Positive electrode active material for lithium-ion secondary batteries, and lithium-ion secondary batteries

Abstract

The present invention provides a positive electrode active material for lithium-ion secondary batteries that has high voltage resistance and a high battery capacity sufficient for practical use in lithium-ion secondary batteries. [Solution] A positive electrode active material for a lithium-ion secondary battery, comprising lithium nickel composite oxide particles and a coating layer covering at least a portion of the surface of the lithium nickel composite oxide particles, wherein the lithium nickel composite oxide contains Li, Ni, Co, and optionally element M as elements other than oxygen, and the molar ratio of these elements is Li:Ni:Co:M=t:1-xy:x:y(0.95≦t≦1.20, 0

Description

[Technical Field]

[0001] This invention relates to a positive electrode active material for lithium-ion secondary batteries and a lithium-ion secondary battery using the same. [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. Lithium-ion rechargeable batteries are one such type of battery. In particular, lithium-ion rechargeable batteries that use layered or spinel-type lithium metal composite oxides as the cathode material are gaining practical use as batteries with high energy density because they can achieve high voltages.

[0003] Currently, typical lithium-ion secondary batteries use lithium metal composite oxides such as LiCoO2, LiNiO2, and LiMn2O4 as the positive electrode active material, and lithium metal, lithium alloys, metal oxides, or carbon as the negative electrode active material.

[0004] Furthermore, the electrolyte used is an organic solvent such as ethylene carbonate, dimethyl carbonate, or ethyl methyl carbonate in which Li salts such as LiClO4 and LiPF6 are dissolved as supporting salts.

[0005] Among the components of lithium-ion secondary batteries, the 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 being actively conducted on all-solid-state lithium-ion secondary batteries (hereinafter also referred to as "all-solid-state batteries"), which improve battery performance by using a solid electrolyte instead of a liquid electrolyte.

[0006] In the research and development, it has been proposed, for example, in Patent Document 1 that sulfide solid electrolytes have high lithium ion conductivity and are preferable for use in all-solid-state batteries. However, in all-solid-state batteries, as disclosed in Non-Patent Document 1 for example, when a sulfide electrolyte comes into contact with an oxide cathode active material, a reaction occurs at the interface between the electrolyte and the cathode active material during charge and discharge, generating a high-resistance phase that inhibits the operation of the battery. In order to suppress the formation of this high-resistance phase, it has been proposed, for example, in Patent Document 2 and the like, to provide a coating layer made of LiNbO3 on the surface of the cathode active material.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Non-Patent Documents

[0008]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0009] The energy density of a lithium-ion secondary battery improves as the charge-discharge voltage increases. As a result of intensive research on lithium-ion secondary batteries operating at a high voltage of 4.5 V or higher, the present inventors have found that a positive electrode active material having a coating layer made of LiNbO3 is not stable under high voltage.

[0010] On the other hand, for example, Patent Document 3 describes a positive electrode active material for a lithium secondary battery including a coated positive electrode active material in which the surface of the positive electrode active material is directly coated with lithium metaphosphate. According to this positive electrode active material, it is said that even when the operating voltage of the lithium secondary battery is set to a high potential, oxidation of the solvent at the positive electrode can be suppressed, and gas generation can be suppressed.

[0011] Further, Patent Document 4 describes a positive electrode including a positive electrode active material and a positive electrode active material layer containing (100 - x)LiPO3·xLiPO3·xLiVO3 (0 < x ≤ 60) and a current collector. This positive electrode is said to be excellent in its own formability and also excellent in forming an interface with an oxide-type solid electrolyte by containing a LiPO3-LiVO3-based glass in the positive electrode active material layer.

[0012] Therefore, the present inventors conducted research on coating of the positive electrode active material with lithium metaphosphate (LiPO3) as in Patent Documents 3 and 4 above, but found that the positive electrode active material coated with LiPO3 has high resistance and is not suitable for practical use. This is presumably due to the low lithium ion conductivity of LiPO3 itself.

[0013] The present invention has been made in view of the above problems, and an object thereof is to provide a positive electrode active material for a lithium-ion secondary battery that has high voltage resistance and a high battery capacity sufficient for practical use in a lithium-ion secondary battery.

Means for Solving the Problems

[0014] In a first aspect of the present invention, there is provided a positive electrode active material for a lithium-ion secondary battery, comprising particles of a lithium nickel composite oxide and a coating layer covering at least a part of the surface of the particles of the lithium nickel composite oxide. The lithium nickel composite oxide contains, as elements other than oxygen, Li, Ni, Co, and optionally an element M (M is at least one element selected from the group consisting of Mg, Al, Ca, Si, Sc, Ti, V, Fe, Mn, Cu, Cr, Zn, Zr, Nb, Mo, and W). The molar ratio of these elements is represented by Li:Ni:Co:M = t:1 - x - y:x:y (0.95 ≤ t ≤ 1.20, 0 < x ≤ 0.22, 0 ≤ y ≤ 0.15). The coating layer contains a compound containing Li, P, and an element X (X is at least one element selected from the group consisting of elements that form hexavalent cations).

[0015] Further, it is preferable that the composition ratio of elements other than oxygen in the above compound is represented by Li:P:X = 1 - a:1 - a:a (0.01 ≤ a ≤ 0.1). Also, the total amount of the amount of substances of P and X derived from the coating layer per 1 m² of the surface area of the lithium nickel composite oxide is preferably 50 μmol or more and 410 μmol or less. Also, X in the coating layer is preferably Mo or W or both. Also, the carbon content of the positive electrode active material for a lithium-ion secondary battery is preferably 0.5% by mass or less, and the moisture content is preferably 0.2% by mass or less. Also, the specific surface area of the lithium nickel composite oxide is preferably 0.1 m² / g or more and 2.0 m² / g or less. Also, the lithium nickel composite oxide is preferably layered rock salt-type compound particles having a crystal structure belonging to the space group R-3m. 2 [[ID=##]] 2 / g or more and 2.0 m² 2 / g or less. Also, the lithium nickel composite oxide is preferably layered rock salt-type compound particles having a crystal structure belonging to the space group R-3m.

[0016] In a second aspect of the present invention, there is provided a lithium-ion secondary battery including a positive electrode containing the above positive electrode active material for a lithium-ion secondary battery, a negative electrode, and an electrolyte.

Advantages of the Invention

[0017] According to the present invention, it is possible to provide a positive electrode active material for lithium-ion secondary batteries that has high voltage resistance and high battery capacity. [Brief explanation of the drawing]

[0018] [Figure 1] Figure 1 shows an example of a method for producing a positive electrode active material according to this embodiment. [Figure 2] Figure 2 shows an example of a method for producing lithium nickel composite oxide according to this embodiment. [Figure 3] This is an explanatory diagram of the cross-sectional configuration of the evaluation battery used for battery evaluation. [Modes for carrying out the invention]

[0019] As a result of diligent research to solve the above problems, the inventors have found that by coating lithium nickel composite oxide particles and at least a portion of the surface of said lithium nickel composite oxide particles with a compound containing lithium, phosphorus, and a hexavalent cation element compound, it is possible to achieve a high level of both high voltage resistance and high capacitance, and have completed the present invention. An example of an embodiment of the present invention will be described below.

[0020] Furthermore, the embodiments described below do not unduly limit the scope of the present invention as described in the claims, and modifications are permitted without departing from the spirit of the invention. In addition, not all of the configurations described in these embodiments are necessarily essential as solutions to the problems of the present invention.

[0021] <1. Cathode Active Material for Lithium-ion Secondary Batteries> A positive electrode active material for a lithium-ion secondary battery according to one embodiment of the present invention (hereinafter referred to as "positive electrode active material") comprises lithium nickel composite oxide particles and a coating layer that covers at least a portion of the surface of the lithium nickel composite oxide particles. The positive electrode active material for a lithium-ion secondary battery according to this embodiment (hereinafter also simply referred to as "positive electrode active material") will be described in detail below.

[0022] <1-1. Particles of Lithium Nickel Composite Oxide> The particles of the lithium nickel composite oxide are a composite oxide containing Li, Ni, Co, and optionally an element M (M is at least one element selected from the group consisting of Mg, Al, Ca, Si, Sc, Ti, V, Fe, Mn, Cu, Cr, Zn, Zr, Nb, Mo, and W).

