Coated particle and method for manufacturing same, positive electrode active material for nonaqueous electrolyte secondary battery, and nonaqueous electrolyte secondary battery

Abstract

Provided are coated particles with which, when the coated particles are used as a positive electrode active material for a nonaqueous electrolyte secondary battery, the nonaqueous electrolyte secondary battery in which the coated particles are used exhibits superior charging / discharging capacity and cycle characteristics.　Coated particles according to the present invention each include: a first layer at least including lithium and nickel, the first layer including a lithium metal composite oxide in the form of a primary particle or a secondary particle, as well as nickel (II) oxide on at least a portion of the surface of the lithium metal composite oxide; and a second layer including an oxide containing lithium and one or more substances selected from the group consisting of boron, phosphorus, and sulfur on at least a portion of the surface of the first layer.

Description

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

[0001] The present disclosure relates to coated particles and a method for producing the same, a positive electrode active material for a non-aqueous electrolyte secondary battery, and a non-aqueous electrolyte secondary battery.

[0002] 2. Description of the Related Art Lithium ion secondary batteries have attracted attention as power sources for driving electronic devices such as AV equipment and personal computers because they are small, lightweight, have high energy density, high charge / discharge voltage, and large charge / discharge capacity.

[0003] Lithium ion secondary batteries using layered or spinel-type lithium transition metal composite oxides as the positive electrode active material can obtain a high voltage of 4 V, and are therefore being put into practical use as batteries with high energy density. The main materials proposed are lithium cobalt composite oxide (LiCoO), which is relatively easy to synthesize. ２ ), and lithium nickel composite oxide (LiNiO ２ ), lithium nickel cobalt manganese composite oxide (LiNi １／３ Co １／３ Mn １／３ O ２ ), lithium manganese composite oxide (LiMn ２ O ４ ) etc.

[0004] Such lithium metal composite oxides can generally be produced by mixing a lithium source and a metal source such as nickel and then firing the mixture. The positive electrode of a lithium ion secondary battery is formed, for example, by mixing a positive electrode active material with a binder such as polyvinylidene fluoride (PVDF) and a solvent such as N-methyl-2-pyrrolidone (NMP) to form a positive electrode mixture paste, which is then applied to a current collector such as aluminum foil. When this occurs, if lithium is liberated from the lithium metal composite oxide in the positive electrode mixture paste, it may react with moisture contained in the binder or the like to produce lithium hydroxide.

[0005] When lithium hydroxide is generated in this way, the lithium hydroxide reacts with the binder, causing the positive electrode mixture paste to gel. Gelation of the positive electrode mixture paste leads to poor operability in the manufacturing process and a decrease in yield. This tendency is particularly pronounced when the lithium relative to the transition metal in the lithium transition metal composite oxide, which is the positive electrode active material, is in excess of the stoichiometric ratio, particularly when the proportion of nickel among the transition metals is high.

[0006] Here, the liberation of lithium is thought to be due to lithium that has not been incorporated into the crystal lattice of the lithium metal composite oxide, and due to lithium that has been incorporated into the crystal lattice of the lithium metal composite oxide. When lithium that has been incorporated into the crystal lattice of the lithium metal composite oxide is eluted, the resistance of the lithium metal composite oxide increases, which can reduce the charge / discharge efficiency.

[0007] In order to remove the lithium that has not been incorporated into the crystal lattice of the lithium metal composite oxide, the fired product is subjected to a water washing treatment, as disclosed in, for example, Patent Document 1.

[0008] Furthermore, in order to prevent the elution of lithium incorporated into the crystal lattice of the lithium metal composite oxide, it has been practiced to coat the oxide with a boron compound containing oxygen, as disclosed in Patent Document 2, for example.

[0009] JP 2005-97087 A JP 2019-114560 A

[0010] However, according to the findings of the present inventors, when an attempt is made to remove lithium by subjecting a lithium metal composite oxide to a water washing treatment as in Patent Document 1, the lithium incorporated into the crystal lattice undergoes ion exchange with protons, causing the surface of the lithium metal composite oxide to become lithium-deficient or to form a rock salt phase, thereby reducing the charge / discharge capacity of the lithium ion secondary battery.

[0011] Furthermore, as in Patent Document 2, coating a lithium metal composite oxide with a boron compound containing oxygen has a certain effect in preventing the elution of lithium in the crystal lattice of the lithium metal composite oxide into the electrolyte solution and improving the cycle characteristics, but there is still room for improvement in order to further improve the cycle characteristics.

[0012] The present disclosure has been made in view of the above-described circumstances, and aims to provide coated particles that, when used as a positive electrode active material in non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries, enable the non-aqueous electrolyte secondary batteries using the coated particles to exhibit excellent charge / discharge capacity and cycle characteristics.

[0013] The present inventors conducted extensive research to solve the above-mentioned problems. As a result, they discovered that by using coated particles as a positive electrode active material for a non-aqueous electrolyte secondary battery, the coated particles include: a composite oxide containing at least lithium and nickel, in the form of primary or secondary particles; a first layer containing nickel(II) oxide on at least a portion of the surface of the composite oxide; and a second layer containing an oxide layer containing lithium and one or more elements selected from the group consisting of boron, phosphorus, and sulfur on at least a portion of the surface of the first layer, the non-aqueous electrolyte secondary battery using the coated particles exhibits excellent charge / discharge capacity and cycle characteristics. Specifically, the present disclosure provides the following.

[0014] (1) Coated particles comprising: a lithium metal composite oxide in the form of primary particles or secondary particles, the lithium metal composite oxide containing at least lithium and nickel; a first layer containing nickel (II) oxide on at least a portion of the surface of the lithium metal composite oxide; and a second layer containing an oxide containing lithium and one or more elements selected from the group consisting of boron, phosphorus, and sulfur on at least a portion of the surface of the first layer.

[0015] (2) The lithium metal composite oxide has a layered rock salt structure and is represented by the general formula Li ａ Ni １－ｂ－ｃ Mn ｂ M ｃ O ２(wherein M is one or more elements other than Li, Ni, Mn and O, and 0.95≦a≦1.15, 0≦b+c≦0.70)

[0016] (3) The coated particle according to (1) or (2) above, wherein the average thickness of the first layer is 3 nm or more and 100 nm or less.

[0017] (4) The coated particle according to (1) or (2), wherein the lithium metal composite oxide and the first layer both have a layered structure, and the layered structure of the lithium metal composite oxide and the layered structure of the first layer form a continuous structure.

