2 The amount of Nb loaded per unit can be determined.

[0063] In this example of the positive electrode active material, the amount of niobium supported in the coating layer is regulated solely by the difference in reflectance obtained by subtracting the reflectance at 400 nm from the reflectance at 800 nm in the diffuse reflectance ultraviolet-visible spectrum of the positive electrode active material, and by the change in reflectance between 200 nm and 300 nm in the diffuse reflectance ultraviolet-visible spectrum. However, for example, the amount of niobium supported is determined by the surface area of ​​the lithium transition metal composite oxide particles (1 m²). 2 The ratio can preferably be 0.1 μmol or more and 1.0 μmol or less, and more preferably 0.2 μmol or more and 0.5 μmol or less per unit.

[0064] On the other hand, the amount of lithium supported in the coating layer is determined according to the lithium niobium compound that constitutes the coating layer. For example, if the lithium niobium compound is lithium niobate (LiNbO3), the amount of lithium supported is determined so that the molar ratio of niobium to lithium is Nb:Li = 1:1.

[0065] The liquid coating agent in this example contains not only a niobium compound but also a lithium compound. The liquid coating agent is not particularly limited as long as it contains a niobium compound and a lithium compound. As a liquid coating agent, for uniform coating, a solution in which the niobium compound and lithium compound are dissolved in a solvent, or low-melting-point niobium compound and lithium compound that are liquid at room temperature or melt with low-temperature heat treatment can be preferably used. It is also possible to use a mixture of a solid powder niobium compound (e.g., niobium oxide Nb2O5) and a lithium compound (e.g., lithium hydroxide, lithium carbonate) with a lithium transition metal composite oxide, but it is difficult to achieve a high coverage rate due to poor fluidity on the particle surface. In that case, it is effective to simultaneously add a solvent that does not alter the lithium transition metal composite oxide and in which the niobium compound and lithium compound have a predetermined solubility, but as described above, a higher coverage rate can be obtained by using a liquid coating material.

[0066] Examples of niobium compounds include one or more selected from alkoxides such as niobium ethoxide or aqueous solutions of niobium oxide sol in which fine particles of niobium oxide are uniformly dispersed in an aqueous solution. The lithium compound is not particularly limited, and examples include lithium hydroxide, lithium oxide, lithium nitrate, lithium ethoxide, lithium carbonate, and mixtures thereof. As a liquid coating agent, it is particularly preferable to use a mixture of aqueous lithium hydroxide in niobium oxide sol, as it can be easily prepared and the inclusion of impurities can be suppressed.

[0067] In coating step S2, the lithium transition metal composite oxide particles and a liquid coating agent are mixed to obtain a mixture (mixture preparation step). A general mixer can be used for mixing. After mixing, drying is performed (drying step), and then heat treatment is carried out to fix the lithium niobium compound as a coating layer on at least a portion of the surface of the lithium transition metal composite oxide particles (heat treatment step). From the viewpoint of forming a more uniform coating layer, it is preferable to carry out the mixture preparation step and the drying step in parallel, and for this purpose, it is preferable to use a rolling flow coating apparatus. When a rolling flow coating apparatus is used, the lithium transition metal composite oxide particles and the liquid coating agent, which are flowing due to a heated airflow in the apparatus, are mixed, and the mixture preparation step and the drying step are repeated in parallel, thereby obtaining a more uniform coating layer.

[0068] In the drying step, drying is carried out at a temperature sufficient to remove solvents and other contaminants from the coating. The drying temperature can be arbitrary as long as the solvent is removed, but for example, drying can be carried out at a temperature between 80°C and 300°C.

[0069] The heat treatment conditions for the heat treatment step are not particularly limited, but it is preferable to perform the heat treatment in an oxygen-containing atmosphere, such as an air atmosphere, at a temperature of 200°C to 500°C for 1 to 5 hours. After the heat treatment, the cathode active material consisting of lithium transition metal composite oxide particles having a coating layer can be obtained by cooling to room temperature. The heat treatment step can strengthen the bond between the coating layer and the lithium transition metal composite oxide particles.

