Positive electrode active material for nonaqueous electrolyte secondary battery, and nonaqueous electrolyte secondary battery

Abstract

This invention provides oxide powders comprising coated particles that have: a lithium-metal composite oxide, in the form of primary or secondary particles, comprising at least lithium and nickel; a first layer comprising nickel (II) oxide on at least part of surface of the lithium-metal composite oxide; and a second layer comprising an oxide, including lithium and one or more selected from a group consisting of boron, phosphorus, and sulfur, on at least part of surface of the first layer, wherein the BET specific surface area is 0.3 m2 / g or more, and the LiOH content is 0.35% by mass or less relative to the oxide powder. The positive electrode active materials for a nonaqueous electrolyte secondary battery comprising said coated particles ensure a better charging / discharging capacity and better cycle characteristics in the obtained nonaqueous electrolyte secondary battery, without gelation of the positive electrode active mix paste.

Description

POSITIVE ELECTRODE ACTIVE MATERIAL FOR NONAQUEOUS ELECTROLYTE SECONDARY BATTERY, AND NONAQUEOUS ELECTROLYTE SECONDARY BATTERY[Technical Field]

[0001] The present disclosure relates to an oxide powder, a positive electrode active material for a nonaqueous electrolyte secondary battery, a nonaqueous electrolyte secondary battery, and a method for producing an oxide powder.[Background Art]

[0002] Lithium ion secondary batteries are small and lightweight, also have a high energy density, a high charge / discharge voltage, and a substantial charge / discharge capacity, and have thus garnered attention as power sources for operating AV devices or personal computers and other such electronic devices.

[0003] Lithium ion secondary batteries in which layered or spinel type lithium transition metal composite oxides are employed as the positive electrode active material can provide a high voltage of 4 volts, and are therefore increasingly being put to practical use as high-energy density batteries. Examples of materials that have been primarily proposed include relatively easy-to-synthesize lithium-cobalt composite oxides (UCOO2), lithium-nickel composite oxides (LiN iC>2) (obtained using nickel, which is less expensive than cobalt), as well as lithium-nickel- cobalt-manganese composite oxides (LiNii / sCoi / sMni / sCh), and lithium-manganese composite oxides (LiMn2O4) (obtained using manganese).

[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 electrodes of the lithium ion secondary batteries here are formed by, for example, mixing a positive electrode active material with a binder such as polyvinylidene fluoride (PVDF) or a solvent such as N- methyl-2-pyrrolidone (NMP) into the form of a positive electrode active mix paste, which is applied to a current collector made of aluminum foil, for example. Any lithium that is released at that time from the lithium-metal composite oxide in the positive electrode mix paste may react with moisture in the binder, for example, resulting in lithium hydroxide.

[0005] As lithium hydroxide production increases in this way, reactions between the lithium hydroxide and binder can result in the gelation of the positive electrode mix paste. The gelation of the positive electrode mix paste leads to poor handling during the manufacturing process and to lower yields. This can become quite pronounced when the proportion of lithium in the lithium- transition metal composite oxide serving as the positive electrode active material is greater than the stoichiometric ratio relative to the transition metal, especially when there is a high proportion of nickel in the transition metal.

[0006] The release of lithium here may possibly come from the portion of lithium that has not been incorporated into the crystal lattice of the lithium-metal composite oxide, as well as from lithium that has been incorporated into the crystal lattice of the lithium-metal composite oxide. When the lithium that has been incorporated into the crystal lattice of the lithium-metal compositeoxide is eluted, increases in the resistance of the lithium-metal composite oxide can lower the charging / discharging efficiency.

[0007] As disclosed in Patent Document 1 , for example, the fired material is washed with water to remove the portion of the lithium that has not been incorporated into the crystal lattice of the lithium-metal composite oxide.

[0008] As disclosed in Patent Document 2, for example, the elution of lithium that has been incorporated into the crystal lattice can also be prevented by coating the lithium-metal composite oxide with an oxygen-containing boron compound coating.[Prior Art Documents][Patent Documents]

[0009] [Patent Document 1] JP 2005-97087 A[Patent Document 2] JP 2019-114560 A[Summary of the Invention][Problems to be Solved by the Invention]

[0010] According to the findings of the present inventors, however, when lithium-metal composite oxides are washed with water in an attempt to remove the lithium, as in Patent Document 1 , ionproton exchange in the lithium that has been incorporated into the crystal lattice results in either the loss of lithium on the surface of the lithium-metal composite oxide or in rock-salt phase conversion of the surface of the lithium-metal composite oxide, leading to a lower lithium ion secondary battery charging / discharging capacity.

[0011] Coating lithium-metal composite oxides with an oxygen-containing boron compound, as in Patent Document 2, is effective to some extent in enhancing cycle characteristics by preventing the lithium in the crystal lattice of lithium-metal composite oxides from being eluted into the electrolyte, but there is still room for further improvement in terms of enhancing cycle characteristics.

[0012] In view of the circumstances noted above, an object of the present disclosure is to provide coated particles that, when used as a positive electrode active material for a nonaqueous electrolyte secondary battery such as a lithium ion secondary battery, will ensure a better charging / discharging capacity and better cycle characteristics in the nonaqueous electrolyte secondary battery that is produced, without any gelation of the positive electrode active mix paste.[Means for Solving the Problems]

[0013] The inventors engaged in extensive research to solve the problems noted above. As a result, it was found that a nonaqueous electrolyte secondary battery will have a better charging / discharging capacity and better cycle characteristics, without any gelation of the positive electrode active mix paste, when the material used as the positive electrode active material in the nonaqueous electrolyte secondary battery is an oxide powder comprising coated particles that have: a lithium-metal composite oxide, in the form of primary particles orsecondary particles, that comprises at least lithium and nickel; a first layer comprising nickel (II) oxide on at least part of the surface of the lithium-metal composite oxide; and a second layer comprising an oxide, including lithium and one or more selected from the group consisting of boron, phosphorus, and sulfur, on at least part of the surface of the first layer, wherein the BET specific surface area is 0.3 m2 / g or more, and the LiOH content is 0.35% by mass or less relative to the oxide powder. Specifically, the present disclosure provides the following.