[0018] (5) A positive electrode active material for a non-aqueous electrolyte secondary battery, comprising the coated particles according to (1) or (2).

[0019] (6) A non-aqueous electrolyte secondary battery comprising the positive electrode active material for a non-aqueous electrolyte secondary battery according to (5) above.

[0020] (7) A method for producing coated particles, comprising adding or spraying a compound containing one or more selected from the group consisting of boron, phosphorus, and sulfur to a lithium metal composite oxide in the form of primary particles or secondary particles, the lithium metal composite oxide containing at least lithium and nickel, and then subjecting the resulting mixture to a heat treatment to obtain the coated particles according to claim 1 or 2.

[0021] According to the present disclosure, it is possible to provide coated particles that, when used as a positive electrode active material in a non-aqueous electrolyte secondary battery, suppress gelation of the positive electrode mixture paste while causing the non-aqueous electrolyte secondary battery to exhibit excellent charge / discharge capacity and cycle characteristics, as well as a positive electrode active material for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery using the coated particles.

[0022] 2 is a schematic diagram of a cross section of a coated particle sample according to an embodiment of the present disclosure. 3 is a high-resolution STEM image of a surface layer of a primary particle of a coated particle sample of Example 3. 4 is a diagram for explaining the boundary between the lithium metal composite oxide and the first layer, and the boundary between the first layer and the second layer in the high-resolution STEM image of FIG.

[0023] Hereinafter, embodiments of the present disclosure will be described, but the present disclosure is not limited in any way to the description of the embodiments and can be implemented with appropriate modifications.

[0024] <Coated Particle> The coated particle according to an embodiment of the present disclosure contains at least lithium and nickel and has a lithium metal composite oxide in the form of primary particles or secondary particles, a first layer containing nickel (II) oxide on at least a portion of the surface of the lithium metal composite oxide, and a second layer containing an oxide containing lithium and one or more elements selected from the group consisting of boron, phosphorus, and sulfur on at least a portion of the surface of the first layer.

[0025] Fig. 1 is a schematic diagram of a cross section of a coated particle sample according to an embodiment of the present disclosure. The coated particle 1 shown in Fig. 1 includes a lithium metal composite oxide 2 containing at least lithium and nickel and in the form of primary or secondary particles, a first layer 3 containing nickel (II) oxide on at least a portion of the surface of the lithium metal composite oxide 2, and a second layer 4 containing an oxide containing lithium and one or more elements selected from the group consisting of boron, phosphorus, and sulfur on at least a portion of the surface of the first layer 3. Note that, although Fig. 1 shows the lithium metal composite oxide 2 as a circle and the outer contours of the first layer 3 and the second layer 4 as being circular, the cross-sectional shape is not limited to being circular.

[0026] In the coated particle 1, the lithium metal composite oxide acting as the positive electrode active material is coated with two layers: a first layer 3 containing thermally and mechanically stable nickel(II) oxide, and a second layer 4 containing an oxide highly resistant to elution into the electrolyte. When this coated particle 1 is used as the positive electrode active material, the nonaqueous electrolyte secondary battery can exhibit excellent charge / discharge capacity and cycle characteristics. While the reason for this is not entirely clear, it is believed that the presence of the thermally and mechanically stable nickel(II) oxide layer coated on the surface of the lithium metal composite oxide 2 can prevent the coated particle 1 serving as the positive electrode active material in a highly charged state from decomposing into a rock salt structure or a spinel structure from the surface toward the interior, thereby preventing deterioration of the cycle characteristics. It is also believed that the presence of the second layer can prevent the coated particle serving as the positive electrode active material from directly contacting the electrolyte and deteriorating, resulting in deterioration of the cycle characteristics.

[0027] (Lithium Metal Composite Oxide) The lithium metal composite oxide 2 contains at least lithium and nickel, and is in the form of primary particles or secondary particles.

[0028] The chemical composition of the lithium metal composite oxide 2 is not particularly limited, but it may have a layered rock salt structure and be represented by the general formula Li ａ Ni １－ｂ－ｃ Mn ｂ M ｃ O ２ (wherein M is one or more elements other than Li, Ni, Mn and O, and 0.95≦a≦1.15, 0≦b+c≦0.70) is preferably used.

[0029] In the general formula, the value of a is not particularly limited as long as it is within the range of 0.95≦a≦1.15, and may be, for example, 0.955 or more, 0.96 or more, 0.965 or more, 0.97 or more, 0.975 or more, or 0.98 or more. On the other hand, the value of a may be 1.145 or less, 1.14 or less, 1.135 or less, 1.13 or less, 1.125 or less, 1.12 or less, 1.115 or less, 1.11 or less, 1.105 or less, 1.10 or less, 1.095 or less, 1.09 or less, 1.085 or less, 1.08 or less, 1.075 or less, or 1.07 or less.

[0030] In the general formula, the value of b+c is not particularly limited as long as it is within the range of 0≦b+c≦0.70, and may be, for example, 0.05 or more, 0.1 or more, and the value of b+c may be 0.65 or less, 0.6 or less, 0.55 or less, 0.5 or less, 0.45 or less, 0.4 or less, 0.35 or less, 0.3 or less, 0.25 or less, 0.2 or less, or 0.15 or less.

[0031] In the general formula, the element M is not particularly limited as long as it is one or more elements other than Li, Ni, Mn, and O, and examples that can be used include Co, Al, Ti, Mg, Zn, Nb, W, Mo, Sb, V, Cr, Ca, Fe, Ga, Sr, Y, Ru, In, Sn, Ta, Bi, Zr, and B. The type of element M may be selected depending on the purpose of addition. When multiple elements are contained as element M, the value of c represents the total amount of the multiple elements.

[0032] (First Layer) The first layer 3 is disposed on at least a part of the surface of the lithium metal composite oxide described above, and contains nickel (II) oxide.

[0033] It is preferable that the lithium metal composite oxide 2 and the first layer 3 form a continuous structure. Here, "the lithium metal composite oxide 2 and the first layer 3 form a continuous structure" means that, when a lattice image of the coated particle is observed using a high-resolution STEM image as shown in Fig. 3, the oxygen atoms constituting the lithium metal composite oxide and the oxygen atoms constituting the first layer (both of which are shown as relatively white dots, with typical parts surrounded by squares) are continuously arranged even across the boundary between them.