[0023] (Composition) The particles of the lithium nickel composite oxide can contain the above elements other than oxygen, namely lithium (Li), nickel (Ni), cobalt (Co), and element M (M) in a molar ratio of Li:Ni:Co:M = t:1 - x - y:x:y (0.95 ≤ t ≤ 1.20, 0 < x ≤ 0.22, 0 ≤ y ≤ 0.15).

[0024] In the molar ratio of each element in the above lithium nickel composite oxide, the value of t representing the molar ratio of lithium (Li) can be 0.95 or more and 1.20 or less, preferably 0.98 or more and 1.10 or less, and more preferably 1.00 or more and 1.10 or less.

[0025] By setting the value of t to 0.95 or more, the internal resistance of a secondary battery using a positive electrode active material containing the lithium nickel composite oxide can be suppressed, and the output characteristics can be improved. Also, by setting the value of t to 1.20 or less, the initial discharge capacity of a secondary battery using a positive electrode active material containing the lithium nickel composite oxide can be increased. That is, by setting the value of t within the above range, the output characteristics and capacity characteristics of a secondary battery using a positive electrode active material containing the lithium nickel composite oxide can be improved.

[0026] Nickel (Ni) is an element that contributes to increasing the capacity of secondary batteries using positive electrode active materials containing lithium nickel composite oxide. Among the molar ratios of each element in the lithium nickel composite oxide described above, the value of (1-xy), which indicates the molar ratio of nickel (Ni), is between 0.63 and less than 1, and is a value that varies depending on the proportions of cobalt (Co) and element M, which will be described later. The value of (1-xy) may be 0.65 or higher, 0.7 or higher, or 0.8 or higher.

[0027] Cobalt (Co) is an element that contributes to reducing the irreversible capacity of secondary batteries using a positive electrode active material containing lithium nickel composite oxide. In the molar ratio of each element in the lithium nickel composite oxide described above, the value of x, which indicates the cobalt content, can be greater than 0 and less than or equal to 0.22, preferably between 0.10 and 0.22, and more preferably between 0.10 and 0.20.

[0028] By setting the value of x above to greater than 0, the irreversible capacity, which is the difference between the charging capacity and the discharging capacity, can be reduced in a secondary battery using a positive electrode active material containing the lithium nickel composite oxide. Furthermore, when the value of x is 0.22 or less, the Ni content becomes relatively higher, and a higher battery capacity can be obtained.

[0029] Furthermore, the lithium nickel composite oxide may contain, in addition to the above-mentioned metal elements, an additive element M. As the above-mentioned element M, at least one element selected from magnesium (Mg), aluminum (Al), calcium (Ca), silicon (Si), scandium (Sc), titanium (Ti), vanadium (V), iron (Fe), manganese (Mn), copper (Cu), chromium (Cr), zinc (Zn), zirconium (Zr), niobium (Nb), molybdenum (Mo), and tungsten (W) can be used. Element M is appropriately selected depending on the application and required performance of the secondary battery constructed using the positive electrode active material.

[0030] Since there are some elements M that do not contribute to the redox reaction themselves, in the molar ratio of each element in the above lithium nickel composite oxide, the value of y indicating the content of element M can be 0.15 or less, preferably 0.10 or less, and more preferably 0.05 or less. Since the lithium nickel composite oxide may not contain element M, the lower limit value of y indicating the content of element M can be 0. When element M contains Mn, the range of Mn (y1) may be, for example, 0 < y1 ≤ 0.15, or may be 0 < y1 ≤ 0.10. When element M contains Al, the range of Al (y2) may be, for example, 0 < y2 ≤ 0.1, or may be 0 < y2 ≤ 0.05.

[0031] In addition, the lithium nickel composite oxide may contain elements other than the above Li, Ni, Co, element M, and oxygen when it does not inhibit the effects of the present invention. The lithium nickel composite oxide may also be composed of the above Li, Ni, Co, element M, and oxygen.

[0032] (Crystal structure) When X-ray diffraction (XRD) measurement is performed on the particles of the lithium nickel composite oxide, it is preferable that peaks attributable to the "R-3m" structured layered rock salt type crystal structure are detected from the diffraction pattern obtained. Particularly, it is more preferable that only peaks attributable to the "R-3m" structured layered rock salt type crystal structure are detected from the diffraction pattern. This is because the layered rock salt type oxide with the "R-3m" structure can particularly suppress the internal resistance when used as the positive electrode active material of a secondary battery and is preferable.

[0033] However, a single-phase lithium nickel composite oxide having a layered rock salt type crystal structure cannot be obtained, and an impurity phase may be mixed in. Even when an impurity phase is mixed in this way, the intensity of the peaks attributable to the impurity phases other than the "R-3m" structured layered rock salt type structure preferably does not exceed the peak intensity attributable to the "R-3m" structured layered rock salt type structure.

[0034] (Particle structure) The particles of the lithium nickel composite oxide are preferably single primary particles, secondary particles formed by aggregation of a plurality of primary particles, or a mixture thereof. Here, the primary particles and secondary particles can be confirmed by an electron microscope such as SEM or TEM. Further, inside each secondary particle, there may be a space or void surrounded by one or more primary particles.

[0035] (Specific surface area) The lithium nickel composite oxide preferably has a specific surface area of 0.1 m 2 / g or more and 2.0 m 2 / g or less, more preferably 0.1 m 2 / g or more and 1.0 m 2 / g or less. The release / absorption of lithium ions that occurs in the lithium nickel composite oxide during charge / discharge of the secondary battery occurs through the interface between the particles of the lithium nickel composite oxide and the electrolyte, that is, the surface of the lithium nickel composite oxide. Therefore, by setting the specific surface area of the lithium nickel composite oxide contained in the positive electrode active material to 0.1 m 2 / g or more, the release / absorption of such lithium ions can be sufficiently promoted, and the internal resistance of the secondary battery during charge / discharge can be lowered, which is preferable. Further, as described above, the electrolyte may be decomposed by a side reaction that occurs at the interface between the positive electrode active material and the electrolyte, and a coating layer serving as a buffer layer is required. By setting the specific surface area of the particles of the lithium nickel composite oxide to 1.0 m 2 / g or less, the amount of the electrochemically inactive coating layer component can be kept small.

[0036] The specific surface area of the lithium nickel composite oxide can be measured, for example, by the BET method using nitrogen adsorption.

[0037] (Volume average particle size) The volume-average particle size of the lithium nickel composite oxide according to one embodiment of the present invention is preferably 2 μm or more and 20 μm or less, more preferably 2 μm or more and 15 μm or less, and even more preferably 3 μm or more and 15 μm or less. This is because, when the volume-average particle size of the lithium nickel composite oxide particles is within the above range, a secondary battery using a positive electrode active material containing the lithium nickel composite oxide particles as the positive electrode can achieve a sufficiently large battery capacity per unit volume, and excellent battery characteristics such as high safety and high output can be obtained. Here, the volume-average particle size can be measured using a laser diffraction scattering particle size analyzer or the like.

[0038] <1-2. Covering layer> The coating layer contains a compound comprising lithium, phosphorus, and element X (where X is an element that forms a hexavalent cation), and coats at least a portion of the lithium nickel composite oxide particles. By including element X in addition to lithium and phosphorus, the coating layer can improve high-voltage resistance.

[0039] Compounds containing lithium, phosphorus, and a hexavalent cationic element are preferably composed of phosphorus (P) and the hexavalent cationic element X in a molar ratio such that P:X = 1 - a:a (0.01 ≤ a ≤ 0.1). When a is within this range, the high voltage resistance is improved. The ratio of lithium (Li) to phosphorus (P) (Li:P, molar ratio) may be 1 to 3:1, and is preferably 1:1. The above compound may also be an oxide.