[0070] The oxygen concentration in the oxygen-containing atmosphere is preferably equal to or greater than the oxygen concentration in the air atmosphere, i.e., an oxygen concentration of 20% by volume or more. By making the oxygen-containing atmosphere during heat treatment equal to or greater than the oxygen concentration in the air atmosphere, the occurrence of oxygen defects inside the resulting positive electrode active material can be further suppressed. The oxygen-containing atmosphere may be an oxygen atmosphere, and the upper limit of the oxygen concentration in the oxygen-containing atmosphere is 100% by volume.

[0071] By setting the heat treatment temperature to 200°C or higher, the retention of impurities contained in the coating agent within the positive electrode active material can be particularly suppressed, and the niobium compound and lithium compound in the coating layer can react to exist as a lithium niobium compound. Furthermore, by setting the heat treatment temperature to 500°C or lower, excessive diffusion of the components of the coating layer can be suppressed, and the shape of the coating layer can be maintained.

[0072] By extending the heat treatment time to one hour or more, it becomes particularly effective in suppressing the retention of impurities contained in the coating agent within the positive electrode active material. Furthermore, since no significant changes are observed in the resulting positive electrode active material even when heat treatment is performed for longer than five hours, it is preferable to limit the heat treatment time to five hours or less from the viewpoint of energy efficiency.

[0073] If slight sintering is observed in the positive electrode active material obtained after the coating process S2, a crushing treatment can be applied.

[0074] (3) Non-aqueous electrolyte secondary battery An example of the configuration of a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure will be described. The non-aqueous electrolyte secondary battery in this example has a configuration comprising a positive electrode using a positive electrode active material according to one embodiment of the present disclosure, a negative electrode, and a non-aqueous electrolyte (solid electrolyte or non-aqueous electrolyte solution).

[0075] <Positive electrode> The positive electrode is formed by molding a positive electrode mixture. The positive electrode is then processed as appropriate according to the battery being used. For example, pressure compression using a press may be performed to increase electrode density.

[0076] The positive electrode mixture used in all-solid-state batteries is formed by mixing a powdered positive electrode active material with a solid electrolyte.

[0077] Solid electrolytes are added to electrodes to provide them with appropriate ionic conductivity. The material of the solid electrolyte is not particularly limited, but examples include Li3PS4 and Li7P3S. 11 Li 10 GeP2S 12 Sulfide solid electrolytes such as Li7La3Zr2O 12 Li 0.34 La 0.51 TiO 2.94 Oxide solid electrolytes such as PEO and polymer-based electrolytes such as PEO can be used.

[0078] Binding agents and conductive additives may also be added to the positive electrode mixture. Conductive additives are added to provide the electrode with appropriate conductivity. The material of the conductive additive is not particularly limited, but for example, graphite such as natural graphite, artificial graphite, and expanded graphite, or carbon black-based materials such as acetylene black and Ketjenblack (registered trademark) can be used. The content of the conductive additive is not particularly limited and is appropriately determined according to the performance of the battery to be used. For example, if the total solid content of the positive electrode mixture is 100% by mass, the content of the conductive additive can be 0.5% by mass or more and 20% by mass or less, preferably 1% by mass or more and 10% by mass or less.

[0079] The binder plays the role of holding the positive electrode active material together. The binder used in the positive electrode mixture is not particularly limited, but for example, one or more selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ethylene propylene diene rubber, styrene butadiene, cellulose resin, polyacrylic acid, etc., can be used. The binder content is not particularly limited and is appropriately determined according to the performance of the battery to be used. For example, if the total solid content of the positive electrode mixture is 100% by mass, the binder content can be 1% by mass or more and 30% by mass or less, preferably 2% by mass or more and 15% by mass or less.

[0080] The mixing ratio of the positive electrode active material to the solid electrolyte in the positive electrode mixture is not particularly limited. For example, the content of the positive electrode active material in the positive electrode mixture can be 50 parts by mass or more and 90 parts by mass or less, and the content of the solid electrolyte can be 10 parts by mass or more and 50 parts by mass or less.

[0081] The cathode mixture may also contain other additives (such as thickeners).

[0082] In this example, even when a sulfide solid electrolyte is included as the solid electrolyte constituting the positive electrode, the presence of a coating layer containing a lithium niobium compound on at least a portion of the surface of the positive electrode active material makes direct contact between the positive electrode active material and the sulfide solid electrolyte difficult, and sufficiently suppresses the increase in resistance caused by the reaction between the positive electrode active material and the sulfide solid electrolyte.