[0034] Since the first layer 3 contains nickel(II) oxide, high-resolution STEM-EDS was used to confirm that it has a rock salt structure derived from nickel(II) oxide and that the layer contains nickel, and this layer was identified as the first layer. For the second layer 4, XPS was used to confirm that an oxide layer of lithium and boron, etc., exists on the surface, and then high-resolution STEM-EDS was used to identify the layer containing boron and oxygen as the second layer 4.

[0035] The average thickness of the first layer 3 is not particularly limited, but is preferably 3 nm or more, 4 nm or more, or 5 nm or more. The average thickness of the first layer may be 100 nm or less, 95 nm or less, 90 nm or less, 85 nm or less, 80 nm or less, 75 nm or less, 70 nm or less, 65 nm or less, 60 nm or less, 55 nm or less, 50 nm or less, 45 nm or less, 40 nm or less, 35 nm or less, 30 nm or less, 25 nm or less, 20 nm or less, 19 nm or less, 18 nm or less, 17 nm or less, 16 nm or less, 15 nm or less, 14 nm or less, 13 nm or less, 12 nm or less, or 11 nm or less. When the average thickness of the first layer 3 is within the required range, the resistance of the battery can be reduced without becoming a resistance component that impairs lithium mobility. The average thickness is measured at a magnification of 10,000,000 times using a high-resolution STEM (JEM-ARM200F Dual-X, manufactured by JEOL Ltd.). Specifically, after identifying the boundaries of the lithium metal composite oxide 2, the first layer 3, and the second layer 4 as described above, 22 points are set on the boundary between the lithium metal composite oxide 2 and the first layer 3, dividing the length of the boundary into 23 equal parts. The shortest distance from each of 20 points, excluding the two points at both ends, to the boundary between the first layer 3 and the second layer 4 is defined as the thickness at each point, and the average of the 20 points is defined as the average thickness.

[0036] The first layer 3 may contain a compound other than nickel (II) oxide. In one embodiment, the first layer 3 contains a lithium metal composite oxide 2 and at least one constituent element other than lithium (Ni 、 Mn 、 It is preferable that M and O) are the same (contain the same element as the lithium metal composite oxide). However, it is not excluded that the first layer 3 contains lithium, and the first layer 3 may contain lithium.

[0037] (Second Layer) The second layer 4 contains lithium and one or more elements selected from the group consisting of boron, phosphorus, and sulfur on at least a portion of the surface of the first layer 3 described above.

[0038] The average particle diameter of the coated particles 1 (which refers to the primary particle diameter in the case of primary particles and the secondary particle diameter in the case of secondary particles, and may be abbreviated as D50) is not particularly limited, and may be, for example, 80 nm or more, 100 nm or more, 120 nm or more, 150 nm or more, 170 nm or more, 200 nm or more, 250 nm or more, 300 nm or more, 350 nm or more, 400 nm or more, or 450 nm or more. On the other hand, the D50 of coated particles 1 is 25 μm or less, 24.5 μm or less, 24 μm or less, 23.5 μm or less, 23 μm or less, 22.5 μm or less, 22 μm or less, 21.5 μm or less, 21 μm or less, 20.5 μm or less, 20 μm or less, 19.5 μm or less, 19 μm or less, 18.5 μm or less, 18 μm or less, 17.5 μm or less, 17 μm or less, 16.5 μm or less, 16 μm or less, 15.5 μm or less D50 may be 15 μm or less, 14.5 μm or less, 14 μm or less, 13.5 μm or less, 13 μm or less, 12.5 μm or less, 12 μm or less, 11.5 μm or less, 11 μm or less, 10.5 μm or less, 10 μm or less, 9.5 μm or less, 9 μm or less, 8.5 μm or less, 8 μm or less, 7.5 μm or less, 7 μm or less, 6.5 μm or less, 6 μm or less, 5.5 μm or less, 5 μm or less, or 4.5 μm or less. D50 is measured on a volume basis by a wet laser method using a laser particle size distribution analyzer (Microtrac HRA, manufactured by Nikkiso Co., Ltd.).

[0039] <Method for manufacturing positive electrode active material for non-aqueous electrolyte secondary battery> The positive electrode active material for non-aqueous electrolyte secondary battery according to the embodiment of the present disclosure can be manufactured, for example, by performing the following steps in this order, but can also be manufactured by other methods. Furthermore, even if the steps are performed in the order below, depending on the combination of conditions, the positive electrode active material for non-aqueous electrolyte secondary battery according to the embodiment of the present disclosure may not be manufactured. However, the positive electrode active material for non-aqueous electrolyte secondary battery according to the embodiment of the present disclosure can be manufactured at least under the conditions shown in the examples described below.

[0040] Step 1: A precursor composite compound containing at least nickel is synthesized, and the precursor composite compound is mixed with a lithium compound to prepare a mixture. Step 2: The mixture prepared in Step 1 is fired. Step 3: If necessary, the composite oxide obtained by firing in Step 2 is subjected to a water washing treatment. Step 4: The composite oxide obtained in Step 2 or 3 is subjected to a surface treatment using a compound containing one or more elements selected from the group consisting of boron, phosphorus, and sulfur.

[0041] [Step 1] First, a precursor composite compound is synthesized as an aggregate of primary particles containing at least a transition metal. The method for synthesizing the precursor composite compound is not particularly limited, and a method can be used in which, for example, an aqueous solution containing a transition metal aqueous solution and various aqueous solutions of compounds containing other elements corresponding to the composition of the target composite oxide is dropped into a reaction vessel in which an alkaline aqueous solution such as a sodium hydroxide aqueous solution or an ammonia solution is stirred as a mother liquid, and while also dropping sodium hydroxide or the like, the pH is monitored and controlled to be within an appropriate range, and co-precipitation is carried out by a wet reaction to obtain, for example, a hydroxide, an oxide obtained by calcining the hydroxide, a carbonate, or the like.

[0042] In the synthesis reaction, after preparing the aqueous alkaline solution to serve as the mother liquid, it is preferable to create a nitrogen atmosphere in the reaction tank using an inert gas, or industrially preferably nitrogen gas, to reduce the oxygen concentration in the reaction tank system and in the solution as much as possible. If the oxygen concentration is too high, there is a risk that the coprecipitated hydroxide may be excessively oxidized by a predetermined amount or more of remaining oxygen, or that the formation of aggregates by crystallization may be hindered.

[0043] The aqueous solution of the transition metal is not particularly limited, but for example, an acidic aqueous solution is preferably used, and in the case of a nickel compound, it is more preferable to use an aqueous sulfuric acid solution such as an aqueous nickel sulfate solution. In addition, one or more kinds of aqueous solutions of the transition metal can be used.