[0040] Furthermore, for compounds containing lithium, phosphorus, and hexavalent cation elements, it is preferable that the composition ratio of elements other than oxygen is expressed as Li:P:X = 1-a:1-a:a (0.01 ≤ a ≤ 0.1). When a in the composition ratio is within the above range, the battery has a high capacity and improved high voltage resistance. The coating layer may also be composed of an oxide with a composition expressed as Li:P:X = 1-a:1-a:a (0.01 ≤ a ≤ 0.1). If a is less than 0.01, there may be no Li holes in the LiPO3 framework, and a high-resistance coating layer may be formed. On the other hand, if a exceeds 0.1, high voltage resistance may be impaired and the charge / discharge capacity may decrease.

[0041] The element X contained in the above compound is not particularly limited as long as it is an element that forms a hexavalent cation, and examples include Mo, W, S, Cr, Te, and Se, among which Mo or W or both are particularly preferred, and Mo or W may also be used.

[0042] Furthermore, the coating layer and the lithium nickel composite oxide particles may or may not have a clear boundary. In this specification, the coating layer refers to the region on the surface side of the positive electrode active material of this embodiment in which the concentration of phosphorus and element X (e.g., Mo, W, etc.) is higher than that of the lithium nickel composite oxide particles, which are the coated material, i.e., the central region. In addition, the elements constituting the coating layer may be partially in solid solution with the lithium nickel composite oxide.

[0043] The content of phosphorus and element X in the positive electrode active material is not particularly limited, but it is preferable to adjust their content according to the specific surface area of ​​the lithium nickel composite oxide to be coated.

[0044] For example, the coating layer has a surface area of ​​1 m² of lithium nickel composite oxide particles. 2 Preferably, the mixture contains phosphorus and element X in a proportion of 50 μmol to 410 μmol per unit. Surface area of ​​lithium nickel composite oxide particles 1 m² 2By ensuring that the total content of phosphorus and element X per unit is 50 μmol or more, the coating layer can be more uniformly distributed across the entire surface of the lithium nickel composite oxide particles.

[0045] Furthermore, while having a coating layer on the positive electrode active material can improve cycle characteristics and suppress battery swelling, it may also increase internal resistance. In the positive electrode active material according to this embodiment, the surface area of ​​the lithium nickel composite oxide particles is 1 m². 2 It is preferable to keep the total content of phosphorus and element X per unit to 410 μmol or less, as this suppresses the coating layer from hindering the lithium release / intersorption reaction of lithium nickel composite oxide and reduces internal resistance.

[0046] The method for evaluating and calculating the total content of phosphorus and element X in the coating layer is not particularly limited, but for example, the content of phosphorus and element X (μmol / g) in 1g of the obtained positive electrode active material is first measured by chemical analysis or other methods. For example, it can be measured by ICP (Inductively Coupled Plasma). In addition, the specific surface area (m²) of the lithium nickel composite oxide particles before coating with phosphorus and molybdenum compounds is measured. 2 The phosphorus and element X content (μmol / g) in 1g of the obtained positive electrode active material is measured by the BET method using nitrogen adsorption, etc. Then, the specific surface area (m²) of the lithium nickel composite oxide particles before coating treatment is measured. 2 Dividing by ( / g) gives the surface area of ​​lithium nickel composite oxide particles 1 m² 2 Content of phosphorus and element X per unit (μmol / m³) 2 ) can be calculated.

[0047] If the lithium nickel composite oxide before coating contains the same elements as element X contained in the coating layer (i.e., element M and element X are the same element), it is preferable to use the difference in the content of lithium and element M (or element X) before and after coating as the amount of element X used for coating.

[0048] Furthermore, the lithium, phosphorus, and element X compound in the coating layer may react with the lithium nickel composite oxide, resulting in some of the phosphorus and element X used in the coating treatment being dissolved in the lithium nickel composite oxide. For example, heat treatment can be performed after the coating treatment, and depending on the conditions at that time, the lithium, phosphorus, and molybdenum in the coating layer may be dissolved in the lithium nickel composite oxide.

[0049] The solid solution of lithium, phosphorus, and some of the elements constituting the compound containing element X in the coating layer into the lithium nickel composite oxide prevents lithium nickel composite oxide particles that are not solid-dissolved in the lithium phosphomolybdenum compound from directly contacting the solid electrolyte. This not only reduces the opportunity for interfacial reactions but also suppresses the reaction between the lithium nickel composite oxide and the solid electrolyte itself. However, for the positive electrode active material, it is preferable to adjust the degree of solid solution so that the effect of improving cycle characteristics is also fully realized.

[0050] <1-3. Characteristics of the positive electrode active material> The positive electrode active material according to this embodiment preferably has the following characteristics.

[0051] The positive electrode active material according to this embodiment may consist only of the lithium nickel composite oxide particles and the coating layer, but small amounts of impurities may be mixed in during the manufacturing of the positive electrode active material. Among the impurities, moisture and carbon in particular are impurities that may increase due to the coating process when forming the coating layer. Since moisture and carbon may affect the cycle characteristics, it is preferable that they be controlled within a predetermined range.

[0052] (Carbon content) The positive electrode active material according to this embodiment preferably has a carbon content of, for example, 0.5% by mass or less, more preferably 0.05% by mass or more and 0.5% by mass or less, and even more preferably 0.2% by mass or less. By setting the carbon content within the above range, the generation of carbon dioxide caused by, for example, the decomposition of organic components contained in the coating layer during the charging and discharging of a secondary battery using the positive electrode active material can be sufficiently suppressed, thereby preventing battery swelling and other problems. The lower limit of the carbon content is, for example, 0.05% by mass. The above carbon content can be evaluated by, for example, infrared absorption spectroscopy.

[0053] (moisture content) Furthermore, the positive electrode active material according to one embodiment of the present invention preferably has a moisture content of 0.2% by mass or less, and may also be 0.1% by mass or less. By setting the moisture content within the above range, the deterioration of battery characteristics can be further suppressed in a secondary battery using the positive electrode active material.

[0054] For example, in the case of all-solid-state batteries, hydrolysis of the solid electrolyte generates hydrogen sulfide gas and a phosphate-based resistive layer. However, by keeping the moisture content of the positive electrode active material below 0.2% by mass, the hydrolysis reaction of the electrolyte in the electrolyte solution can be more reliably suppressed, thereby preventing such degradation. The moisture content of the positive electrode active material is evaluated using the Karl Fischer method with a heating temperature of 300°C.

[0055] <2. Method for manufacturing positive electrode active material for lithium-ion secondary batteries> Next, an example of a method for manufacturing the positive electrode active material for lithium-ion secondary batteries will be described with reference to the drawings. Figure 1 is a diagram showing an example of a method for manufacturing the positive electrode active material for lithium-ion secondary batteries, and Figure 2 is a process diagram (schematic diagram) showing an example of a method for manufacturing lithium nickel composite oxide. As shown in Figure 1, the method for manufacturing the positive electrode active material for lithium-ion secondary batteries according to this embodiment includes a coating step S4 in which lithium nickel composite oxide particles and a coating agent containing lithium, phosphorus, and element X are mixed and dried (S41a, b) to form a coating layer.

[0056] Furthermore, as shown in Figure 2, lithium-ion nickel composite oxide can be obtained, for example, by a method comprising a precursor crystallization step S1, an oxidation roasting step S2, and a lithium-nickel composite oxide synthesis step S3. Each step will be described in detail below. Note that the following description is merely an example of a manufacturing method and does not limit the manufacturing method.

[0057] <2-1. Precursor crystallization step S1> In the precursor crystallization step S1 for obtaining nickel composite hydroxide, nickel composite hydroxide, which is a precursor of lithium nickel composite oxide, is prepared by a crystallization reaction.

[0058] Specifically, for example, a mixed aqueous solution can be prepared using water-soluble raw materials of each element such that the molar ratio of each element is Ni:Co:M = 1-xy:x:y, and this mixture can be reacted in a reaction vessel with an alkali metal aqueous solution or the like to obtain a nickel composite hydroxide.

[0059] Furthermore, the values ​​of x and y in the above formula can be within the same preferred ranges as those described for lithium nickel composite oxide particles.

[0060] The following describes an example of the precursor crystallization step S1. First, a metal compound containing nickel, a metal compound containing cobalt, and, if applicable, a metal compound containing element M are dissolved in water in predetermined proportions to prepare a raw material mixed aqueous solution (raw material mixed aqueous solution preparation step).