[0014] (1) An oxide powder comprising coated particles that have: a lithium-metal composite oxide, in the form of primary particles or secondary particles, that comprises at least lithium and nickel; a first layer comprising nickel (II) oxide on at least part of the surface of the lithium-metal composite oxide; and a second layer comprising an oxide, including lithium and one or more selected from the group consisting of boron, phosphorus, and sulfur, on at least part of the surface of the first layer, wherein the BET specific surface area is 0.3 m2 / g or more, and the LiOH content is 0.35% by mass or less relative to the oxide powder.

[0015] (2) The oxide powder according to (1), wherein the lithium-metal composite oxide has a layered rock-salt structure and is represented by the general formula LiaNii-b-cMnbMcO2 (in the formula, M is one or more elements other than Li, Ni, Mn, and O; 0.95<a<1.15; and 0<b+c<0.70).

[0016] (3) The oxide powder according to (1) or (2), wherein the average thickness of the first layer is 3 nm to 100 nm.

[0017] (4) The oxide powder according to (1) or (2), wherein the lithium-metal composite oxide and the first layer have a continuous structure.

[0018] (5) A positive electrode active material for a nonaqueous electrolyte secondary battery, comprising the oxide powder according to (1) or (2).

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

[0020] (7) A method for producing an oxide powder, wherein to a lithium-metal composite oxide, in the form of primary particles or secondary particles, that contains at least lithium and nickel, is added or sprayed a compound including one or more selected from the group consisting of boron, phosphorus, and sulfur, along with at least 5% by mass of water relative to the lithium-metal composite oxide, and heat treatment is then carried out to obtain the oxide powder according to (1).

[0021] (8) The method for producing an oxide powder, wherein a compound including one or more selected from the group consisting of boron, phosphorus, and sulfur is added to or sprayed onto a lithium-metal composite oxide that comprises at least lithium and nickel, that is in the form of primary particles or secondary particles, and that has anLiOH content of 0.4% by mass or less relative to the lithium-metal composite oxide, and heat treatment is then carried out to obtain the oxide powder according to (1).

[0022] (9) The method for producing an oxide powder according to (7) or (8), wherein the ratio of the BET specific surface area of the oxide powder after the heat treatment relative to the BET specific surface area of the lithium-metal composite oxide before the heat treatment is 1.2 or more.[Effects of the Invention]

[0023] The present disclosure can provide coated particles that, when used as a positive electrode active material for a nonaqueous electrolyte secondary battery such as a lithium ion secondary battery, will ensure a better charging / discharging capacity and better cycle characteristics in the nonaqueous electrolyte secondary battery that is produced, without any gelation of the positive electrode active mix paste.[Brief Description of the Drawings]

[0024] [Fig. 1] is a cross-sectional schematic diagram of coated particles according to one embodiment of the present disclosure.[Fig. 2] is a high-resolution STEM image of the primary particle surface layer of a sample of the coated particles obtained in Example 10.[Fig. 3] illustrates the boundary between the lithium-metal composite oxide and the first layer, as well as the boundary between the first layer and the second layer, in the high- resolution STEM image of Fig. 2.[Embodiments of the Invention]

[0025] Embodiments of the present disclosure are described below, but the present disclosure is not limited in any way by the description of the embodiments and can be carried out with additional modifications, as appropriate.

[0026] <Oxide Powder>The oxide powder according to an embodiment of the present disclosure comprises coated particles that have: a lithium-metal composite oxide, in the form of primary particles or secondary particles, that comprises at least lithium and nickel; a first layer comprising nickel (II) oxide on at least part of the surface of the lithium-metal composite oxide; and a second layer comprising an oxide, including lithium and one or more selected from the group consisting of boron, phosphorus, and sulfur, on at least part of the surface of the first layer, wherein the BET specific surface area is 0.3 m2 / g or more, and the LiOH content is 0.35% by mass or less relative to the oxide powder.

[0027] [Coated Particles]The coated particles according to an embodiment of the present disclosure have: a lithium- metal composite oxide, in the form of primary particles or secondary particles, that comprises at least lithium and nickel; a first layer comprising nickel (II) oxide on at least part of the surface of the lithium-metal composite oxide; and a second layer comprising an oxide, including lithium and one or more selected from the group consisting of boron, phosphorus, and sulfur, on atleast part of the surface of the first layer.

[0028] Fig. 1 is a cross-sectional schematic diagram of coated particles according to one embodiment of the present disclosure. The coated particles 1 shown in Fig. 1 have: a lithium- metal composite oxide 2, in the form of primary particles or secondary particles, that comprises at least lithium and nickel; a first layer 3 comprising nickel (II) oxide on at least part of the surface of the lithium-metal composite oxide 2; and a second layer 4 comprising an oxide, including lithium and one or more selected from the group consisting of boron, phosphorus, and sulfur, on at least part of the surface of the first layer 3. In Fig. 1 , the lithium-metal composite oxide 2 is shown in circular form, and the outer contours of the first layer 3 and the second layer 4 are also shown in circular form, but the cross-sectional shape is not limited to circular forms.

[0029] In the coated particles, the lithium-metal composite oxide serving as the positive electrode active material is coated with two layers, a first layer comprising thermally and mechanically stable nickel (II) oxide, and a second layer comprising an oxide that is highly resistant to elution into the electrolyte, so that the use thereof as a positive electrode active material can ensure that a nonaqueous electrolyte secondary battery has a better charging / discharging capacity and better cycle characteristics. Although the reasons for this are not necessarily fully understood, it is believed that the presence of the thermally and mechanically stable coated nickel (II) oxide layer in the form of a coating on the surface of the lithium-metal composite oxide can prevent the cycle properties from becoming compromised as a result of decomposition into rock-salt structures or spinel structures, from the surface into the interior, of the coated particles serving as the highly charged positive electrode active material. It is also believed that the presence of the second layer prevents the cycle properties from becoming compromised as a result of direct contact between the electrolyte and the coated particles serving as the positive electrode active material.

[0030] (Lithium-Metal Composite Oxide)The lithium-metal composite oxide comprises at least lithium and nickel, and is in the form of primary particles or secondary particles.

[0031] In terms of the chemical composition, the lithium-metal composite oxide is preferably, but not particularly limited to, one that has a layered rock-salt structure and that is represented by the general formula LiaNii-b-cMnbMcO2 (in the formula, M is one or more elements other than Li, Ni, Mn, and O; 0.95<a<1.15; and 0<b+c<0.70).