[0044] The nickel compound is not particularly limited, but may be one or more selected from, for example, nickel sulfate, nickel oxide, nickel hydroxide, nickel nitrate, nickel carbonate, nickel chloride, nickel iodide, and metallic nickel.

[0045] The cobalt compound is not particularly limited, but for example, one or more compounds selected from cobalt sulfate, cobalt oxide, cobalt hydroxide, cobalt nitrate, cobalt carbonate, cobalt chloride, cobalt iodide, and metallic cobalt can be used.

[0046] The manganese compound is not particularly limited, but may be one or more selected from, for example, manganese sulfate, manganese oxide, manganese hydroxide, manganese nitrate, manganese carbonate, manganese chloride, manganese iodide, and manganese metal.

[0047] The aluminum compound is not particularly limited, but examples thereof include aluminum sulfate, aluminum oxide, aluminum hydroxide, aluminum nitrate, aluminum carbonate, aluminum chloride, aluminum iodide, sodium aluminate, and metallic aluminum.

[0048] The titanium compound is not particularly limited, but for example, one or more compounds selected from titanyl sulfate, titanium oxide, titanium hydroxide, titanium nitrate, titanium carbonate, titanium chloride, titanium iodide, and metallic titanium can be used.

[0049] The iron compound is not particularly limited, but may be one or more selected from, for example, iron sulfate, iron oxide, iron hydroxide, iron nitrate, iron carbonate, iron chloride, iron iodide, metallic iron, and the like.

[0050] The niobium compound is not particularly limited, but for example, one or more compounds selected from niobium oxide, niobium chloride, lithium niobate, niobium iodide, and the like can be used.

[0051] The tungsten compound is not particularly limited, but for example, one or more compounds selected from tungsten oxide, sodium tungstate, ammonium paratungstate, tungsten hexacarbonyl, tungsten sulfide, and the like can be used.

[0052] The magnesium compound is not particularly limited, but for example, one or more compounds selected from magnesium sulfate, magnesium oxide, magnesium hydroxide, magnesium nitrate, magnesium carbonate, magnesium chloride, magnesium iodide, and metallic magnesium can be used.

[0053] The zinc compound is not particularly limited, but may be one or more selected from, for example, zinc sulfate, zinc oxide, zinc hydroxide, zinc nitrate, zinc carbonate, zinc chloride, zinc iodide, and metallic zinc.

[0054] As for other elements, one or more selected from sulfates, oxides, hydroxides, nitrates, carbonates, chlorides, iodides, metals, and the like can be used.

[0055] The blending ratio of each compound may be adjusted so that the amount of each element is in the desired ratio, taking into consideration the composition of the target composite oxide.

[0056] The appropriate pH range for synthesizing the precursor composite compound is not particularly limited and can be determined so as to obtain the desired secondary particle size and shape, such as the degree of coarseness and density, and is generally in the range of about 10 to 13.

[0057] The precursor composite compound obtained by the wet reaction is preferably subjected to a washing treatment, dehydrated, and then dried.

[0058] By subjecting the precursor composite compound to a washing treatment, impurities such as sulfate groups, carbonate groups, and sodium that have been incorporated into the aggregated particles or attached to the surface during the reaction can be washed away. For small amounts of the washing, a Nutsche washing method using a Buchner funnel can be used, or a method in which the suspension after the reaction is sent to a press filter for washing with water and dehydration can be used. For example, pure water, an aqueous sodium hydroxide solution, an aqueous sodium carbonate solution, etc. can be used in the washing treatment, but pure water is preferably used industrially. However, if a large amount of residual sulfate groups remains, an aqueous sodium hydroxide solution with its pH adjusted according to the amount of the remaining sulfate groups may be used.

[0059] Next, the precursor composite compound thus synthesized and a lithium compound are mixed in a predetermined ratio to prepare a mixture. The mixing may be a solvent-based mixing in which the precursor composite compound and the lithium compound are each made into solutions such as aqueous solutions and these solutions are mixed in a predetermined ratio, or a non-solvent-based mixing in which a powder of the precursor composite compound and a powder of the lithium compound are weighed out to give a predetermined ratio and then mixed together in a dry state.

[0060] The lithium compound is not particularly limited, and various lithium salts can be used. Specifically, the lithium compound can be, for example, one or more selected from anhydrous lithium hydroxide, lithium hydroxide hydrate, lithium nitrate, lithium carbonate, lithium acetate, lithium bromide, lithium chloride, lithium citrate, lithium fluoride, lithium iodide, lithium lactate, lithium oxalate, lithium phosphate, lithium pyruvate, lithium sulfate, and lithium oxide. Among these, it is preferable to use one or more selected from anhydrous lithium hydroxide and lithium hydroxide hydrate.

[0061] The blending ratio of the lithium compound and the precursor composite compound is not particularly limited, but may be adjusted appropriately so that the amount of lithium and the total amount of each element are in the desired ratio, taking into consideration the composition of the target composite oxide.

[0062] [Step 2] As described above, when producing a composite oxide containing at least a transition metal, a lithiation reaction and crystal growth occur during firing, and the lithiation reaction requires a certain oxygen partial pressure. The lithiation reaction produces a composite oxide containing lithium. Thereafter, the temperature is raised to a predetermined temperature to promote crystal growth.

[0063] The maximum temperature of the mixture during firing is preferably 650°C to 1100°C, 670°C to 1000°C, or 700°C to 980°C. The firing time at the maximum temperature is preferably 1 to 24 hours, 1 to 20 hours, 1 to 15 hours, 1 to 10 hours, 2 to 9 hours, or 3 to 8 hours. By setting the firing temperature to be equal to or higher than the melting point of the lithium compound in the mixture and the maximum temperature and time at which the lithium-containing composite oxide undergoes desired crystal growth or particle growth, the desired composite compound can be obtained.

[0064] In general, firing is performed by weighing out a lithium compound, a precursor composite compound, and, if necessary, a compound of element M, mixing them in a mixer, and then filling the resulting mixed powder into a container such as a crucible or a sagger. However, particularly in the lithiation reaction, it becomes difficult to vent the generated gas to the outside and to diffuse the required oxygen concentration, especially toward the bottom of the container where the mixed powder is filled. As a result, it becomes difficult to control the uniformity of the reaction and the primary particle size.