[0083] The positive electrode mixture paste used in non-aqueous electrolyte secondary batteries using a non-aqueous electrolyte is prepared by mixing a binder and a conductive additive with the positive electrode active material, and further adding solvents such as activated carbon and viscosity adjusters as needed, and then kneading these together. The mixing ratio of each component in the positive electrode mixture paste can be, for example, if the solid content of the positive electrode mixture excluding the solvent is 100 parts by mass, the content of the positive electrode active material can be 60 parts by mass or more and 95 parts by mass or less, the content of the conductive additive can be 1 part by mass or more and 20 parts by mass or less, and the content of the binder can be 1 part by mass or more and 20 parts by mass or less.

[0084] In this case, the resulting positive electrode mixture paste is applied to the surface of a current collector, for example, made of aluminum foil, and dried to allow the solvent to evaporate. If necessary, pressure is applied using a roll press or the like to increase the electrode density. In this way, a sheet-like positive electrode is produced.

[0085] Furthermore, the configuration and manufacturing method of the positive electrode are not limited to those exemplified above, and any other known configuration and manufacturing method for the positive electrode can also be applied.

[0086] <Negative electrode> The negative electrode is formed by molding a negative electrode mixture. Although the components and their proportions of the negative electrode mixture differ, the negative electrode is essentially formed by the same manufacturing method as the positive electrode described above, and various treatments are performed as necessary, just as with the positive electrode.

[0087] The negative electrode mixture used in all-solid-state batteries can be prepared by mixing a negative electrode active material with a solid electrolyte. The negative electrode mixture used in non-aqueous electrolyte secondary batteries is prepared by mixing a binder with the negative electrode active material, adding a suitable solvent to form a paste, applying the paste to the surface of a metal foil current collector such as copper, drying it, and compressing it as needed. As the negative electrode active material, for example, an intercalating material capable of intercalating and deintercalating lithium ions can be used.

[0088] The absorbed material is not particularly limited, but for example, one or more selected from natural graphite, artificial graphite, calcined organic compounds such as phenolic resin, and powdered carbon materials such as coke can be used. When such an absorbed material is used as the negative electrode active material, a sulfide electrolyte such as Li3PS4 can be used as the solid electrolyte, similar to the positive electrode.

[0089] Furthermore, the negative electrode can also be composed of a sheet-like member made of a material containing a metal that alloys with lithium, such as metallic lithium or indium.

[0090] <Non-aqueous electrolytes> Among non-aqueous electrolytes, solid electrolytes consist of solids that are lithium-ion conductive and possess the property of being able to withstand high voltages. Examples of solid electrolytes include inorganic solid electrolytes and organic solid electrolytes.

[0091] Inorganic solid electrolytes include sulfide solid electrolytes and oxide solid electrolytes.

[0092] The sulfide solid electrolyte is not particularly limited and can be used as long as it contains sulfur (S) and has lithium ion conductivity and electronic insulating properties. Examples of sulfide solid electrolytes include Li2S-P2S5, Li2S-SiS2, LiI-Li2S-SiS2, LiI-Li2S-P2S5, LiI-Li2S-B2S3, Li3PO4-Li2S-Si2S, Li3PO4-Li2S-SiS2, LiPO4-Li2S-SiS, LiI-Li2S-P2O5, and LiI-Li3PO4-P2S5.

[0093] The oxide solid electrolyte is not particularly limited and can be used as long as it contains oxygen (O) and has lithium ion conductivity and electronic insulating properties. Examples of oxide solid electrolytes include lithium phosphate (Li3PO4) and Li3PO4N. X LiBO2N X , LiNbO3, LiTaO3, Li2SiO3, Li4SiO4-Li3PO4, Li4SiO4-Li3VO4, Li2O-B2O3-P2O5, Li2O-SiO2, Li2O-B2O3-ZnO, Li 1+X Al X Ti 2-X (PO4)3(0≦X≦1), Li 1+X Al X Ge 2-X (PO4)3(0≦X≦1), LiTi2(PO4)3, Li 3X La 2 / 3-X TiO3(0≦X≦2 / 3), Li5La3Ta2O 12 Li7La3Zr2O 12 Li6BaLa2Ta2O 12 Li 3.6 Si0.6 P 0.4 Examples include O4.