[0032] The value of a in the general formula is not particularly limited, provided that it is in the range between 0.95<a<1.15, such as 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. Meanwhile, 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.

[0033] The value of b+c in the general formula is not particularly limited, provided that it is in the range between 0<b+c<0.70, such as 0.05 or more, or 0.1 or more. 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 orless, 0.25 or less, 0.2 or less, or 0.15 or less.

[0034] In the general formula, the element M is not particularly limited, provided that it is one or more elements other than Li, Ni, Mn and O, where 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 should be selected depending on the purpose for which it is being added. When a plurality of elements are included as the element M, the value of c represents the total amount of the plurality of elements.

[0035] (First Layer)The first layer is disposed on at least part of the surface of the above lithium-metal composite oxide, and comprises nickel (II) oxide.

[0036] The lithium-metal composite oxide and the first layer preferably have a continuous structure. The expression “the lithium-metal composite oxide and the first layer have a continuous structure” here means that the oxygen atoms constituting the lithium-metal composite oxide and the oxygen atoms constituting the first layer (both represented by relatively white dots, a typical portion of which is surrounded by a rectangle) are disposed continuously, even across the boundary therebetween, as shown by the high-resolution STEM image of a coated particle lattice in Fig. 3.

[0037] The first layer contains nickel (II) oxide and was thus confirmed, using high-resolution STEM- EDS, to have a rock-salt structure derived from nickel (II) oxide, and the layer was confirmed to contain nickel, allowing it to be identified as the first layer. In the second layer, the presence of an oxide layer of lithium and boron, etc., on the surface was confirmed via XPS, and the layer containing boron and oxygen was then identified as the second layer via high-resolution STEM- EDS.

[0038] The average thickness of the first layer 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 is also 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. Ensuring that the average thickness of the nickel (II) oxide is within the prescribed range will allow the resistance to be lowered during battery usage without producing any resistance components that might impede lithium mobility. The average thickness is determined via high- resolution STEM (JEM-ARM200F Dual-X, manufactured by JEOL Ltd.) at 10 000 OOOx magnification. Specifically, the boundaries of the lithium-metal composite oxide, the first layer, and the second layer are identified as noted above, and 22 points (equally dividing the boundary length into 23 equal parts) are then established on the boundary between the lithium- metal composite oxide and the first layer. The shortest distance from each of 20 points (minus the 2 points at both ends) to the boundary between the first layer and second layer is used as the thickness at each point, and the average of the 20 points is used as the average thickness.

[0039] The first layer may also include a compound other than nickel (II) oxide. In one embodiment, the first layer preferably shares in common at least one constituent element other than lithium (Ni, Mn, M, and O) with the lithium-metal composite oxide (contains the same element(s) as the lithium-metal composite oxide). The inclusion of lithium in the first layer is not excluded, however, and the first layer may include lithium.

[0040] (Second Layer)The second layer is present on at least part of the surface of the first layer, and includes lithium and one or more selected from the group consisting of boron, phosphorus, and sulfur.

[0041] [Oxide Powder]The average particle size of the oxide powder (which refers to the primary particle size when the powder is primary particles, and refers to the secondary particle size when the powder is secondary particles; sometimes abbreviated as D50) may be, but is not particularly limited to, 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, or450 nm or more. Meanwhile, the D50 may be 25 pm or less, 24.5 pm or less, 24 pm or less,23.5 pm or less, 23 pm or less, 22.5 pm or less, 22 pm or less, 21.5 pm or less, 21 pm or less,20.5 pm or less, 20 pm or less, 19.5 pm or less, 19 pm or less, 18.5 pm or less, 18 pm or less,17.5 pm or less, 17 pm or less, 16.5 pm or less, 16 pm or less, 15.5 pm or less, 15 pm or less,14.5 pm or less, 14 pm or less, 13.5 pm or less, 13 pm or less, 12.5 pm or less, 12 pm or less,11.5 pm or less, 11 pm or less, 10.5 pm or less, 10 pm or less, 9.5 pm or less, 9 pm or less, 8.5 pm or less, 8 pm or less, 7.5 pm or less, 7 pm or less, 6.5 pm or less, 6 pm or less, 5.5 pm or less, 5 pm or less, or 4.5 pm or less. The D50 is determined on a volume basis by a wet laser method using a laser type particle size distribution analyzer (Microtrac HRA, manufactured by Nikkiso Co., Ltd.).

[0042] The BET specific surface area of the oxide powder is not particularly limited, provided that it is 0.3 m2 / g or more, but is preferably, for example, 0.31 m2 / g or more, 0.32 m2 / g or more, 0.33 m2 / g or more, 0.34 m2 / g or more, 0.35 m2 / g or more, 0.36 m2 / g or more, 0.37 m2 / g or more, 0.38 m2 / g or more, 0.39 m2 / g or more, or 0.4 m2 / g or more. Meanwhile, the BET specific surface area of the oxide powder may be 5 m2 / g or less, 4.5 m2 / g or less, 4 m2 / g or less, 3.5 m2 / g or less, 3 m2 / g or less, 2.5 m2 / g or less, 2 m2 / g or less, 1.5 m2 / g or less, or 1 m2 / g or less.

[0043] The LiOH content of the oxide powder is not particularly limited, provided that it is 0.35% by mass or less relative to the oxide powder, but is preferably, for example, 0.34% by mass or less, 0.33% by mass or less, 0.32% by mass or less, 0.31 % by mass or less, or 0.30% by mass or less. Meanwhile, the LiOH content of the oxide powder may be 0% by mass or more, 0.05% by mass or more, 0.10% by mass or more, 0.12% by mass or more, or 0.15% by mass or more.

[0044] <Method for Producing Oxide Powder>The oxide powder according to an embodiment of the present disclosure can be produced, for example, by carrying out the following steps, but can also be produced by other methods. Even if the following steps are carried out in that order, it sometimes may not be possible to produce the oxide powder according to embodiments of the present disclosure, depending on thecombination of conditions, but the oxide powder according to embodiments of the present disclosure can be produced provided that at least the conditions shown in the Examples described below are used.