[0065] Therefore, when producing the composite oxide according to the embodiment of the present disclosure, it is preferable to first use a method in which pre-calcination is performed under the following predetermined conditions in step 2, and then main calcination is performed under the following predetermined conditions. However, pre-calcination is not an essential step.

[0066] In the pre-firing of step 2, it is preferable to adopt a firing method that particularly promotes the lithiation reaction. Specifically, a method can be used that makes the mixture more susceptible to heat, easily expels gas generated from the lithium compound, and diffuses gas with a high oxygen partial pressure into the mixture (particles). For example, by pre-firing a smaller amount of the mixture, it is possible to achieve the desired properties.

[0067] In step 2, the mixture can be pre-fired by filling it into a sagger or crucible and firing it in a stationary furnace, roller hearth kiln, or pusher furnace, but a rotary kiln in which the mixture is fired while being fluidized can also be used.

[0068] The maximum temperature of the mixture to be pre-baked is not particularly limited, and is preferably adjusted depending on the type of lithium compound used in preparing the mixture. This ensures that the precursor composite compound and the lithium compound in the mixture react with each other, that the lithiation reaction proceeds reliably and uniformly, and that no heterogeneous phases are generated, thereby enabling the production of the desired composite oxide.

[0069] The atmosphere for the pre-baking is not particularly limited as long as it is an oxidizing atmosphere in which the lithiation reaction proceeds reliably and uniformly. For example, it is preferable to use a decarbonated oxidizing gas atmosphere with a carbon dioxide concentration of 30 ppm or less, or an oxygen atmosphere with an oxygen concentration of 80 vol % or more, or 90 vol % or more.

[0070] The pre-baking time is not particularly limited as long as the lithiation reaction proceeds reliably and uniformly, and is preferably 1 to 10 hours, or 2 to 8 hours, for example.

[0071] The pre-calcined mixture is then subjected to main calcination at a higher temperature to promote crystal growth and particle growth. During this process, it is necessary to ensure that crystal growth proceeds uniformly and reliably to obtain a composite oxide with the desired crystal structure.

[0072] The atmosphere for the main firing is not particularly limited, as long as it ensures reliable and uniform crystal growth, has an oxygen partial pressure that does not reduce the transition metal contained in the mixture to be fired, and preferably has a low moisture content and carbon dioxide concentration. For example, it is preferable to use a decarbonated oxidizing gas atmosphere with a carbon dioxide concentration of 30 ppm or less, or an oxygen atmosphere with an oxygen concentration of preferably 80 vol% or more, or 90 vol% or more.

[0073] The temperature of the main firing is not particularly limited as long as it is higher than the temperature of the pre-firing, and can be adjusted depending on the composition of the composite oxide to be obtained. For example, it is preferable to adjust the maximum temperature to 700°C to 1100°C, 710°C to 1000°C, or 720°C to 980°C. By keeping the maximum temperature within the required range, it is possible to obtain a composite oxide having a desired crystal structure with reduced unreacted components, and it is also possible to prevent a decrease in the battery characteristics of a nonaqueous electrolyte secondary battery using the obtained composite oxide in the positive electrode. Furthermore, for example, when obtaining a composite oxide with a Ni content of 20 mol% to 80 mol% of elements other than Li, it is preferable to fire the mixture at a maximum temperature not exceeding 1100°C.

[0074] The time for the main firing is not particularly limited as long as it is long enough to form a composite oxide having a desired crystal structure, and is preferably, for example, 1 to 15 hours, 2 to 12 hours, or 2 to 10 hours.

[0075] [Step 3] The lithium metal composite oxide obtained in Step 2 may contain impurities such as unreacted lithium compounds and lithium compounds that appear on the particle surface due to the crystalline structure during the calcination process. If the amount of such lithium compounds is large, a compound containing one or more elements selected from the group consisting of boron, phosphorus, and sulfur (hereinafter also referred to as "boron, etc.") used in Step 4 described below (hereinafter also referred to as "boron, etc.") may form an oxide of lithium and boron, etc., but may not coat the lithium metal composite oxide. Therefore, to remove or reduce these impurities, the product may be washed with water, followed by heat treatment and drying. Note that Step 3 is not an essential step.

[0076] As described above, in order to produce the coated particles of the embodiment of the present disclosure, it is preferable to wash the lithium metal composite oxide obtained in step 2. However, if the lithium metal composite oxide is washed too much, the lithium contained in the crystal lattice of the lithium metal composite oxide may be deeply removed, which may have an adverse effect on conductivity.

[0077] On the other hand, when washing is not possible due to reasons such as process design, an alternative is to increase the amount of boron or other compounds added. The boron or other compounds react with lithium not contained in the crystal lattice to form a compound, which suppresses the generation of lithium hydroxide and prevents gelation of the positive electrode mixture.

[0078] [Step 4] A compound such as boron is added to or mixed with the lithium metal composite oxide obtained in Step 2 or 3, or the compound is sprayed thereon to bring the lithium metal composite oxide into contact with the compound such as boron, followed by heat treatment under specific conditions. This allows the production of coated particles according to an embodiment of the present disclosure. It is believed that this reaction occurs when the compound such as boron reacts with water or protons in the crystals to become weakly acidic, causing a partial disproportionation reaction on the surface of the lithium metal composite oxide to transform it into a nickel oxide-like substance, and that subsequent heat treatment forms a mechanically and thermally stable nickel(II) oxide layer. It is also believed that the compound such as boron reacts with lithium to form an oxide of lithium and boron, etc., which has high ionic conductivity, and coats the surface of the nickel(II) oxide layer.

[0079] The compound added for the above-mentioned surface treatment is at least one selected from boron compounds, phosphorus compounds and sulfur compounds.

[0080] As the boron compound, boric acid, lithium tetraborate, sodium tetraborate, sodium perborate, diboron trioxide, boric trifluoride, etc. can be used.

[0081] Examples of phosphorus compounds that can be used include diammonium hydrogen phosphate, ammonium dihydrogen phosphate, calcium phosphate, calcium dihydrogen phosphate, and phosphoric acid.

[0082] As the sulfur compound, ammonium sulfate, lithium sulfate, sodium sulfate, sulfur fluoride, sulfuric acid, etc. can be used.

[0083] Such a lithium metal composite oxide can be used as a positive electrode active material for non-aqueous electrolyte secondary batteries such as lithium secondary batteries.

[0084] The nonaqueous electrolyte secondary battery according to the embodiment of the present disclosure includes a positive electrode containing the lithium metal composite oxide described above as a positive electrode active material. The nonaqueous electrolyte secondary battery is composed of a positive electrode, a negative electrode, and an electrolyte solution containing an electrolyte.