[0061] The molar ratio of each metal in the aforementioned raw material mixed aqueous solution will be the same as the molar ratio of the nickel composite hydroxide finally obtained. Therefore, it is preferable to adjust the molar ratio of each metal in the raw material mixed aqueous solution by adjusting the proportion of the metal compound dissolved in water so that it is the same ratio as the molar ratio of each metal in the target nickel composite hydroxide particles. Any water-soluble metal compound is acceptable, and sulfates, chlorides, nitrates, etc., can be used, but sulfates are preferred from a cost standpoint. If a suitable water-soluble metal compound cannot be found for element M, etc., it may be added in the oxidation roasting step S2 or the lithium nickel composite oxide synthesis step S3 described later, without being added to the raw material mixed aqueous solution.

[0062] Next, water is added to the reaction vessel, and an appropriate amount of alkaline substance and ammonium ion supply is added to prepare the initial aqueous solution (initial aqueous solution preparation step). At this time, it is preferable to prepare the initial aqueous solution so that the pH value is 11.2 or higher and 13.0 or lower based on a liquid temperature of 25°C, and the ammonia concentration is 2 g / L or higher and 20 g / L or lower.

[0063] When preparing nickel composite hydroxide by performing the precursor crystallization step S1, impurities originating from anions constituting the metal compounds in the raw material mixture aqueous solution may be mixed into the nickel composite hydroxide. However, it is preferable to set the pH value of the initial aqueous solution to 11.2 or higher to suppress the inclusion of impurities originating from anions constituting the metal compounds of the raw materials. Furthermore, by setting the pH value of the initial aqueous solution to 13.0 or lower, the resulting nickel composite hydroxide particles can be suppressed from becoming micronized and can be made to an optimal size.

[0064] Furthermore, it is preferable to set the ammonia concentration of the initial aqueous solution to 2 g / L or higher, as this makes it easier for the resulting nickel composite hydroxide particles to form spherical shapes. It is also preferable to set the ammonia concentration of the initial aqueous solution to 20 g / L or lower, as this prevents an excessive increase in the solubility of nickel that forms the ammonia complex, and allows for a more reliable achievement of the target molar ratio of the resulting nickel composite hydroxide.

[0065] The alkaline substance used when preparing the initial aqueous solution is not particularly limited, but it is preferably one or more selected from sodium carbonate, sodium bicarbonate, potassium carbonate, sodium hydroxide, and potassium hydroxide. It is preferable to add it in the form of an aqueous solution because the amount added can be easily adjusted. Furthermore, the ammonium ion supplier is not particularly limited, but it is preferable to use one or more selected from aqueous ammonium carbonate, aqueous ammonia, aqueous ammonium chloride, and aqueous ammonium sulfate.

[0066] In the precursor crystallization step S1, the above raw material mixed aqueous solution can be added dropwise to the initial aqueous solution to form the reaction aqueous solution.

[0067] The atmosphere inside the reaction vessel is preferably a non-oxidizing atmosphere, for example, an atmosphere with an oxygen concentration of 1% by volume or less. This is preferable because a non-oxidizing atmosphere inside the reaction vessel suppresses the oxidation of the raw material compounds, etc. For example, it can prevent the precipitation of oxidized cobalt as fine particles.

[0068] In the precursor crystallization step S1, the temperature inside the reaction vessel is preferably maintained at 40°C to 60°C, and more preferably at 45°C to 55°C. In order to maintain the reaction vessel in this temperature range, it is also preferable that the initial aqueous solution and reaction aqueous solution placed inside the reaction vessel are maintained within the same temperature range.

[0069] Because the temperature of the reaction vessel naturally rises due to the heat of the reaction and the energy of stirring, maintaining the temperature inside the reaction vessel above 40°C eliminates the need to consume extra energy for cooling. Furthermore, by keeping the temperature of the reaction vessel below 60°C, the evaporation of ammonia from the initial aqueous solution and the reaction aqueous solution can be suppressed, making it easier to maintain the target ammonia concentration.

[0070] Next, as described above, the initial aqueous solution is placed in the reaction vessel, and after adjusting the temperature and other parameters, the mixed aqueous solution of raw materials is continuously added to the reaction vessel at a constant rate to proceed with the crystallization reaction of the precursor nickel composite hydroxide particles (crystallization step).

[0071] It is preferable that the pH value and ammonia concentration of the reaction aqueous solution are within the same suitable range as described for the initial aqueous solution. Therefore, when adding the mixed aqueous solution dropwise to the initial aqueous solution or the reaction aqueous solution, it is preferable to add the ammonium ion supplier and alkaline substance to the initial aqueous solution or the reaction aqueous solution dropwise at a constant rate. Furthermore, it is preferable to control the pH value of the reaction aqueous solution to be maintained at 11.2 to 13.0 and the ammonia concentration to be maintained at 2 g / L to 20 g / L based on a liquid temperature of 25°C. In addition, the crystallization process may be performed in two stages, clearly distinguishing between a nucleation process and a particle growth process. In this case, the process may include a nucleation process in which the pH value of the reaction aqueous solution at a liquid temperature of 25°C is adjusted to a range of 12.0 to 14.0 and nucleation is performed, and a particle growth process in which the pH value of the reaction aqueous solution containing the nuclei obtained in the nucleation process at a liquid temperature of 25°C is controlled to be lower than the pH value of the nucleation process and to be between 10.5 and 12.0 to grow the nuclei. Furthermore, it is preferable that the pH and ammonia concentration be maintained at a constant value within the above range during the crystallization reaction.

[0072] After the crystallization reaction is complete, the slurry containing nickel composite hydroxide particles in the reaction vessel can be filtered and dried to obtain the precursor, powdered nickel composite hydroxide particles. Alternatively, the nickel composite hydroxide particles obtained after filtration may be washed with water to remove at least some of the impurities.

[0073] <2-2. Oxidation roasting process S2> An oxidation roasting step S2 may be performed after the precursor crystallization step S1. In the oxidation roasting step S2, the nickel composite hydroxide obtained in the precursor crystallization step S1 is oxidized and roasted to obtain a nickel composite oxide. In the oxidation roasting step S2, the nickel composite oxide can be obtained by calcining in an oxygen-containing atmosphere and then cooling to room temperature.

[0074] The roasting conditions in the oxidation roasting process S2 are not particularly limited, but it is preferable to roast in an oxygen-containing atmosphere, for example, an air atmosphere, at a temperature of 500°C to 700°C for 1 to 12 hours. This is because a roasting temperature of 500°C or higher allows for the complete conversion of nickel composite hydroxide particles into nickel composite oxide, which is preferable. Furthermore, a roasting temperature of 700°C or lower is preferable because it prevents the specific surface area of ​​the nickel composite oxide from becoming excessively small.

[0075] A firing time of one hour or more is preferable because it allows for a uniform temperature within the firing vessel, enabling the reaction to proceed uniformly. Furthermore, since no significant changes are observed in the resulting nickel composite oxide even when firing is performed for longer than 12 hours, it is preferable to limit the firing time to 12 hours or less from the viewpoint of energy efficiency.

[0076] The oxygen concentration in the oxygen-containing atmosphere during heat treatment 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. Since an oxygen atmosphere can also be used, the upper limit of the oxygen concentration in the oxygen-containing atmosphere can be 100% by volume.

[0077] If, for example, the compound containing element M is not coprecipitated in the precursor crystallization step S1, the compound containing element M may be added to the nickel composite hydroxide subjected to the oxidative roasting step S2 in such a ratio as the intended composition and then heat-treated. The compound containing element M to be added is not particularly limited, and for example, oxides, hydroxides, carbonites, or mixtures thereof can be used.

[0078] If slight sintering is observed in the nickel composite oxide particles after the oxidation roasting process, a crushing treatment may be added.

[0079] <2-3. Lithium nickel composite oxide synthesis process S3> In the lithium nickel composite oxide synthesis step S3, the nickel composite hydroxide and / or nickel composite oxide obtained in the precursor crystallization step S1 and / or oxidation roasting step S2 are mixed with a lithium compound and calcined to obtain a lithium nickel composite oxide. An example of the lithium nickel composite oxide synthesis step S3 will be described below.