[0094] In addition, inorganic solid electrolytes other than those mentioned above may be used; for example, Li3N, LiI, Li3N-LiI-LiOH, etc., can also be used.

[0095] The organic solid electrolyte is not particularly limited as long as it is a polymer compound exhibiting ionic conductivity; for example, polyethylene oxide, polypropylene oxide, or copolymers thereof can be used. Furthermore, the organic solid electrolyte may contain a supporting salt (lithium salt).

[0096] Among non-aqueous electrolytes, non-aqueous electrolytes can be prepared by dissolving a lithium salt, which is a supporting salt, in an organic solvent. Organic solvents used in non-aqueous electrolytes can be selected from the following: cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate; linear carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate; ether compounds such as tetrahydrofuran, 2-methyltetrahydrofuran, and dimethoxyethane; sulfur compounds such as ethylmethyl sulfone and butanesultone; and phosphorus compounds such as triethyl phosphate and trioctyl phosphate. One of these can be used alone or in mixtures of two or more.

[0097] As supporting salts, LiPF6, LiBF4, LiClO4, LiAsF6, LiN(CF3SO2)2, and composite salts thereof can be used. The non-aqueous electrolyte may also contain radical scavengers, surfactants, and flame retardants.

[0098] <Shape and structure of non-aqueous electrolyte secondary batteries> This section describes examples of the arrangement and configuration of components in the non-aqueous electrolyte secondary battery of this example. The non-aqueous electrolyte secondary battery of this example, which comprises a positive electrode, a negative electrode, and a solid electrolyte or non-aqueous electrolyte, can take on various shapes, such as coin-type or stacked type. In any case, a structure can be adopted in which the positive electrode and negative electrode are stacked with a solid electrolyte in between, or a structure in which the positive electrode and negative electrode are stacked with a separator in between to form an electrode body, and the resulting electrode body is impregnated with a non-aqueous electrolyte. The positive electrode current collector and the positive electrode terminal that is open to the outside, and the negative electrode current collector and the negative electrode terminal that is open to the outside are connected using current collector leads, etc., and the battery can be sealed in a battery case to form a non-aqueous electrolyte secondary battery.

[0099] <Characteristics of non-aqueous electrolyte secondary batteries> The non-aqueous electrolyte secondary battery of this example, using a positive electrode active material according to one embodiment of the present disclosure, can exhibit high charge and discharge capacities. In particular, the all-solid-state battery can have a charge and discharge capacity equivalent to that of a non-aqueous electrolyte secondary battery using a non-aqueous electrolyte.

[0100] Specifically, a test battery was constructed using the positive electrode active material of this embodiment as the positive electrode, and the current density was set to 0.2 mA / cm². 2 Preferably, the initial discharge capacity, which is the discharge capacity when charged to a cutoff voltage of 3.7V (vs. Li-In), discharged to a cutoff voltage of 1.9V (vs. Li-In) after a 1-hour rest, is 132mAh / g or more, and more preferably 140mAh / g or more.

[0101] The applications of the non-aqueous electrolyte secondary battery in this example are not particularly limited and can be suitably used in applications requiring various power sources. Furthermore, even when an all-solid-state battery is used as the non-aqueous electrolyte secondary battery in this example, it has the same charge / discharge capacity as a non-aqueous electrolyte secondary battery using a non-aqueous electrolyte, and can be miniaturized. Therefore, the non-aqueous electrolyte secondary battery in this example, whether an all-solid-state battery or a non-aqueous electrolyte secondary battery using a non-aqueous electrolyte is used, is suitable as a power source for electric vehicles where mounting space is limited. [Examples]

[0102] A positive electrode active material for a non-aqueous electrolyte secondary battery, a method for producing the same, and a non-aqueous electrolyte secondary battery according to an example of one embodiment of this disclosure will be described in detail with reference to examples in which these are applied to an all-solid-state battery. This disclosure is not limited to these examples. The analytical methods for the metals contained in the positive electrode active material and the various evaluation methods for the positive electrode active material in the examples and comparative examples are as follows.

[0103] The composition of the lithium-ion transition metal composite oxide was measured using an ICP emission spectrometer (VARIAN 725ES).