[0085] When manufacturing a positive electrode, a conductive agent and a binder are added to and mixed with the lithium metal composite oxide according to the embodiment of the present disclosure. The conductive agent is preferably, for example, acetylene black, carbon black, graphite, or the like. The binder is preferably, for example, polytetrafluoroethylene, polyvinylidene fluoride, or the like.

[0086] The negative electrode is not particularly limited, and examples thereof include negative electrode active materials such as lithium metal, graphite, and low-crystalline carbon materials, as well as one or more non-metallic or metallic elements selected from Si, Al, Sn, Pb, Zn, Bi, and Cd, alloys containing them, or chalcogen compounds containing them.

[0087] The solvent for the electrolytic solution is not particularly limited, but may be, for example, an organic solvent containing one or more selected from carbonates such as ethylene carbonate, propylene carbonate, dimethyl carbonate, and diethyl carbonate, and ethers such as dimethoxyethane.

[0088] The electrolyte is not particularly limited, but may be, for example, lithium hexafluorophosphate (LiPF ６ ), lithium salts such as lithium perchlorate and lithium tetrafluoroborate can be dissolved in a solvent and used.

[0089] The present disclosure will be described in more detail below with reference to examples, but the present disclosure is not limited to these examples.

[0090] <Preparation of Samples> Samples of Examples 1 to 13 and Comparative Examples 1 and 2 were produced according to the methods described below.

[0091] Example 1 (Production of Precursor Compound) A metal aqueous solution was obtained by mixing an aqueous nickel sulfate solution, an aqueous cobalt sulfate solution, and an aqueous manganese sulfate solution so that the molar ratio of Ni to Co to Mn was Ni:Co:Mn = 89:6:5. 10 L of pure water containing 300 g of sodium hydroxide solution and 500 g of ammonia water was prepared in advance in a reaction vessel as a mother liquid. A nitrogen atmosphere was created in the reaction vessel by supplying nitrogen gas at a flow rate of 0.7 L / min, and the reaction was also carried out in a nitrogen atmosphere.

[0092] Thereafter, while rotating the stirring blade at 1000 rpm, the metal aqueous solution, the sodium hydroxide aqueous solution, and the ammonia water were simultaneously dropped at a predetermined rate, and the amount of the alkaline solution dropped was adjusted so that the pH became 11.5. Through a crystallization reaction, Ni, Co, and Mn were crystallized and coprecipitated to form aggregated particles, thereby obtaining a coprecipitate.

[0093] Thereafter, the slurry in the reactor was subjected to solid-liquid separation and further washed with pure water to reduce remaining impurities, and the coprecipitate in a cake state was then dried at 110°C for 12 hours in an air environment to obtain a precursor compound.

[0094] (Production of Lithium Metal Composite Oxide) The obtained precursor compound, anhydrous lithium hydroxide, and aluminum hydroxide were weighed out so that the ratio (molar ratio) of Al to the total amount of Ni, Co, and Mn was Al / (Ni+Co+Mn+Al) = 0.02, and the ratio (molar ratio) of Li to the total amount of Ni, Co, Mn, and Al was Li / (Ni+Co+Mn+Al) = 1.07, and these were mixed using a mixer to prepare a raw material mixture.

[0095] The mixture was then fired in an electric furnace under an oxygen atmosphere (oxygen concentration: 97 vol%) at a maximum temperature of 770°C for 5 hours, cooled, and pulverized in a pulverizer to obtain a lithium metal composite oxide. The lithium metal composite oxide had an average secondary particle size of approximately 12.8 μm. The moisture content, as determined by the Karl Fischer method, was 131 ppm.

[0096] The obtained lithium metal composite oxide was added to pure water (water temperature 25°C) in a reaction tank (volume 10 L) and stirred for 10 minutes to prepare a slurry. Here, the ratio of the amount of lithium metal composite oxide to the amount of pure water (solid-liquid ratio) was adjusted to 2000 g / L. The obtained slurry was filtered using a Buchner funnel to obtain a cake of lithium metal composite oxide. The moisture content of the cake was 6.2 wt%.

[0097] The resulting cake was heat-treated at 100°C for 60 minutes using a vacuum dryer to dry the cake, thereby obtaining a dry powder of lithium metal composite oxide. As a result of the ICP measurement described later, the final composition was as per the charge ratio, and Li １．０７ Ni ０．８７２２ Mn ０．０４９ Co ０．０５８８ Al ０．０２ O ２ This is expressed by the general formula, Li ａ Ni １－ｂ－ｃ Mn ｂ M ｃ O ２ In this case, a = 1.07, b = 0.049, and c = 0.0788. The moisture content of the obtained dried powder was 653 ppm, and as a result of neutralization titration by the Walder method, the amount of residual LiOH was 0.29 wt %.

[0098] (Production of coated particles) Then, boric acid powder having an average particle size of 13 μm was weighed into the dried powder so that the amount of B was 1000 ppm relative to the final coated particle sample, and mixed using a mixer to obtain a mixed powder. The obtained mixed powder was annealed for 3 hours in an oxygen atmosphere (oxygen concentration: 97 vol%) using an electric furnace so that the maximum temperature of the mixed powder was 300° C., and after cooling, it was pulverized in a pulverizer to obtain a coated particle sample.

[0099] Example 2 A coated particle sample was obtained in the same manner as in Example 1, except that the amount of boric acid powder mixed with the dry powder was changed so that the detected amount of B was 500 ppm.

[0100] Example 3 A coated particle sample was obtained in the same manner as in Example 1, except that the amount of boric acid powder mixed with the dry powder was changed so that the detected amount of B was 2000 ppm.

[0101] Example 4 A coated particle sample was obtained in the same manner as in Example 1, except that the drying time in the vacuum dryer when drying the cake was changed to 30 minutes.

[0102] Example 5 A coated particle sample was obtained in the same manner as in Example 1, except that the drying time in the vacuum dryer when drying the cake was changed to 90 minutes.

[0103] Example 6 A coated particle sample was obtained in the same manner as in Example 1, except that the solid-liquid ratio in preparing the lithium metal composite oxide slurry was set to 2500 g / L.

[0104] Example 7 A coated particle sample was obtained in the same manner as in Example 1, except that the solid-liquid ratio in preparing the lithium metal composite oxide slurry was set to 1000 g / L.