[0080] First, a lithium mixture can be obtained by adding a lithium compound to the nickel composite hydroxide and / or nickel composite oxide obtained in the precursor crystallization step S1 and / or oxidation roasting step S2, such that the amount of lithium is 95% or more and 120% or less relative to the total amount of substance of the component metal elements contained in the composite oxide (lithium mixture preparation step). Alternatively, in the lithium mixture preparation step, a compound containing element M may be mixed in the same composition ratio as the desired composition, and then the calcination step described later may be performed.

[0081] The lithium compound to be added is not particularly limited, and for example, lithium hydroxide, lithium nitrate, lithium carbonate, or mixtures thereof can be used. Among the lithium compounds, lithium hydroxide, which has a particularly low melting point and high reactivity, is preferred.

[0082] Next, the obtained lithium mixture is calcined in an oxygen-containing atmosphere and then cooled to room temperature to obtain a lithium nickel composite oxide (calcination step). The calcination conditions are not particularly limited, but it is preferable to calcine at a temperature of 700°C to 800°C for 1 hour to 24 hours.

[0083] Furthermore, it is preferable that the oxygen-containing atmosphere contains 80% by volume or more oxygen. This is because a concentration of 80% by volume or more of oxygen in the atmosphere particularly suppresses cation mixing, where Ni atoms are mixed into the Li sites in the resulting lithium nickel composite oxide, which is preferable. Since an oxygen-only atmosphere can also be used, the upper limit of the oxygen concentration in the oxygen-containing atmosphere can be 100% by volume.

[0084] By setting the firing temperature to 700°C or higher, the crystal structure of the lithium nickel composite oxide can be sufficiently grown. Conversely, by setting the firing temperature to 800°C or lower, the aforementioned cation mixing can be suppressed.

[0085] A firing time of one hour or more is preferable because it allows for a uniform temperature within the firing vessel, enabling the reaction to proceed uniformly. Furthermore, since no significant changes are observed in the resulting lithium nickel composite oxide even when firing is performed for longer than 24 hours, it is preferable to limit the firing time to 24 hours or less from the viewpoint of energy efficiency.

[0086] If slight sintering is observed in the lithium nickel composite oxide obtained after the lithium nickel composite oxide synthesis step S3, a crushing treatment may be added.

[0087] <2-4. Covering process S4> In coating step S4, a coating layer containing a compound comprising lithium (Li), phosphorus (P), and element X (where X is at least one element selected from the group consisting of elements that form hexavalent cations) is formed on at least a portion of the surface of the lithium nickel composite oxide particles obtained in lithium nickel composite oxide synthesis step S3.

[0088] Furthermore, in coating step S4, the total amount of phosphorus and element X in the above compound is determined by the surface area of ​​the lithium nickel composite oxide (1 m²). 2It is preferable to add a compound containing phosphorus and element X in an amount of 50 μmol to 410 μmol per unit. An example of coating step S4 will be described below.

[0089] First, the specific surface area of ​​the lithium nickel composite oxide obtained in the lithium nickel composite oxide synthesis step S3 is measured, and a liquid coating agent is prepared according to the surface amount of phosphorus and element X of the target coating layer (coating agent preparation step S40).

[0090] The coating agent is not particularly limited as long as it contains lithium, phosphorus, and element X, or a compound containing lithium, phosphorus, and element X. For uniform coating, it is preferable to use a coating agent that contains lithium, phosphorus, and element X dissolved in a solvent, a low-melting-point compound containing lithium, phosphorus, and molybdenum that is liquid at room temperature or melts with low-temperature heat treatment, or a sol-type material in which nanoparticles are dispersed in a solvent.

[0091] Examples of compounds containing phosphorus and lithium include lithium metaphosphate (LiO3P), lithium dihydrogen phosphate (LiH2PO4), Li2HPO4, Li4P2O7, and Li3PO4. Examples of compounds containing element X include ammonium molybdate and ammonium tungstate.

[0092] As a coating agent, a solution of lithium metaphosphate, ammonium molybdate, and / or ammonium tungstate dissolved in water is particularly preferred, from the viewpoint of being easy to prepare and suppressing the inclusion of impurities.

[0093] Next, the lithium nickel composite oxide particles and the coating agent can be mixed. A general mixer can be used for mixing (mixture preparation step S41a). Then, drying is performed after mixing (drying step S41b).

[0094] In the drying process described above, drying can be carried out at a temperature sufficient to remove the solvent and other components of the coating agent. For example, drying can be performed at temperatures between 80°C and 300°C.

[0095] The mixture preparation step S41a and the drying step S41b may be carried out in parallel. In this case, for example, a rolling flow coating apparatus can be used. When a rolling flow coating apparatus is used, the coating liquid is sprayed onto lithium nickel composite oxide particles that are flowing due to a heated airflow in the apparatus, so the mixture preparation step and the drying step can be repeated in parallel, and a more uniform coating layer with fewer gaps can be obtained.

[0096] Next, heat treatment may be performed as needed (heat treatment step S42). By heat treatment, a compound containing lithium, phosphorus, and element X can be fixed as a coating layer. The heat treatment conditions are not particularly limited, but it is preferable to perform the heat treatment in an oxygen-containing atmosphere, for example, an air atmosphere, at a temperature of 200°C to 600°C for 1 hour to 10 hours. After heat treatment, the material is cooled to room temperature to obtain a positive electrode active material, which is lithium nickel composite oxide particles having a coating layer as the final product.

[0097] The oxygen concentration in the oxygen-containing atmosphere during heat treatment 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. This is preferable because it particularly suppresses the occurrence of oxygen defects in the resulting positive electrode active material by using an oxygen-containing atmosphere with an oxygen concentration equal to or greater than that of the air atmosphere. Since an oxygen atmosphere can also be used, the upper limit of the oxygen concentration in the oxygen-containing atmosphere can be set to 100% by volume.

[0098] The maximum temperature during heat treatment should preferably be 200°C or higher, as this is preferable because it particularly suppresses the retention of impurities contained in the coating agent within the positive electrode active material. Conversely, setting the maximum temperature to 600°C or lower is preferable because it suppresses excessive diffusion of the coating layer components into the positive electrode active material, thereby maintaining the shape of the coating layer. The maximum temperature during heat treatment should preferably be selected in accordance with the target surface amounts of phosphorus and element X in the coating layer, etc., so that the coating layer can sufficiently maintain its thickness.

[0099] It is preferable to extend the heat treatment time to one hour or more, as this particularly suppresses 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 firing is performed for longer than 10 hours, it is preferable to limit the treatment time to 10 hours or less from the viewpoint of energy efficiency.

[0100] If slight sintering is observed in the positive electrode active material obtained after the coating process S4, a crushing treatment may be added.

[0101] Furthermore, heat treatment is not required. In other words, the positive electrode active material can be manufactured by performing only the mixture preparation. This is because, even without heat treatment, a uniform and strong coating layer can be formed on the surface of the lithium nickel composite oxide particles by using a liquid coating agent. Even if the above heat treatment is not performed, it is preferable to dry the coating agent to reduce or remove the solvent and moisture as needed.

[0102] <3. Lithium-ion rechargeable batteries> A secondary battery according to one embodiment of the present invention may have a configuration comprising a positive electrode using the above-described positive electrode active material, a negative electrode, and an electrolyte. Furthermore, the secondary battery may be any secondary battery that performs charging and discharging by desorption and insertion of lithium ions, for example, a non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, or an all-solid-state lithium secondary battery comprising a positive electrode, a negative electrode, and a solid electrolyte. Below, an example of an all-solid-state battery will be mainly described. Note that the embodiments described below are merely examples, and secondary batteries can be implemented in various modified and improved forms based on the knowledge of those skilled in the art, including the embodiments described below. Furthermore, the secondary battery is not particularly limited in its application.

[0103] <3-1. Positive electrode> The positive electrode can be formed by molding a positive electrode mixture. The positive electrode is then processed as appropriate according to the type of battery being used. For example, pressure compression using a press may be performed to increase electrode density.