[0104] The diffuse reflectance ultraviolet-visible spectrum of the positive electrode active material was measured using an ultraviolet-visible near-infrared spectrophotometer (JASCO Corporation, V-770iRM) under the following conditions.

[0105] Equipment: JASCO V-770iRM Integrating sphere unit ISN-923 model Measurement mode: Reflectance Bandwidth: 5nm Response: Medium Data acquisition interval: 0.1nm Scan speed: 400nm / min Wavelength range: 200nm~800nm Light source switching wavelength: 340nm Background (baseline): Barium sulfate (special grade) manufactured by Kanto Chemical Co., Ltd. was placed in the same powder cell as the sample and measured. Measurement method: Powder cells were used, with 0.2g of each sample enclosed.

[0106] [Example 1] 1. Manufacturing of lithium transition metal composite oxide particles A nickel-based hydroxide powder was mixed with lithium hydroxide, and then calcined to obtain lithium transition metal composite oxide particles. The composition of the obtained lithium transition metal composite oxide was Li 1.0 Ni 1 / 3Co 1 / 3 Mn 1 / 3 It was O2.

[0107] 2. Manufacturing of coating agents 150 g of niobium oxide sol (manufactured by Taki Chemical Co., Ltd., Viral Nb-G6000) was mixed with an aqueous lithium hydroxide solution so that the molar ratio of niobium to lithium in the coating agent was Nb:Li = 1:1. The amount of coating solution applied was such that the amount of niobium in the niobium oxide sol was 1.0 mol% relative to the total amount of Ni, Co, and Mn (total amount of metal elements other than Li) contained in the lithium metal composite oxide powder, and the amount of lithium in the coating solution was 1.0 mol%.

[0108] 3. Manufacturing of positive electrode active material 500 g of lithium transition metal composite oxide particles were placed in a rolling flow coating apparatus (manufactured by Powrec Co., Ltd., model: FP-MP-01D), the supply air temperature was set to 120°C, and the coating agent was added at a rate of 7 μL / min per gram of lithium transition metal composite oxide particles for 171 minutes while drying with mixing. (Mixture preparation step and drying step).

[0109] The material was heat-treated at 300°C for 5 hours in an oxygen-containing atmosphere using an atmospheric firing furnace (manufactured by Siliconit Co., Ltd., model: BM-50100M) (heat treatment step). After that, it was cooled to room temperature to obtain lithium transition metal composite oxide particles having a coating layer which is the positive electrode active material.

[0110] The difference in reflectance of the obtained positive electrode active material, obtained by subtracting the reflectance at 400 nm from the reflectance at 800 nm in the diffuse reflectance ultraviolet-visible spectrum, was 0.6%, and the change in reflectance in the range from 200 nm to 300 nm in the diffuse reflectance ultraviolet-visible spectrum was -0.01% / nm. Table 1 shows the composition, BET specific surface area, presence or absence of coating layer, niobium content, lithium content, and reflectance at 800 nm of the positive electrode active material. Table 2 shows the reflectance at 400 nm, the difference in reflectance obtained by subtracting the reflectance at 400 nm from the reflectance at 800 nm (difference in reflectance = (reflectance at 800 nm) - (reflectance at 400 nm)), and the change in reflectance in the range from 200 nm to 300 nm (change in reflectance = [(reflectance at 300 nm) - (reflectance at 200 nm)]). The same applies to Examples 2-7 and Comparative Examples 1-5.

[0111] 4. Fabrication of all-solid-state batteries To evaluate the capacity of the obtained positive electrode active material, a battery with the structure shown in Figure 2 (hereinafter referred to as the "test battery") was used. The test battery 1 consists of a case and compacted powder cells 2 housed inside the case.

[0112] The case comprises a hollow negative electrode can 3 with one end open, and a positive electrode can 4 positioned in the opening of the negative electrode can 3. When the positive electrode can 4 is positioned in the opening of the negative electrode can 3, a space for housing the compacted powder cell 2 is formed between the positive electrode can 4 and the negative electrode can 3. The positive electrode can 4 is fixed to the negative electrode can 3 with a wing nut 5 and a nut 6.