[0105] [Example 8] A coated particle sample was obtained in the same manner as in Example 1, except that the dried powder was mixed with boric acid and then annealed in an electric furnace under an oxygen atmosphere (oxygen concentration: 97 vol%) so that the maximum temperature of the mixed powder reached 250°C.

[0106] [Example 9] A coated particle sample was obtained in the same manner as in Example 1, except that the dried powder was mixed with boric acid and then annealed in an electric furnace under an oxygen atmosphere (oxygen concentration: 97 vol%) so that the maximum temperature of the mixed powder reached 350°C.

[0107] [Example 10] A coated particle sample was obtained in the same manner as in Example 1, except that the dried powder was mixed with boric acid and then annealed in an electric furnace in a low-humidity decarbonated air atmosphere (oxygen concentration: 21 vol%, carbon dioxide concentration: 10 ppm or less) so that the maximum temperature of the mixed powder reached 300°C.

[0108] [Example 11] A dry powder was produced in the same manner as in Example 1. An aqueous solution of boric acid was sprayed onto this dry powder. The water content after spraying was 20 wt%. The boric acid aqueous solution was prepared by mixing and dissolving pure water and boric acid so that the amount of added B was 1000 ppm relative to the amount of the coated particle sample finally obtained.

[0109] Thereafter, the mixture was dried by heat treatment at 100°C for 60 minutes using a vacuum drying apparatus, and then annealed for 3 hours using an electric furnace in an oxygen atmosphere (oxygen concentration: 97 vol%) so that the maximum temperature of the mixed powder reached 300°C, thereby obtaining a coated particle sample.

[0110] Example 12 A coated particle sample was obtained in the same manner as in Example 11, except that the amount of boric acid added was changed so that the detected amount of B was 500 ppm.

[0111] Example 13: Diammonium hydrogen phosphate powder having an average particle size of 11 μm was weighed out and mixed with the dry powder in a mixer so that the amount of P in the final coated particle sample was 500 ppm. The mixed powder was then annealed in an electric furnace in an oxygen atmosphere (oxygen concentration: 97 vol%) for 3 hours so that the maximum temperature of the mixed powder reached 300° C. After cooling, the mixed powder was pulverized in a pulverizer to obtain a coated particle sample.

[0112] Comparative Example 1 A lithium metal composite oxide was obtained in the same manner as in Example 1. This lithium metal composite oxide was used as a particle sample.

[0113] Comparative Example 2 A dry powder was obtained in the same manner as in Example 1. This dry powder was used as a particle sample.

[0114] <Evaluation of Samples> The samples of Examples 1 to 13 and Comparative Examples 1 and 2 were evaluated according to the methods described below. The results are also shown below.

[0115] [Composition of Precursor Compound and Lithium Metal Composite Oxide] 0.2 g of the precursor compound or lithium metal composite oxide was dissolved in 25 mL of 20% hydrochloric acid solution by heating, cooled, and then transferred to a 100 mL measuring flask. Pure water was added to prepare a liquid. The constituent elements of this liquid were quantified using an ICP-AES [Optima 8300, manufactured by PerkinElmer Co., Ltd.], and it was confirmed that the precursor compound and lithium metal composite oxide had the same composition ratio for each metal element.

[0116] [Measurement by XPS] Information on the vicinity of the particle surface of the sample was obtained using XPS (Quantera SXM, manufactured by Ulvac-PHI). Specifically, soft X-rays were irradiated onto the sample surface in an ultra-high vacuum, and photoelectrons emitted from the surface were detected using an analyzer to obtain information at a detection depth of several nm. Elemental information on the surface was obtained from the binding energy values ​​of bound electrons in the material, and the boron etc. / lithium ratio was quantified using the peak area ratio based on information on the valence and bonding state from the energy shift of each peak. Measurement conditions: Apparatus: Quantera SXM (Ulvac-PHI); Excitation X-ray: monochromatic Al K 1,2 ray (1486.6 eV); X-ray diameter: 200 m; Photoelectron detection angle: 45 degrees (tilt of the detector relative to the sample surface).

[0117] [Measurement by high-resolution STEM] The sample was sampled in a glove box with a dew point of -70°C or lower, and transferred to a processing and analysis device using a transfer vessel without exposure to the atmosphere. First, a thin analytical sample for TEM observation was prepared using an FIB. Then, a high-resolution STEM image was acquired at an accelerating voltage of 200 kV using an atomic resolution analytical electron microscope (JEM-ARM200F Dual-X (with aberration correction), manufactured by JEOL Ltd.), and EDX mapping analysis was performed to obtain an elemental mapping image.

[0118] Examination of the Fourier transform patterns of the obtained high-resolution STEM images revealed that the samples of Examples 1 to 13 had a structure in which the surface of the lithium metal composite oxide was coated with two layers, a first layer and a second layer. More specifically, the lithium metal composite oxide had a layered rock salt structure, and the first layer, a nickel (II) oxide layer, had a NaCl structure (rock salt structure). Representative images are shown in FIGS. 1 and 2. FIG. 1 is a high-resolution STEM image of the surface layer of a primary particle of the coated particle sample of Example 3. FIG. 2 is a diagram illustrating the boundary between the lithium metal composite oxide and the first layer, and the boundary between the first layer and the second layer, in the high-resolution STEM image of FIG. 1. A closer examination of these figures revealed that the layered rock salt structure of the lithium metal composite oxide and the rock salt structure of the first layer were continuous. As shown in FIG. 2, when a lattice image was observed using a high-resolution STEM, oxygen atoms constituting the lithium metal composite oxide and oxygen atoms constituting the first layer (both of which are shown as relatively white dots, with typical parts surrounded by squares) were found to be continuously arranged even beyond their boundaries.

[0119] The average thicknesses of the first layer and the second layer are measured at a magnification of 10,000,000 times using a high-resolution STEM. Specifically, after identifying the boundaries between the complex oxide, the first layer, and the second layer as described above, 22 points are set on the boundary between the complex oxide and the first layer, dividing the length of the boundary into 23 equal parts. The shortest distance from each of the 20 points, excluding the two points at both ends, to the boundary between the first layer and the second layer is taken as the thickness at each point, and the average of the 20 points is taken as the average thickness of the first layer. The thickness of the second layer is measured at 22 points, dividing the length of the boundary into 23 equal parts, and the shortest distance from each of the 20 points, excluding the two points at both ends, to the surface of the second layer is taken as the thickness at each point, and the average of the 20 points is taken as the average thickness of the second layer.