[0104] The positive electrode mixture can be formed, for example, by mixing the above-mentioned positive electrode active material, which is in powder form, with a solid electrolyte. The solid electrolyte is added to provide the electrode with appropriate ionic conductivity.

[0105] The material of the solid electrolyte is not particularly limited, but examples include Li3PS4 and Li7P3S. 11 Li 10 GeP2S 12 Sulfide-based solid electrolytes such as Li7La3Zr2O 12 Li 0.34 La 0.51 TiO 2.94 Oxide-based solid electrolytes such as PEO and polymer-based electrolytes such as PEO can be used.

[0106] In addition to the components mentioned above, binders and conductive additives may also be added to the positive electrode mixture.

[0107] 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 types selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ethylene propylene diene rubber, styrene butadiene, cellulose resin, polyacrylic acid, etc. can be used.

[0108] Conductive materials are added to electrodes to provide them with appropriate conductivity. The material of the conductive material 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.

[0109] Furthermore, the mixing ratio of each substance 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.

[0110] However, the method for preparing the positive electrode is not limited to the examples given above, and other methods may also be used.

[0111] <3-2. Negative electrode> The negative electrode can be formed by molding a negative electrode mixture.

[0112] Although the components and their proportions of the negative electrode mixture differ, the negative electrode is formed in essentially the same way as the positive electrode described above, and various treatments are performed as necessary, just as with the positive electrode.

[0113] The negative electrode mixture can be prepared by mixing a negative electrode active material with a solid electrolyte. For example, a storage material capable of intercalating and deintercalating lithium ions can be used as the negative electrode active material.

[0114] The absorbed material is not particularly limited, but one or more selected from, for example, 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.

[0115] The negative electrode can also be a sheet-like component made of a material containing a metal that alloys with lithium, such as metallic lithium or indium.

[0116] <3-3. Electrolytes>

[0117] (solid electrolyte) Solid electrolytes are solids that have Li+ ion conductivity. As solid electrolytes, one type selected from sulfides, oxides, polymers, etc., can be used alone, or two or more types can be used in mixtures.

[0118] The sulfide-based solid electrolyte is not particularly limited and can be used as long as it contains sulfur (S) and has lithium-ion conductivity and electronic insulation properties. Examples of sulfide-based 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.

[0119] The oxide-based 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.

[0120] Examples of oxide-based solid electrolytes include lithium phosphate (Li3PO4), Li3PO4NX, LiBO2NX, 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, Li3XLa 2 / 3-X TiO3(0≦X≦2 / 3), Li5La3Ta2O 12 Li7La3Zr2O 12 Li6BaLa2Ta2O 12 Li 3.6 Si 0.6 P 0.4 Examples include O4.

[0121] Furthermore, other inorganic solid electrolytes may be used besides those mentioned above; for example, Li3N, LiI, Li3N-LiI-LiOH, etc., may be used.

[0122] The polymer-based 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). When using a solid electrolyte, the solid electrolyte may also be mixed into the cathode material to ensure contact between the electrolyte and the cathode active material.

[0123] (Non-aqueous electrolyte) For the non-aqueous electrolyte, a lithium salt can be used in which the supporting salt is dissolved in an organic solvent.

[0124] As organic solvents, one or more selected from 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 ethyl methyl sulfone and butanesultone; and phosphorus compounds such as triethyl phosphate and trioctyl phosphate can be used alone or in combination of two or more.

[0125] Supporting salts that can be used include LiPF6, LiBF4, LiClO4, LiAsF6, LiN(CF3SO2)2, and their composite salts. In addition to the above, the non-aqueous electrolyte may also contain radical scavengers, surfactants, and flame retardants.

[0126] <3-4. Separator> Furthermore, the secondary battery may be equipped with a separator. The separator is placed between the positive electrode and the negative electrode and has the function of separating the positive and negative electrodes and holding the electrolyte. As for such a separator, for example, a thin film made of polyethylene or polypropylene with many fine pores can be used, but it is not particularly limited as long as it has the above function.

[0127] <3-5. Shape and composition of secondary batteries> Next, an example of the arrangement and configuration of the components of the secondary battery in this embodiment will be described.

[0128] The secondary battery of this embodiment, which consists of a positive electrode, a negative electrode, and a solid electrolyte as described above, can be made into various shapes, such as coin-type or stacked type. In any shape, the positive electrode and negative electrode can be stacked with the solid electrolyte in between. Then, 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 or the like, and the battery can be sealed in a battery case to make a secondary battery.

[0129] <3-5. Characteristics of Lithium-ion Secondary Batteries> According to the positive electrode active material for lithium-ion secondary batteries of this embodiment, it is possible to provide a positive electrode active material for lithium-ion secondary batteries that has high capacity and excellent high voltage resistance.

[0130] For example, when the positive electrode active material of this embodiment is used as the positive electrode of the test battery shown in Figure 3, it is preferable that the initial discharge capacity measured under the following conditions is 160 mAh / g or more. That is, the test battery shown in Figure 3 is constructed, and the current density is 0.2 mA / cm² in a 25°C environment. 2 The battery is charged to a cutoff voltage of 4.5V (vs. Li) (initial charge capacity), then left idle for 1 hour, and discharged to a cutoff voltage of 2.5V (vs. Li) to measure the initial discharge capacity.

[0131] Furthermore, the ratio of the initial discharge capacity to the initial charge capacity is called the Coulomb efficiency, and it serves as an indicator of the dielectric strength characteristics of the positive electrode active material. For example, when the positive electrode active material of this embodiment is used as the positive electrode of the test battery shown in Figure 3, the Coulomb efficiency is preferably 80% or higher, and may be 83% or higher. The excess capacity relative to the discharge during the initial charge contains components resulting from side reactions related to the deterioration of the coating layer; therefore, the closer the Coulomb efficiency is to 100%, the higher the dielectric strength characteristics of the positive electrode active material. [Examples]

[0132] Next, an active material for a non-aqueous electrolyte secondary battery, a method for producing a positive electrode active material for a non-aqueous secondary battery, and a non-aqueous electrolyte secondary battery according to one embodiment of the present invention will be described in detail with reference to examples. It should be noted that the present invention is not limited to these examples. The methods for analyzing 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.

[0133] (Example 1) 1. Manufacturing of lithium nickel composite oxide particles The positive electrode active material was manufactured by carrying out the following steps.

[0134] (a) Precursor crystallization step First, 10 L of deionized water was added to the reaction vessel (60 L) and stirred while the vessel temperature was set to 50°C. At this time, the reaction vessel was under a nitrogen atmosphere with an oxygen concentration of 1% by volume or less.

[0135] An initial aqueous solution was prepared by adding appropriate amounts of 25% by mass sodium hydroxide aqueous solution and 25% by mass ammonia aqueous solution to the water in the reaction vessel, so that the pH value was 12.8 and the ammonia concentration was 15 g / L based on a liquid temperature of 25°C.

[0136] Simultaneously, nickel sulfate, cobalt sulfate, and manganese sulfate were dissolved in pure water in a molar ratio of nickel, cobalt, and manganese of Ni:Co:Mn = 0.8:0.1:0.1 to prepare 25 L of a 2.0 mol / L mixed aqueous solution.

[0137] A nickel-cobalt-manganese mixed aqueous solution was added dropwise at a constant rate to the initial aqueous solution in the reaction vessel at a rate of 200 mL to prepare the reaction solution. At the same time, 25% by mass aqueous ammonia and 25% by mass aqueous sodium hydroxide solutions were also added dropwise to the initial aqueous solution at a constant rate, and the pH value of the reaction solution was controlled to be maintained at 12.8 based on a liquid temperature of 25°C.

[0138] Next, sulfuric acid was added dropwise to the reaction vessel to adjust the pH of the reaction solution to 11.5. This procedure was intended to lower the pH value, thereby reducing the rate at which the nickel, cobalt, and manganese composite hydroxide precipitated from the liquid phase to the solid phase in the subsequent precursor crystallization step, and thereby improving the uniformity of the resulting particle size distribution and the sphericity of the particles.