[0113] The negative electrode can 3 is equipped with a negative terminal (not shown), and the positive electrode can 4 is equipped with a positive terminal (not shown). The case is equipped with an insulating sleeve 7, which secures the negative electrode can 3 and the positive electrode can 4 so that they remain in a non-contact state.

[0114] A pressure screw 8 is provided at one closed end of the negative electrode can 3. After fixing the positive electrode can 4 to the negative electrode can 3, the pressure screw 8 is tightened toward the compacted powder cell housing space, thereby maintaining the compacted powder cell 2 under pressure through the hemispherical washer 9. A screw-in plug 10 is provided at the end of the negative electrode can 3 where the pressure screw 8 is located. O-rings 11 are provided between the negative electrode can 3 and the positive electrode can 4, and between the negative electrode can 3 and the plug 10, sealing the gap between the negative electrode can 3 and the positive electrode can 4 and maintaining airtightness within the case.

[0115] The compacted cell 2 consists of a positive electrode layer, a solid electrolyte layer, and a negative electrode layer, and is composed of pellets stacked in this order. The positive electrode layer is in contact with the inner surface of the positive electrode can 4 through the lower current collector 12, and the negative electrode layer is in contact with the inner surface of the negative electrode can 3 through the upper current collector 13, a hemispherical washer 9, and a pressure screw 8, as it is housed in a case. The lower current collector 12, the compacted cell 2, and the upper current collector 13 are protected by a sleeve 14 to prevent the positive and negative electrode layers from making electrical contact.

[0116] A test battery 1 was fabricated as follows.

[0117] First, 80 mg of the synthesized solid electrolyte was pressurized at 25 MPa using a pellet former to obtain a solid electrolyte pellet. Next, 70 mg of the positive electrode active material and 30 mg of the solid electrolyte were mixed in a mortar. The solid electrolyte pellet and 15 mg of the mixture of positive electrode active material and solid electrolyte were placed in the pellet former and pressurized at 360 MPa to form a positive electrode layer on the solid electrolyte pellet. The electrodes were stacked in the following order from bottom to top: the lower electrode, the pellet with the positive electrode layer facing downwards, the indium foil, and the upper electrode, and pressed with 9 kN to form the electrode. The electrode was sealed in a case and the pressure screw was tightened with a torque of 6 N·m to 7 N·m. Test battery 1 was fabricated in a glove box with an Ar atmosphere where the dew point was controlled to -80°C.

[0118] 5. Evaluation of All-Solid-State Batteries The charge and discharge capacity of the fabricated test battery 1 was evaluated as follows. The evaluation results are shown in Table 1. The same process was followed for Examples 2-7 and Comparative Examples 1-5.

[0119] [Initial discharge capacity] The initial discharge capacity of an all-solid-state battery was determined by fabricating a test battery 1 using indium foil as the negative electrode, leaving it for approximately 24 hours, and then measuring the current density to the positive electrode after the open circuit voltage (OCV) had stabilized. The current density was then measured at 0.2 mA / cm². 2 The discharge capacity (initial discharge capacity) was measured after charging to a cutoff voltage of 3.7V (vs. Li-In), letting it sit for 1 hour, and then discharging it to a cutoff voltage of 1.9V (vs. Li-In). The measured result was 140mAh / g.

[0120] [Example 2] Composition is Li 1.03 Ni 0.75 Co 0.11 Mn 0.11 Al 0.03 Except for obtaining lithium transition metal composite oxide particles (O2) and using them, lithium transition metal composite oxide particles, a positive electrode active material, and a secondary battery using the positive electrode active material were obtained under the same conditions as in Example 1. The difference in reflectance of the obtained positive electrode active material was 0.7%, and the change in reflectance was -0.011% / nm. The initial discharge capacity was 145 mAh / g.

[0121] [Example 3] Composition is Li 1.0 Ni 0.5 Co 0.3 Mn 0.2 Except for obtaining lithium transition metal composite oxide particles (O2) and using them, lithium transition metal composite oxide particles, a positive electrode active material, and a secondary battery using the positive electrode active material were obtained under the same conditions as in Example 1. The difference in reflectance of the obtained positive electrode active material was 0.6%, and the change in reflectance was -0.013% / nm. The initial discharge capacity was 143 mAh / g.