[0045] (Method for Producing Oxide Powder According to a First Embodiment)In the method for producing the oxide powder according to a first embodiment of the present disclosure, to a lithium-metal composite oxide, in the form of primary particles or secondary particles, that contains at least lithium and nickel, is added or sprayed a compound that contains one or more selected from the group consisting of boron, phosphorus, and sulfur, along with at least 5% by mass of water relative to the lithium-metal composite oxide, and heat treatment is then carried out to obtain the oxide powder noted above. More specifically, the oxide can be produced by the following Steps 1 through 5.

[0046] 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: The lithium-metal composite oxide obtained by firing the mixture in Step 2 is washed with water, as needed.Step 4: The lithium-metal composite oxide obtained in Step 2 or 3 is surface treated, as needed, using one or more compounds selected from the group consisting of boron, phosphorus, and sulfur.

[0047] [Step 1]First, a precursor composite compound, in the form of an aggregate comprising clusters of primary particles that contain at least a transition metal, is synthesized. The method for synthesizing the precursor composite compound is not particularly limited, and the following method can be used, for example: aqueous solution comprising an aqueous solution of a transition metal as well as a variety of aqueous solutions of compounds including other elements, depending on the composition of the intended lithium-metal composite oxide, is added drop-wise into a reaction tank in which an aqueous alkali solution, such as a sodium hydroxide solution or ammonia solution, is stirred as the mother liquor, for example; the pH is monitored and controlled to within a suitable range as sodium hydroxide, for example, is added drop-wise; and co-precipitation is brought about by means of a wet reaction to obtain a product in the form of, for example, a hydroxide, an oxide obtained by firing the hydroxide, or a carbonate.

[0048] In synthesis-related reactions, after the alkaline aqueous solution serving as the mother liquor has been prepared, the interior of the reactor is purged with an inert gas or preferably nitrogen gas for industrial purposes, to create a nitrogen atmosphere in order to minimize the oxygen concentration within the reaction tank system or in the solution. If the oxygen concentration is excessively high, there is a risk that the co- precipitated hydroxide will be over-oxidized by any remaining oxygen that is over the prescribed amount, and a risk that the formation of aggregates due to crystallization will be compromised.

[0049] The transition metal aqueous solution is not particularly limited, although the use of an acidicaqueous solution, for example, is preferred, and the use of a sulfuric acid aqueous solution such as a nickel sulfate aqueous solution is even more preferred in the case of nickel compounds. One or more transition metal aqueous solutions can also be used.

[0050] Examples of nickel compounds that can be used include, but are not particularly limited to, one or more selected from nickel sulfate, nickel oxide, nickel hydroxide, nickel nitrate, nickel carbonate, nickel chloride, nickel iodide, and metallic nickel.

[0051] Examples of cobalt compounds that can be used include, but are not particularly limited to, one or more selected from cobalt sulfate, cobalt oxide, cobalt hydroxide, cobalt nitrate, cobalt carbonate, cobalt chloride, cobalt iodide, and metallic cobalt.

[0052] Examples of manganese compounds that can be used include, but are not particularly limited to, one or more selected from manganese sulfate, manganese oxide, manganese hydroxide, manganese nitrate, manganese carbonate, manganese chloride, manganese iodide, and metallic manganese.

[0053] Examples of aluminum compounds that can be used include, but are not particularly limited to, aluminum sulfate, aluminum oxide, aluminum hydroxide, aluminum nitrate, aluminum carbonate, aluminum chloride, aluminum iodide, sodium aluminate, and metallic aluminum.

[0054] Examples of titanium compounds that can be used include, but are not particularly limited to, one or more selected from titanyl sulfate, titanium oxide, titanium hydroxide, titanium nitrate, titanium carbonate, titanium chloride, titanium iodide, and metallic titanium.

[0055] Examples of iron compounds that can be used include, but are not particularly limited to, one or more selected from iron sulfate, iron oxide, iron hydroxide, iron nitrate, iron carbonate, iron chloride, iron iodide, and metallic iron.

[0056] Examples of niobium compounds that can be used include, but are not particularly limited to, one or more selected from niobium oxide, niobium chloride, lithium niobate, and niobium iodide.

[0057] Examples of tungsten compounds that can be used include, but are not particularly limited to, one or more selected from tungsten oxide, sodium tungstate, ammonium para-tungstate, tungsten hexacarbonyl, and tungsten sulfide.

[0058] Examples of magnesium compounds that can be used include, but are not particularly limited to, one or more selected from magnesium sulfate, magnesium oxide, magnesium hydroxide, magnesium nitrate, magnesium carbonate, magnesium chloride, magnesium iodide, and metallic magnesium.

[0059] Examples of zirconium compounds that can be used include, but are not particularly limited to, one or more selected from zirconium sulfate, zirconium oxide, zirconium nitrate, zirconium ammonium carbonate, zirconium chloride, zirconium iodide, and metallic zirconium.

[0060] Examples of zinc compounds that can be used include, but are not particularly limited to, one or more selected from zinc sulfate, zinc oxide, zinc hydroxide, zinc nitrate, zinc carbonate, zinc chloride, zinc iodide, and metallic zinc.

[0061] Examples of other elements that can be used include one or more selected from sulfates, oxides, hydroxides, nitrates, carbonates, chlorides, iodides, and metals.

[0062] The proportions in which the various compounds are blended should be adjusted to ensure that the amounts of the various elements are in the desired proportions in the composition of the intended lithium-metal composite oxide.

[0063] The appropriate pH range for when the precursor composite compound is synthesized is not particularly limited, and can be determined so as to achieve the desired secondary particle size and density, but the pH is generally in the range of around 10 to 13.

[0064] The precursor composite compound obtained by means of a wet reaction is preferably washed with water and then dried after being de-watered.

[0065] Washing the precursor composite compound with water will make it possible to wash out impurities such as sulfate radicals or carbonate radicals and sodium fractions that have been incorporated into aggregated particles or that have become stuck on the surface layer during the reaction. Washing treatments that can be used for small amounts of impurities include a procedure in which Nutsche washing using a Buchner funnel is performed, or a procedure in which the reacted suspension is pumped through a press filter, washed with water, and dewatered. Moreover, pure water, a sodium hydroxide aqueous solution, or a sodium carbonate aqueous solution, for example, can be used in the washing treatment, but the use of pure water is preferred for industrial purposes. For sizable residual sulfate radicals, however, a sodium hydroxide aqueous solution in which the pH is controlled according to the amount that remains may be used.