[0139] After pH control, 24.8 L of nickel-cobalt-manganese mixed aqueous solution was added dropwise to the reaction aqueous solution in the reaction vessel at a rate of 103 mL / min. At the same time, 25% by mass aqueous ammonia and 25% by mass aqueous sodium hydroxide solution were also added dropwise to the initial aqueous solution at a constant rate, so that the pH of the reaction aqueous solution was maintained at 11.5 and the ammonia concentration at 15 g / L based on a liquid temperature of 25°C.

[0140] After adding the entire nickel-cobalt-manganese mixed aqueous solution dropwise, the pH of the reaction solution was increased to 13.0 based on a liquid temperature of 25°C. This procedure is intended to precipitate nickel ions, which are complexed with ammonia and dissolved in the liquid phase, onto the hydroxide, thereby obtaining the desired chemical composition.

[0141] Subsequently, the reaction solution was separated into solid and liquid phases using a Buchner funnel, a filter, and a vacuum pump / vacuum filter. The resulting solid phase was then dispersed in 20 L of pure water at 40°C, and the solid-liquid separation process was repeated twice to remove water-soluble impurities such as sodium sulfate from the nickel complex hydroxide.

[0142] After washing and solid-liquid separation, the cake-like solid phase was dried in a stationary dryer at 120°C for 24 hours in an air atmosphere, and then passed through a sieve with a mesh size of 100 μm to obtain a powdered nickel composite hydroxide.

[0143] (b) Oxidation roasting process The composite hydroxide prepared as described above was fired in an atmospheric firing furnace (Siliconit Co., Ltd., BM-50100M) at 600°C for 2 hours in an air atmosphere with an oxygen concentration of 20% by volume, and then cooled to room temperature to obtain nickel composite oxide particles.

[0144] (c) Lithium nickel composite oxide synthesis process A lithium mixture was obtained by adding lithium hydroxide monohydrate, weighed so that the ratio of the amount of lithium to the total amount of nickel, cobalt, and manganese contained in the nickel composite oxide was 1.03, to the above nickel composite oxide and mixing it using a tarbler shaker mixer (Dalton Corporation, T2F) (preparation of lithium mixture).

[0145] The above lithium mixture was placed in an alumina sagger and fired in an atmospheric firing furnace (Siliconit Co., Ltd., BM-50100M) at 750°C for 10 hours in an oxygen-nitrogen mixed gas atmosphere with an oxygen concentration of 90% by volume, and then cooled to room temperature (firing). This yielded lithium nickel composite oxide particles.

[0146] 2. Evaluation of lithium nickel composite oxide particles (a) Chemical composition Quantitative analysis using an ICP emission spectrometer (VARIAN 725ES) confirmed that the lithium nickel composite oxide has a molar ratio of each metal element expressed as Li:Ni:Co:Mn = 1.02:0.81:0.10:0.09.

[0147] (b) Crystal structure The crystal structure of these lithium nickel composite oxide particles was measured using XRD (PANALYTICAL, X'Pert, PROMRD), and it was confirmed that it has a layered rock salt type crystal structure, with peaks attributed to the R-3m structure detected in the diffraction pattern.

[0148] (c) Specific surface area The BET specific surface area of ​​this lithium nickel composite oxide was measured using a fully automated BET specific surface area measuring device (MacSorb, manufactured by Mountec Co., Ltd.). As a result, the specific surface area was 0.50 m². 2 I confirmed that it was / g.

[0149] (d) Volume average particle diameter The volume-average particle size of this lithium nickel composite oxide was measured using a laser diffraction scattering particle size distribution analyzer (Microtrac HRA, manufactured by Nikkiso Co., Ltd.). The result confirmed that it was 5.31 μm.

[0150] 3. Manufacturing of positive electrode active material (d) Covering process The positive electrode active material was manufactured by performing the following coating process on the above lithium nickel composite oxide. The total surface area of ​​the lithium nickel composite oxide to be coated was calculated from the mass and specific surface area of ​​the lithium nickel composite oxide. Then, for the above total surface area, 1 m 2 Lithium metaphosphate and 1 m 2 Ammonium molybdate, weighed to a quantity of 3 mol per unit area (molybdenum surface area), was dissolved in deionized water (coating preparation).

[0151] Using the above coating solution, 500g of lithium nickel composite oxide was coated using a rolling flow coating apparatus (MP-01, manufactured by Powrec).

[0152] 500g of lithium nickel composite oxide was subjected to a flow rate of 0.3m in a heated environment at 120°C. 3 The lithium nickel composite oxide was circulated in a chamber with air at 1 / h, and the coating solution was sprayed onto it at a rate of 1.0 ml / min.

[0153] After spraying the entire coating solution, the lithium nickel composite oxide was recovered from the chamber and heat-treated at 300°C for 10 hours under oxygen flow using an atmospheric firing furnace (Siliconit Co., Ltd., BM-50100M). It was then cooled to room temperature to obtain lithium nickel composite oxide particles having a coating layer that served as the positive electrode active material.

[0154] 4. Evaluation of the positive electrode active material The positive electrode active material obtained in this manner was evaluated as follows.

[0155] (a) composition Analysis using an ICP emission spectrometer (VARIAN 725ES) revealed that this cathode active material contains 0.43% by mass of phosphorus (P) and 0.01% by mass of molybdenum (Mo). The phosphorus content on the surface of the coating layer, determined from the specific surface area before coating, was 280 μmol / m². 2 The molybdenum surface content is 2 μmol / m². 2 It was found that...

[0156] (b) Carbon content The carbon content of the obtained positive electrode active material was measured using a carbon analyzer (LECO, model: CS-600) by high-frequency combustion infrared absorption spectroscopy and was found to be 0.14% by mass.

[0157] (c) Moisture content The moisture content of the obtained positive electrode active material was measured using a Karl Fischer moisture meter (Kyoto Electronics Manufacturing Co., Ltd., Model: MKC210) at a vaporization temperature of 300°C, and was found to be 0.06% by mass.

[0158] 5. Manufacturing a secondary battery To evaluate the capacity of the obtained positive electrode active material, a battery with the structure shown in Figure 3 (hereinafter referred to as the "test battery") was used. As shown in Figure 3, the test battery SBA comprises a case having a negative electrode can NC and a positive electrode can PC, and compacted powder cells C housed within the case.

[0159] The case comprises a hollow negative electrode can NC with one end open, and a positive electrode can PC positioned in the opening of the negative electrode can NC. When the positive electrode can PC is positioned in the opening of the negative electrode can NC, a space for housing the compacted powder cell is formed between the positive and negative electrode cans. The positive electrode can PC is fixed to the negative electrode can NC with a wing nut SW and a nut N.

[0160] The negative electrode can NC has the negative terminal, and the positive electrode can PC has the positive terminal. The case is equipped with an insulating sleeve ISV, which secures the negative electrode can NC and the positive electrode can PC to maintain a non-contact state.

[0161] A pressure screw PSW is provided at one closed end of the negative electrode can NC. After fixing the positive electrode can PC to the negative electrode can NC, the pressure screw PSW is tightened toward the housing space of the compacted powder cell C, thereby holding the compacted powder cell C under pressure through the hemispherical washer W. A screw-in plug P is provided at the end of the negative electrode can NC where the pressure screw PSW is located. O-rings OL are provided between the negative electrode can NC and the positive electrode can PC, and between the negative electrode can NC and the plug P, sealing the gap between the negative electrode can NC and the positive electrode can PC and maintaining airtightness within the case.

[0162] Furthermore, the compacted cell C consists of a positive electrode layer PL, a solid electrolyte layer SEL, and a negative electrode layer NL, and is a pellet stacked in this order. The positive electrode layer PL contacts the inner surface of the positive electrode can PC through the lower current collector LCC, and the negative electrode layer NL contacts the inner surface of the negative electrode can NC through the upper current collector UCC, a hemispherical washer W, and a pressure screw PSW, as it is housed in a case. The lower current collector LCC, compacted cell C, and upper current collector UCC are protected by a sleeve SV to prevent electrical contact between the positive electrode layer PL and the negative electrode layer NL.

[0163] These test batteries (SBAs) were fabricated as follows.

[0164] First, 60 mg of the synthesized solid electrolyte (0.75Li2S-0.25P2S5) was pressurized at 25 MPa in a pelletizer to obtain solid electrolyte pellets.