[0122] [Example 4] In the coating preparation process, the amount of coating solution applied was adjusted to be 2.0 mol% of the niobium in the niobium oxide sol and 2.0 mol% of the lithium in the coating solution relative to the total amount of Ni, Co, and Mn contained in the lithium metal composite oxide particles (total amount of metal elements other than Li). Except for the other conditions being the same as in Example 1, lithium transition metal composite oxide particles, a positive electrode active material, and a secondary battery using the positive electrode active material were obtained. The difference in reflectance of the obtained positive electrode active material was 0.8%, and the change in reflectance was -0.016% / nm. The initial discharge capacity was 142 mAh / g.

[0123] [Example 5] In the coating preparation process, the amount of coating solution applied was adjusted so that the amount of niobium in the niobium oxide sol was 0.7 mol% relative to the total amount of Ni, Co, and Mn contained in the lithium metal composite oxide particles (total amount of metal elements other than Li), and the amount of lithium in the coating solution was 0.7 mol%. Except for these other adjustments, lithium transition metal composite oxide particles, a positive electrode active material, and a secondary battery using the positive electrode active material were obtained under the same conditions as in Example 1. The difference in reflectance of the obtained positive electrode active material was 0.4%, and the change in reflectance was -0.012% / nm. The initial discharge capacity was 143 mAh / g.

[0124] [Example 6] In the coating preparation process, the amount of coating solution applied was adjusted to be 3.0 mol% of the niobium in the niobium oxide sol and 3.0 mol% of the lithium in the coating solution relative to the total amount of Ni, Co, and Mn contained in the lithium metal composite oxide particles (total amount of metal elements other than Li). Except for the other conditions being the same as in Example 1, lithium transition metal composite oxide particles, a positive electrode active material, and a secondary battery using the positive electrode active material were obtained. The difference in reflectance of the obtained positive electrode active material was 0.8%, and the change in reflectance was -0.019% / nm. The initial discharge capacity was 143 mAh / g.

[0125] [Example 7] In the coating preparation process, the amount of coating solution applied was adjusted to be 0.5 mol% of the amount of niobium in the niobium oxide sol and 0.5 mol% of the amount of lithium in the coating solution relative to the total amount of Ni, Co, and Mn contained in the lithium metal composite oxide particles (total amount of metal elements other than Li). Except for the other conditions being the same as in Example 1, lithium transition metal composite oxide particles, a positive electrode active material, and a secondary battery using the positive electrode active material were obtained. The difference in reflectance of the obtained positive electrode active material was 0.3%, and the change in reflectance was -0.011% / nm. The initial discharge capacity was 142 mAh / g.

[0126] [Comparative Example 1] Under the same conditions as in Example 1, except that the coating preparation and coating processes were not performed, lithium transition metal composite oxide particles, a positive electrode active material, and a secondary battery using the positive electrode active material were obtained. Since the coating process was not performed, the lithium transition metal composite oxide particles themselves, which do not have a coating layer, become the positive electrode active material. The difference in reflectance of the obtained positive electrode active material was -0.8%, and the change in reflectance was 0.010% / nm. The initial discharge capacity was 120 mAh / g.

[0127] [Comparative Example 2] In the coating preparation process, the amount of coating solution applied was adjusted to be 0.3 mol% of the niobium in the niobium oxide sol and 0.3 mol% of the lithium in the coating solution relative to the total amount of Ni, Co, and Mn contained in the lithium metal composite oxide particles (total amount of metal elements other than Li). Except for the other conditions being the same as in Example 1, lithium transition metal composite oxide particles, a positive electrode active material, and a secondary battery using the positive electrode active material were obtained. The difference in reflectance of the obtained positive electrode active material was -0.4%, and the change in reflectance was 0.004% / nm. The initial discharge capacity was 130 mAh / g.

[0128] [Comparative Example 3] Except for not performing the coating preparation and coating steps in Example 2, lithium transition metal composite oxide particles, a positive electrode active material, and a secondary battery using the positive electrode active material were obtained under the same conditions as in Example 2. Since the coating step was not performed, the lithium transition metal composite oxide particles themselves, which do not have a coating layer, become the positive electrode active material. The difference in reflectance of the obtained positive electrode active material was -0.8%, and the change in reflectance was 0.009% / nm. The initial discharge capacity was 135 mAh / g.