[0066] The precursor composite compound synthesized in this way and a lithium compound are then mixed in a predetermined ratio to prepare a mixture. The materials may be mixed with the use of a solvent, where the precursor composite compound and the lithium compound are each in the form of a solution, such as an aqueous solution, and the solutions are mixed in a predetermined ratio, or they may be mixed without a solvent, where a powder of the precursor composite compound and a powder of the lithium compound are weighed out in predetermined proportions and mixed by a dry method.

[0067] The lithium compound is not particularly limited, and a variety of lithium salts may be used. Specific examples of lithium compounds that can be used include 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. Of these, the use of one or more selected from anhydrous lithium hydroxides and lithium hydroxide hydrates is preferred.

[0068] The proportions in which the lithium compound and the precursor composite compound are blended are not particularly limited, but should be adjusted, as appropriate, to ensure that the total amounts of the lithium and various other elements are in the desired proportions in the composition of the intended lithium-metal composite oxide.

[0069] [Step 2]Producing a lithium-metal composite oxide containing at least a transition metal as noted above will result in a lithiation reaction and crystal growth during the firing process, but the lithiation reaction will require a certain oxygen partial pressure. The lithiation reaction will produce a lithium-metal composite oxide that contains lithium. The temperature is then increased to a prescribed temperature to promote crystal growth.

[0070] The mixture is preferably fired to a maximum temperature of 650°C to 1100°C, 670°C to 1000°C, or 700°C to 980°C. The mixture is preferably fired at the maximum temperature for 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. The desired composite compound can be obtained by establishing the maximum temperature and time at which the firing temperature will be at or higher than the melting point of the lithium compound in the mixture, and at which the lithium-metal composite oxide in which the lithium is contained will result in the desired crystal growth or particle growth.

[0071] Firing is commonly carried out by weighing out the lithium compound, the precursor composite compound, and a compound of element M, if needed, mixing the contents in a mixer, and loading the resulting powder mixture into a container such as a crucible or sagger, but it will become more and more difficult for the gas that is produced to be externally discharged and for the required oxygen concentration diffusion to be achieved as one gets closer and closer to the bottom of the container loaded with the powder mixture, in particular, during the lithiation reaction, in particular. As a result, it becomes difficult to control the reaction homogeneity and the primary particle size.

[0072] A method in which primary firing under the prescribed conditions is furthermore first preceded by pre-firing under the following prescribed conditions in Step 2 is therefore preferably used when producing the lithium-metal composite oxide according to the embodiment of the present disclosure. The pre-firing is not an essential step, however.

[0073] Incorporating a firing method that promotes the lithiation reaction is particularly desirable for the pre-firing in Step 2. A specific example that may be cited is a method that allows the mixture to be more easily heated, allows the gas that is generated from the lithium compound to be easily discharged, and allows gas having a high oxygen partial pressure to be diffused into the mixture (into the particles). The desired properties can be achieved by, for example, pre-firing less of the mixture.

[0074] For the pre-firing process in Step 2, the mixture can be loaded into a sagger or crucible and fired in a static furnace, roller hearth kiln, or pusher furnace, but a rotary kiln in which the mixture is fired while flowing can also be used.

[0075] The maximum temperature of the mixture being pre-fired is not particularly limited, and is preferably adjusted depending on the type of lithium compound that is being used to prepare the mixture. This can ensure a reliable reaction between the precursor composite compound and lithium compound in the mixture, and can ensure reliable and homogeneous progress of the lithiation reaction in order to prevent the occurrence of foreign phases, thus allowing the desired lithium-metal composite oxide to be obtained.

[0076] The pre-firing atmosphere should be, but is not particularly limited to, an oxidizing atmosphere that ensures reliable and homogeneous progress of the lithiation reaction. For example, the use of an oxidative decarboxylation gas atmosphere having a carbon dioxide gas concentration of 30 ppm or less or an oxygen atmosphere having an oxygen concentration of 80 vol% to 90 vol% is preferred.

[0077] The pre-firing time should be, but is not particularly limited to, a time that ensures reliable and homogeneous progress of the lithiation reaction. For example, a time of 1 to 10 hours or 2 to 8 hours is preferred.

[0078] Primary firing of the pre-fired mixture is carried out in order to bring about crystal growth or particle growth at a higher temperature. Reliable and homogeneous crystal growth progress is required at this time to obtain a lithium-metal composite oxide having a desired crystal structure.

[0079] The primary firing atmosphere should be, but is not particularly limited to, an atmosphere that has an oxygen partial pressure, and preferably a low moisture content or carbon dioxide gas concentration, that will ensure reliable and homogeneous crystal growth, without reducing the transition metal contained in the mixture that is being fired. For example, an oxidative decarboxylation gas atmosphere having a carbon dioxide gas concentration of 30 ppm or less, or an oxygen atmosphere having an oxygen concentration preferably of 80 vol% or more, or 90 vol% or more, is preferably used.

[0080] The primary firing temperature is not particularly limited, provided that it is higher than the pre-firing temperature, but can be adjusted depending on the composition, for example, of the lithium-metal composite oxide that is going to be obtained. The maximum temperature is preferably adjusted to between 700°C and 1100°C, between 710°C and 1000°C, or between 720°C and 980°C, for example. A maximum temperature within the prescribed range will make it possible to obtain a lithium-metal composite oxide that has the desired crystal structure, with fewer unreacted components, and to prevent the loss of the battery characteristics of the nonaqueous electrolyte secondary batteries in which the resulting lithium-metal composite oxide is used as the positive electrode. To obtain, for example, a lithium-metal composite oxide having an Ni content of 20 mol% to 80 mol% among elements other than Li, the mixture is preferably fired at a maximum temperature not to exceed 1100°C.

[0081] The primary firing time is not particularly limited, but should be enough time to form a lithium- metal composite oxide having the desired crystal structure. For example, a time of 1 to 15 hours, 2 to 12 hours, or 2 to 10 hours is preferred.

[0082] [Step 3]The lithium-metal composite oxide obtained in Step 2 may contain impurities, such as unreacted lithium compounds or lithium compounds from the crystal structure that appear on the particle surface layer over the course of the firing step. In the event of a large amount of such lithium compounds, the compound that comprises a compound including one or more selected from the group consisting of boron, phosphorus, and sulfur (sometimes referred to below as “boron, etc.”) (the compound is sometimes referred to below as the “compound of boron, etc.”) that is used in Step 4 will result in the formation of an oxide of lithium and boron, etc., but the surface of the lithium-metal composition oxide may sometimes not be coated. It can therefore be washed with water and then heat treated, for example, in order to remove or minimize such impurities. Note that Step 3 is not mandatory.