[0165] Next, 100 mg of mesocarbon microbeads (MCMB), which are the negative electrode active material, and 100 mg of solid electrolyte were mixed in a mortar. 22 mg of the solid electrolyte pellet and the MCMB + solid electrolyte mixture were placed in a pellet former and pressurized at 25 MPa to form a negative electrode layer NL on the solid electrolyte (SEL) pellet.

[0166] Next, 120 mg of positive electrode active material and 80 mg of solid electrolyte were mixed in a mortar.

[0167] A solid electrolyte (SEL) + negative electrode layer (NL) pellet and a mixture of positive electrode active material + solid electrolyte (15 mg) were placed in a pellet former and pressurized at 360 MPa to form a positive electrode layer (PL) on the side of the solid electrolyte (SEL) pellet opposite the negative electrode layer (NL). The electrodes were sealed in a case, and the pressurizing screw PSW was tightened with a torque of 6-7 N·m to form a compacted powder cell C. The test battery SBA was fabricated in a glove box with an Ar atmosphere where the dew point was controlled to -80°C.

[0168] 6. Evaluation of secondary batteries The charge / discharge capacity and Coulomb efficiency, which indicate the performance of the fabricated test batteries, were evaluated as follows.

[0169] (a) Initial discharge capacity and Coulomb efficiency The initial discharge capacity is determined by placing the test battery in a constant temperature bath at 25°C, and after the open circuit voltage (OCV) stabilizes, the current density to the positive electrode is measured at 0.2 mA / cm². 2 As a cutoff voltage, 4.5V (vs. Li + Charge to (initial charge capacity) / Li) and after a 1-hour rest, cutoff voltage 2.5V (vs.Li) + The evaluation was performed by measuring the discharge capacity (initial discharge capacity) when discharged to 1 / Li. The measurement result was 163 mAh / g, and the Coulomb efficiency (initial discharge capacity / initial charge capacity) was 85%. Table 1 summarizes the manufacturing conditions of the positive electrode active material, morphological characteristics, and the evaluation results of the secondary battery.

[0170] (Example 2) In the coating preparation step, the target phosphorus surface area of ​​the coating layer is 260 μmol / m². 2 The molybdenum surface content is 14 μmol / m². 2 Except for the points mentioned above, lithium nickel 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 evaluation results are shown in Table 1.

[0171] (Example 3) In the coating preparation step, the target phosphorus surface area of ​​the coating layer is 233 μmol / m². 2The molybdenum surface content is 26 μmol / m². 2 Except for the points mentioned above, lithium nickel 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 evaluation results are shown in Table 1.

[0172] (Example 4) In the coating preparation step, the target phosphorus surface area of ​​the coating layer is 52 μmol / m². 2 , molybdenum surface content 3 μmol / m 2 Except for the points mentioned above, lithium nickel 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 evaluation results are shown in Table 1.

[0173] (Example 5) In the coating preparation step, the target phosphorus surface area of ​​the coating layer is 389 μmol / m². 2 , molybdenum surface content 20 μmol / m 2 Except for the points mentioned above, lithium nickel 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 evaluation results are shown in Table 1.

[0174] (Example 6) In the coating preparation step, the total surface area of ​​the lithium nickel composite oxide to be coated was calculated from the mass and specific surface area of ​​the lithium nickel composite oxide. Then, for the above total surface area, 1 m 2 Lithium metaphosphate and 1 m 2 Lithium nickel 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, except that ammonium tungstate, weighed to have a tungsten content of 3 mol per unit area (tungsten content on both sides), was dissolved in deionized water. The evaluation results are shown in Table 1 (coating preparation).

[0175] (Example 7) In the coating preparation step, the target phosphorus surface area of ​​the coating layer is 247 μmol / m². 2The tungsten surface content is 13 μmol / m². 2 Except for the points mentioned above, lithium nickel 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 6. The evaluation results are shown in Table 1.

[0176] (Example 8) In the coating preparation step, the target phosphorus surface area of ​​the coating layer is 213 μmol / m². 2 , tungsten surface content 24 μmol / m 2 Except for the points mentioned above, lithium nickel 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 6. The evaluation results are shown in Table 1.

[0177] (Example 9) In the coating preparation step, the target phosphorus surface area of ​​the coating layer is 49 μmol / m². 2 , tungsten surface content 3 μmol / m 2 Except for the points mentioned above, lithium nickel 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 6. The evaluation results are shown in Table 1.

[0178] (Example 10) In the coating preparation step, the target phosphorus surface area of ​​the coating layer is 371 μmol / m². 2 , tungsten surface content 20 μmol / m 2 Except for the points mentioned above, lithium nickel 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 6. The evaluation results are shown in Table 1.

[0179] (Comparative Example 1) In the coating preparation step, the target phosphorus surface area of ​​the coating layer is 289 μmol / m². 2 Without adding a compound containing element X (molybdenum or tungsten), the amount of element X is 0 μmol / m³. 2 Except for the points mentioned above, lithium nickel 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 evaluation results are shown in Table 1.

[0180] [Table 1]

[0181] (Evaluation results) The positive electrode active material of the example demonstrated improved high voltage resistance, exhibiting higher discharge capacity and higher Coulomb efficiency when charged and discharged at a cutoff voltage of 4.5V, compared to the positive electrode active material of Comparative Example 1, which did not contain element X in the coating layer. [Explanation of Symbols]

[0182] SBA… Test battery PC... Positive electrode can NC... Negative electrode can ISV…Insulating Sleeve C... Compacted powder cell PL…Positive electrode layer NL…Negative electrode layer SEL…Solid electrolyte layer LCC... Lower current collector UCC... Upper current collector P...plug PSW...Pressure screw W... Hemispherical washer Office lady... O-ring SV... Sleeves SW... Screw N...nut

Claims

1. A positive electrode active material for a lithium-ion secondary battery, comprising lithium nickel composite oxide particles and a coating layer covering at least a portion of the surface of the lithium nickel composite oxide particles, The lithium nickel composite oxide contains, as elements other than oxygen, Li, Ni, Co, and optionally element M (where M is at least one element selected from the group consisting of Mg, Al, Ca, Si, Sc, Ti, V, Fe, Mn, Cu, Cr, Zn, Zr, Nb, Mo, and W), and the molar ratio of these elements is expressed as Li:Ni:Co:M = t:1-x-y:x:y (0.95 ≤ t ≤ 1.20, 0 < x ≤ 0.22, 0 ≤ y ≤ 0.15). The coating layer contains a compound comprising Li, P, and element X (where X is at least one element selected from the group consisting of elements that form hexavalent cations). Positive electrode active material for lithium-ion secondary batteries.

2. The compound is a positive electrode active material for a lithium-ion secondary battery according to claim 1, wherein the composition ratio of elements other than oxygen is expressed as Li:P:X = 1-a:1-a:a (0.01 ≤ a ≤ 0.1).

3. Surface area of ​​the lithium nickel composite oxide 1 m² ２ The positive electrode active material for a lithium-ion secondary battery according to claim 1 or 2, wherein the total amount of P and element X derived from the coating layer per unit is 50 μmol or more and 410 μmol or less.

4. The positive electrode active material for a lithium-ion secondary battery according to claim 1 or 2, wherein the element X is Mo or W, or both.

5. The positive electrode active material for a lithium-ion secondary battery according to claim 1 or 2, wherein the carbon content of the positive electrode active material for a lithium-ion secondary battery is 0.5% by mass or less, and the water content is 0.2% by mass or less.

6. The specific surface area of ​​the aforementioned lithium nickel composite oxide is 0.1 m². ２ / g or more 2.0m ２ The positive electrode active material for a lithium-ion secondary battery according to claim 1 or 2, wherein the amount is less than or equal to / g.

7. The lithium nickel composite oxide is a layered rock salt type compound particle having a crystalline structure belonging to space group R-3m, as described in claim 1 or 2, positive electrode active material for lithium-ion secondary battery.

8. A lithium-ion secondary battery comprising a positive electrode containing the positive electrode active material for lithium-ion secondary batteries described in claim 1 or 2, a negative electrode, and an electrolyte.

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