[0129] [Comparative Example 4] In Example 3, lithium transition metal composite oxide particles, a positive electrode active material, and a secondary battery using the positive electrode active material were obtained under the same conditions as in Example 2, except that the coating preparation and coating processes in Example 3 were not performed. Since the coating process was not performed, the lithium transition metal composite oxide particles themselves, which do not have a coating layer, become the positive electrode active material. The difference in reflectance of the obtained positive electrode active material was -0.9%, and the change in reflectance was 0.005% / nm. The initial discharge capacity was 130 mAh / g.

[0130] [Comparative Example 5] In the coating preparation process, the amount of coating solution applied was adjusted to 5.0 mol% of the niobium content in the niobium oxide sol and 5.0 mol% of the lithium content in the coating solution relative to the total amount of Ni, Co, and Mn contained in the lithium metal composite oxide particles (total amount of metal elements other than Li). Except for the other conditions being the same as in Example 1, lithium transition metal composite oxide particles, a positive electrode active material, and a secondary battery using the positive electrode active material were obtained. The thickness of the coating layer of the obtained positive electrode active material was too large, so the diffuse reflectance ultraviolet-visible spectrum of the positive electrode active material was not measured. The initial discharge capacity was 132 mAh / g.

[0131] [Table 1]

[0132] [Table 2]

[0133] The initial discharge capacities of the all-solid-state batteries in Examples 1-3 were higher than those of Comparative Examples 1, 3, and 4. This is thought to be because coating the surface of the positive electrode active material with lithium niobate suppressed the increase in interfacial resistance between the positive electrode active material and the solid electrolyte. Furthermore, the initial discharge capacities of the all-solid-state batteries in Examples 1, 4-7 were higher than those of Comparative Examples 2 and 5. This is thought to be because the amount of niobium compound added was appropriate, improving the coating state and suppressing the increase in interfacial resistance between the positive electrode active material and the solid electrolyte. [Explanation of symbols]

[0134] 1 Test battery 2 Compacted powder cell 3 Negative electrode can 4 Positive electrode can 5 wing nuts 6 nuts 7. Insulating sleeve 8 Pressure screw 9. Hemispherical Washer 10 plugs 11 O-rings 12 Lower current collector 13 Upper current collector 14 sleeves

Claims

1. A positive electrode active material for a non-aqueous electrolyte secondary battery, comprising lithium transition metal composite oxide particles and a coating layer covering at least a portion of the surface of the particles, The lithium transition metal composite oxide particles contain Li, Ni, Co, and element M in a molar ratio of Li:Ni:Co:M = t:(1-x-y):x:y (where M is at least one element selected from the group consisting of Mg, Al, Ca, Si, Mn, Ti, V, Fe, Cu, Cr, Zn, Zr, Nb, Mo, and W, with 0.95 ≤ t ≤ 1.20, 0.15 < x ≤ 0.4, 0 ≤ y ≤ 0.4, and 0.33 ≤ (1-x-y) < 0.8). The coating layer contains a lithium niobium compound. The difference in reflectance obtained by subtracting the reflectance at 400 nm from the reflectance at 800 nm in the diffuse reflectance ultraviolet-visible spectrum of the positive electrode active material is greater than 0, and the change in reflectance in the range from 200 nm to 300 nm in the diffuse reflectance ultraviolet-visible spectrum of the positive electrode active material is less than 0% / nm. Positive electrode active material for non-aqueous electrolyte secondary batteries.

2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the difference in reflectance is 0.2% or more, and the change in reflectance is -0.005% / nm or less.

3. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the element M is Mn.

4. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, applicable to the positive electrode of an all-solid-state battery in which a solid electrolyte is used as the non-aqueous electrolyte.

5. A method for obtaining a positive electrode active material for a non-aqueous electrolyte secondary battery as described in claim 1, The process includes mixing the lithium transition metal composite oxide particles with a liquid coating agent containing a lithium compound and a niobium compound, drying the mixture, and then performing a heat treatment. A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery.

6. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 5, wherein the heat treatment is performed in an oxygen-containing atmosphere at a temperature of 200°C to 500°C for 1 hour to 5 hours.

7. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a solid electrolyte, or comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, wherein the positive electrode active material used in the positive electrode is the positive electrode active material for a non-aqueous electrolyte secondary battery described in claim 1.

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