[0083] To produce the coated particles according to the embodiment of the present disclosure in the above manner, the lithium-metal composite oxide obtained in Step 2 is preferably washed with water, but over-washing the lithium-metal composite oxide may result in extensive removal of even the lithium contained in the crystal lattice of the lithium-metal composite oxide, which may adversely affect conductivity.

[0084] On the other hand, in cases where process design, for example, precludes cleaning, another option is to increase the amount in which the compound of boron, etc., is added. A compound of boron, etc., will react with any lithium that is not included in the crystal lattice to form a compound, thus allowing lithium hydroxide production and gelation of the positive electrode mix to be suppressed.

[0085] [Step 4]The compound of boron, etc., is added to or sprayed, along with 5% by mass or more of water, onto the lithium-metal composite oxide that has been obtained in Step 2 or 3. After the lithium-metal composite oxide has thus come into contact with the compound of boron, etc., heat treatment (annealing treatment) is carried out under specific conditions. This will allow the oxide powder of the embodiment of the present disclosure to be produced. When added or sprayed, the compound of boron, etc., is preferably added or sprayed as the lithium-metal composite oxide flows (while mixed with the compound of boron, etc.).

[0086] In this sort of reaction, it is believed that the compound of boron, etc., reacts with protons in the crystal or water and becomes weakly acidic, that the surface of the lithium-metal composite oxide partially changes into nickel oxide via a disproportionation reaction, and that the subsequent heat treatment results in the formation of a mechanically and thermally stable nickel oxide (II) layer. It is also believed that the compound of boron, etc., reacts with lithium to form a highly ion conductive oxide of lithium and boron, etc., which coats the surface of the nickel oxide layer.

[0087] In particular, the compound of boron,, etc., is added or sprayed, along with a prescribed amount of water, in Step 4. The compound of boron, etc., is thus brought into contact therewith, along with water, thereby allowing the particles of the lithium-metal composite oxide to bereadily permeated by the compound of boron, etc., and a more homogeneous layer to be formed.

[0088] One or more selected from boron compounds, phosphorus compounds, and sulfur compounds, for example, can be used as the elemental compound added for the surface treatment noted above.

[0089] Examples of boron compounds that can be used include boric acid, lithium tetraborate, sodium tetraborate, sodium perborate, diboron trioxide, and boric trifluoride.

[0090] Examples of phosphorus compounds that can be used include diammonium hydrogen phosphate, ammonium dihydrogen phosphate, calcium phosphate, calcium dihydrogen phosphate, and phosphoric acid.

[0091] Examples of sulfur compounds that can be used include ammonium sulfate, lithium sulfate, sodium sulfate, sulfur fluoride, aluminum sulfate, and sulfuric acid.

[0092] When the compound of boron, etc., is admixed or sprayed, the amount of water relative to the lithium-metal composite oxide is not particularly limited, provided that the amount is at least 5% by mass, but is preferably 6% by mass or more, 7% by mass or more, 8% by mass or more, 9% by mass or more, 10% by mass or more, 11% by mass or more, 12% by mass or more, 13% by mass or more, 14% by mass or more, 15% by mass or more, 16% by mass or more, 17% by mass or more, 18% by mass or more, 19% by mass or more, or 20% by mass or more. Meanwhile, the amount of water relative to the lithium-metal composite oxide may be 50% by mass or less, 45% by mass or less, 40% by mass or less, 35% by mass or less, 30% by mass or less, or 25% by mass or less. Ensuring that the amount of water is at least the required amount will allow the compound of boron, etc., that is added to the lithium-metal composite oxide to be uniformly distributed. Ensuring that the amount of water is not greater than the required amount will make it possible to prevent loss of quality caused by elution of the lithium from the lithium-metal composite oxide.

[0093] The LiOH content of the lithium-metal composite oxide when the compound of boron, etc. is added to or sprayed onto the lithium metal composite oxide is not particularly limited, but is, for example, preferably 5% by mass or less, 4% by mass or less, 3% by mass or less, 2% by mass or less, 1% by mass or less, 0.9% by mass, 0.8% by mass or less, 0.7% by mass or less, 0.6% by mass or less, 0.5% by mass or less, 0.4% by mass or less, 0.37% by mass or less, 0.35% by mass or less, 0.32% by mass or less, or 0.3% by mass or less. The LiOH content may be 0% by mass or more (including 0% by mass).

[0094] The atmosphere for heat treatment is not particularly limited, and may be, for example, the atmosphere, where an oxidizing gas atmosphere for decarboxylation that has a carbon dioxide concentration of 30 ppm or less, or an oxygen atmosphere that preferably has an oxygen concentration of 80% by volume or more, or 90% by volume or more, may be used.

[0095] The heat treatment temperature is not particularly limited, but is preferably adjusted to, forexample, a range between 200 °C or 400 °C, or between 250 °C and 350°C.

[0096] The heat treatment time is not particularly limited, but should be enough time to form a lithium-metal composite oxide having the desired crystal structure. For example, a time of 1 to 15 hours, 2 to 12 hours, or 2 to 10 hours is preferred.

[0097] The ratio of the BET specific surface area of the oxide powder after the heat treatment relative to the BET specific surface area of the lithium-metal composite oxide before the heat treatment is not particularly limited, but is preferably, for example, 1.2 or more, 1.3 or more, 1.4 or more, 1.5 or more, 1.6 or more, 1.7 or more, 1.8 or more, 1.9 or more, or 2 or more. Meanwhile, the ratio of the BET specific surface area of the oxide powder after the heat treatment relative to the BET specific surface area of the lithium-metal composite oxide before the heat treatment may be 10 or less, 9.5 or less, 9 or less, 8.5 or less, 8 or less, 7.5 or less, 7 or less, 6.5 or less, 6 or less, 5.5 or less, 5 or less, 4.5 or less, or 4 or less.