[0120] [Measurement of Water Content] The water content (ppm) of a sample was determined as the amount of water generated up to 300°C based on the Karl Fischer method (coulometric titration method).

[0121] [Measurement of the amount of residual lithium hydroxide] The amount of residual lithium hydroxide in each sample was measured and calculated based on the Warder method in neutralization titration. Specifically, 20 g of the sample particle powder was added to 100 ml of water, stirred at room temperature for 20 minutes, and the solids were filtered off and removed. The supernatant was then titrated with 0.2 N hydrochloric acid to determine the amount of residual lithium hydroxide. On a pH curve plotted with the titration amount (ml) on the horizontal axis and the pH of the supernatant on the vertical axis, the two points with the largest slope were designated as the first and second titration points, in order from the smallest titration amount. The value was calculated using a formula from the titration amounts at these points.

[0122] [Battery Characteristics of Nonaqueous Electrolyte Secondary Battery] (Fabrication of Coin Cell Using Positive Electrode Active Material) A 2032 type coin cell using the positive electrode active material was fabricated using a positive electrode, a negative electrode, and an electrolyte solution fabricated by the following method.

[0123] Positive electrode: Acetylene black and graphite were used as conductive agents in a weight ratio of 1:1, and polyvinylidene fluoride was used as a binder. A sample as a positive electrode active material, conductive agent, and binder were blended in a weight ratio of 90:6:4 (positive electrode active material:conductive agent:binder). These were mixed with N-methylpyrrolidone to form a slurry, which was then applied to aluminum foil. This was dried at 110°C to prepare a sheet, which was then punched into a 15 mm diameter sheet. The density of the composite was 3.0 g / cm. ３ The resultant was rolled to form a positive electrode.

[0124] Negative Electrode A lithium foil having a thickness of 500 μm and punched to a diameter of 16 mm was used as the negative electrode.

[0125] Electrolyte: A mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) was prepared so that the volume ratio of EC:DMC was 1:2, and 1M LiPF 6 was added to the mixed solvent. ６ The mixed solution was used as the electrolyte.

[0126] (Initial charge capacity and initial charge-discharge efficiency) Using the coin cell manufactured by the method described above, constant current charging was performed at a current density of 18 mA / g up to 4.30 V (upper voltage limit) in an environment of 25° C., followed by constant voltage charging until the current reached 1.8 mA / g. The capacity at this time was defined as the initial charge capacity (mAh / g).

[0127] Next, after a 5-minute pause, constant-current discharge was performed at a current density of 18 mA / g up to 3.00 V under the same environment, and after a 5-minute pause, the initial discharge capacity (mAh / g) was measured. The series of steps up to the measurement of the initial discharge capacity was defined as a charge-discharge cycle under condition A.

[0128] Using the measured values ​​of the initial charge capacity and the initial discharge capacity, the initial charge / discharge efficiency was calculated according to the following formula: Initial charge / discharge efficiency (%)=(initial discharge capacity / initial charge capacity)×100

[0129] (Cycle Retention Rate) Using the manufactured coin cell, 104 charge / discharge cycles were performed under the following conditions in an environment of 60°C. Note that 1C was defined as a current density of 180 mA / g. During charging, constant current charging was performed up to the cutoff voltage, followed by constant voltage charging until the current reached 1.8 mA / g. - 1st, 2nd, and 104th cycles: 0.2C constant current charging to 4.3V, followed by constant voltage charging to 0.01C, followed by a 5-minute pause, followed by 0.2C constant current discharging to 3.0V, followed by a 5-minute pause. - 3rd to 103rd cycles: 0.5C constant current charging to 4.3V, followed by constant voltage charging to 0.01C, followed by a 5-minute pause, followed by 1C constant current discharging to 3.0V, followed by a 5-minute pause. Using the measured discharge capacity values ​​at the 2nd cycle and the 104th cycle, the cycle retention rate at 60°C was calculated according to the following formula: Cycle retention rate (%) = (discharge capacity at 104th cycle / discharge capacity at 2nd cycle) × 100

[0130] (Initial Reaction Resistance) Using the manufactured coin cell, the initial reaction resistance was measured under the following conditions in an environment of 25°C. - 1st cycle: 0.1C constant current charge to 4.3V, then constant voltage charge to 0.01C, 5 minute pause, 0.1C constant current discharge to 2.5V, 5 minute pause - 2nd cycle: 0.1C constant current charge to 4.3V, then constant voltage charge to 0.01C Using the cell after the second cycle of charging was completed, impedance measurement was performed under the following conditions in an environment of 25°C. This result was taken as the impedance measurement result of the second cycle (second cycle reaction resistance (initial reaction resistance)). Frequency range: 300k to 0.01Hz (76 points) Amplitude: 10mV

[0131] The production conditions and evaluation results of Examples 1 to 13 and Comparative Examples 1 and 2 are shown in Table 1 below.

[0132]

[0133] 1 coated particle 2 lithium metal composite oxide 3 first layer 4 second layer

Claims

1. Coated particles comprising: a lithium metal composite oxide in the form of primary or secondary particles, containing at least lithium and nickel; a first layer containing nickel (II) oxide on at least a portion of the surface of the lithium metal composite oxide; and a second layer containing an oxide containing lithium and one or more elements selected from the group consisting of boron, phosphorus and sulfur on at least a portion of the surface of the first layer.

2. The lithium metal composite oxide has a layered rock salt structure and has the general formula Li ａ Ni １－ｂ－ｃ Mn ｂ M ｃ O ２ 2. The coated particle according to claim 1, represented by the formula: (wherein M is one or more elements other than Li, Ni, Mn, and O, and 0.95≦a≦1.15, 0≦b+c≦0.70).

3. The coated particle according to claim 1 or 2, wherein the average thickness of the first layer is 3 nm or more and 100 nm or less.

4. The coated particle according to claim 1 or 2, wherein the lithium metal composite oxide and the first layer form a continuous structure.

5. A positive electrode active material for a non-aqueous electrolyte secondary battery comprising the coated particles according to claim 1 or 2.

6. A non-aqueous electrolyte secondary battery comprising the positive electrode active material for non-aqueous electrolyte secondary batteries according to claim 5.

7. A method for producing coated particles, comprising adding or spraying a compound containing one or more selected from the group consisting of boron, phosphorus and sulfur to a lithium metal composite oxide in the form of primary particles or secondary particles, which contains at least lithium and nickel, and then subjecting the resulting mixture to a heat treatment to obtain the coated particles according to claim 1 or 2.

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