[0098] (Method for Producing Oxide Powder According to a Second Embodiment)In the method for producing the oxide powder according to a second embodiment of the present disclosure, a compound including one or more selected from the group consisting of boron, phosphorus, and sulfur is added to or sprayed onto a lithium-metal composite oxide that comprises at least lithium and nickel, that is in the form of primary particles or secondary particles, and that has an LiOH content of 0.4% by mass or less relative to the lithium-metal composite oxide, and heat treatment is then carried out to obtain the oxide powder noted above.

[0099] Steps 1 through 3 are the same as in the method for producing an oxide powder according to the first embodiment. The description of the step for carrying out the annealing treatment (Step 4’) after the lithium-metal composite oxide has been brought into contact with the compound of boron, etc., in the method for producing the oxide powder according to the second embodiment of the present disclosure will be limited to only those parts that are different from Step 4 of the method for producing the oxide powder according to the first embodiment. For everything other than the parts described here, please refer to the descriptions in the section on the method for producing the oxide powder according to the first embodiment.

[0100] In the method for producing the oxide powder according to the first embodiment, the compound of boron, etc. is added or sprayed, along with 5% by mass or more of water, but there is no need for the compound of boron, etc., to be added or sprayed, along with 5% by mass or more of water, in the method for producing the oxide powder according to the second embodiment. There is also no need to add water along with the compound of boron, etc. However, whereas the LiOH content of the lithium-metal composite oxide when the compound of boron, etc., is added or sprayed is not limited in the method for producing the oxide powder according to the first embodiment, the content is 0.4% by mass or less in the method for producing the oxide powder according to the second embodiment.

[0101] The LiOH content of the lithium-metal composite oxide when the compound of boron, etc., is added or sprayed is not particularly limited, provided that it is 0.4% by mass or less relative to the lithium-metal composite oxide, but is preferably 0.37% by mass or less, 0.35% by mass orless, 0.32% by mass or less, or 0.3% by mass or less. The LiOH content may be 0% by mass or more (including 0% by mass).

[0102] The amount of the added water relative to the lithium-metal composite oxide when the compound of boron, etc., is added or sprayed may be 0% by mass (excluding inevitably included water), but is preferably 0.1% by mass or more, 0.5% by mass or more, 1% by mass or more, 2% by mass or more, 3% by mass or more, 4% by mass or more, 5% by mass or more, 6% by mass or more, 7% by mass or more, 8% by mass or more, 9% by mass or more, 10% by mass or more, 11% by mass or more, 12% by mass or more, 13% by mass or more, 14% by mass or more, 15% by mass or more, 16% by mass or more, 17% by mass or more, 18% by mass or more, 19% by mass or more, or 20% by mass or more. Meanwhile, the amount of water relative to the lithium-metal composite oxide may be 50% by mass or less, 45% by mass or less, 40% by mass or less, 35% by mass or less, 30% by mass or less, or 25% by mass or less. Ensuring that the amount of water is at least the required amount will allow the compound of boron, etc., that is added to the lithium-metal composite oxide to be uniformly distributed. Ensuring that the amount of water is not greater than the required amount will make it possible to prevent loss of quality caused by elution of the lithium from the lithium-metal composite oxide.

[0103] The above lithium-metal composite oxide can be used, for example, as a positive electrode active material for nonaqueous electrolyte secondary batteries such as lithium secondary batteries.

[0104] <Nonaqueous Electrolyte Secondary Battery>The nonaqueous electrolyte secondary battery according to embodiments of the present disclosure comprises a positive electrode containing the above oxide powder as a positive electrode active material for a nonaqueous electrolyte secondary battery. The nonaqueous electrolyte secondary battery is composed of a positive electrode, a negative electrode, and an electrolyte.

[0105] When the positive electrode is produced, a conductive agent and a binder are added to and mixed with the oxide powder according to the embodiment of the present disclosure. Acetylene black, carbon black or graphite, etc., is preferably used as the conductive agent, for example. Polytetrafluoroethylene or polyvinylidene fluoride, etc., is preferably used as the binder, for example.

[0106] The negative electrode is not particularly limited, but it is possible to use not only a negative electrode active material such as lithium metal, graphite, or a low-crystallinity carbon material, for example, but also one or more non-metal or metal elements selected from Si, Al, Sn, Pb, Zn, Bi and Cd, or alloys comprising same, or chalcogen compounds comprising same, etc.

[0107] The solvent of the electrolytic solution is not particularly limited, but it is possible to use an organic solvent comprising one or more selected from carbonates, such as ethylene carbonate, propylene carbonate, dimethyl carbonate, and diethyl carbonate, or ethers such as dimethoxyethane, for example.

[0108] The electrolyte is not particularly limited, but one or more selected from lithium salts such as lithium hexafluorophosphate (LiPFe), lithium perchlorate, and lithium tetrafluoroborate can be used while dissolved in a solvent.[Examples]

[0109] The present disclosure is illustrated in greater detail below using examples but is not limited to these examples.

[0110] <Sample preparation>Samples of Examples 1 through 10 and Comparative Examples 1 through 5 were prepared by the methods given below.

[0111] [Example 1](Production of Precursor Composite Compound)A nickel sulfate aqueous solution as well as cobalt sulfate and manganese sulfate aqueous solution were mixed to an Ni and Co and Mn ratio (molar ratio) of Ni:Co:Mn=89:6:5, giving a metal aqueous solution. 10 L of pure water to which 300 g of a sodium hydroxide aqueous solution and 500 g of aqueous ammonia had been added were prepared in advance as the mother liquor in a reactor, the interior of the reactor was purged with nitrogen gas at a flow rate of 0.7 L / min to create a nitrogen atmosphere, and the reaction was also carried out under a nitrogen atmosphere.

[0112] The metal aqueous solution, the sodium hydroxide aqueous solution, and the aqueous ammonia were then simultaneously added drop-wise at a predetermined rate as a stirring blade was rotated at 1000 rpm, and the Ni, Co and Mn were co- precipitated by being crystallized into particle aggregates through a crystallization reaction in which the amount of the alkaline solution drops was adjusted to a pH of 11.5, thus giving a co-precipitate.

[0113] The slurry inside the reactor was then separated into liquid and solids, which were washed with pure water to lower residual impurities, and the co-precipitate in the form of cake was then dried for 12 hours at 110°C in the atmosphere to obtain a precursor composite compound.

[0114] (Production of Lithium-Metal Composite Oxide)The resulting precursor composite compound, anhydrous lithium hydroxide, and aluminum hydroxide were weighed out so that the proportions (molar ratio) of the total amount of Al to Ni, Co, and Mn were AI / (Ni+Co+Mn+AI)=0.02 and the proportions (molar ratio) of the total amount of Li to Ni, Co, Mn, and Al were Li / (Ni+Co+Mn+AI)=1.07, and the materials were mixed using a mixer to prepare a mixture.

[0115] The mixture was then fired over a 5-hour period at a maximum temperature of 770°C in an oxygen atmosphere (oxygen concentration: 97 vol%) using an electric furnace, was quenched, and was then milled using a grinding mill to obtain a lithium-metal composite oxide. The average particle diameter of the secondary particles of the lithium-metal composite oxide was about 12.8 pm. The BET specific surface area was 0.23 m2 / g. The moisture content, as determined by the Karl Fischer method, was 131 ppm. The amount of residual lithium hydroxide, as determined bythe Warder method, was 0.55% by mass.

[0116] (Coating Treatment)A boric acid solution, obtained by completely dissolving boric acid powder (measured out to a boron concentration of 1000 ppm relative to oxide powder sample) in pure water (water temperature: 25°C), was mixed, while being sprayed using a mixer, with the resulting lithium- metal composite oxide to obtain a mixed powder having a moisture content of 20% by mass. The resulting mixed powder was then annealed (heat treated) over a 3-hour period to a maximum mixed powder temperature of 300°C in an oxygen atmosphere (oxygen concentration: 97 vol%) using an electric furnace, was quenched, and was then milled using a grinding mill to obtain oxide powder samples.

[0117] [Example 2]Oxide powder samples were obtained in the same manner as in Example 1 , except that, in the coating treatment, the amount of boric acid added to the resulting lithium-metal composite oxide was changed to 500 ppm based on boron.

[0118] [Example 3]Oxide powder samples were obtained in the same manner as in Example 1 , except that, in the coating treatment, the boric acid solution added to the resulting lithium-metal composite oxide was mixed, as the solution was added drop-wise, using a mixer, and the moisture content of the mixed powder was 15% by mass.

[0119] [Example 4]Oxide powder samples were obtained in the same manner as in Example 1 , except that, in the coating treatment, the maximum temperature in the annealing (heat treatment) of the mixed powder was 350°C.

[0120] [Example 5]Oxide powder samples were obtained in the same manner as in Example 1 , except that, in the coating treatment, the maximum temperature in the annealing (heat treatment) of the mixed powder was 250°C.

[0121] [Example 6]Oxide powder samples were obtained in the same manner as in Example 1 , except that, in the coating treatment, the amount of boric acid added to the resulting lithium-metal composite oxide was changed to 1800 ppm based on boron.

[0122] [Example 7]Oxide powder samples were obtained in the same manner as in Example 1 , except that, in the coating treatment, mixed powder having a moisture content of 10% by mass was used.

[0123] [Example 8]Oxide powder samples were obtained in the same manner as in Example 1, except that the lithium-metal composite oxide that had been produced was then washed with water as follows.

[0124] (Water Washing Process)The lithium-metal composite oxide was introduced into pure water (water temperature: 25°C) in a reaction tank (10 L capacity), and the contents were stirred over a 10-minute period to prepare a slurry. Here, the ratio of the amount of the lithium-metal composite oxide relative to the amount of pure water (solid-liquid ratio) was adjusted to 2000 g / L. The resulting slurry was filtered using a Buchner funnel to obtain lithium-metal composite oxide cake. The cake had a moisture content of 6.2% by weight.

[0125] The resulting cake was heat treated for 60 minutes at 100°C using a vacuum dryer, and the cake was dried to obtain lithium-metal composite oxide in the form of a dry powder. At this time, the amount of residual lithium hydroxide, as determined by the Warder method, was 0.29% by mass.

[0126] [Example 9]Oxide powder samples were obtained in the same manner as in Example 1 , except that, in the coating treatment, the agent added to the resulting lithium-metal composite oxide was ammonium powder in hydrogen phosphate, and was added in a concentration of 800 ppm based on phosphorus.

[0127] [Example 10]The lithium-metal composite oxide that had been produced was then washed with water as follows, and boric acid powder having an average particle size of 13 pm (measured out to a boron concentration of 2000 ppm relative to a sample of the final product coated particles) was mixed, using a mixer, with the resulting lithium-metal composite oxide to obtain a mixed powder. The resulting mixed powder was then annealed (heat treated) over a 3-hour period to a maximum mixed powder temperature of 300°C in an oxygen atmosphere (oxygen concentration: 97 vol%) using an electric furnace, was quenched, and was then milled using a grinding mill to obtain oxide powder samples.

[0128] (Water Washing Process)The lithium-metal composite oxide was introduced into pure water (water temperature: 25°C) in a reaction tank (10 L capacity), and the contents were stirred over a 10-minute period to prepare a slurry. Here, the ratio of the amount of the lithium-metal composite oxide relative to the amount of pure water (solid-liquid ratio) was adjusted to 2000 g / L. The resulting slurry was filtered using a Buchner funnel to obtain lithium-metal composite oxide cake. The cake had a moisture content of 6.2% by mass.

[0129] The resulting cake was heat treated for 60 minutes at 100°C using a vacuum dryer, and the cake was dried to obtain lithium-metal composite oxide in the form of a dry powder. At this time, the amount of residual lithium hydroxide, as determined by the Warder method, was 0.29% by mass.

[0130] [Comparative Example 1]Oxide powder samples were obtained in the same manner as in Example 1 , except that nocoating treatment was carried out.

[0131] [Comparative Example 2]Lithium-metal composite oxide was produced in the same manner as in Example 1, the lithium-metal composite oxide was introduced into pure water (water temperature: 25°C) in a reaction tank (10 L capacity), and the contents were stirred over a 10-minute period to prepare a slurry. Here, the ratio of the amount of the lithium-metal composite oxide relative to the amount of pure water (solid-liquid ratio) was adjusted to 2000 g / L. The resulting slurry was filtered using a Buchner funnel to obtain lithium-metal composite oxide cake. The cake had a moisture content of 6.1 % by mass.

[0132] The resulting cake was heat treated for 60 minutes at 100°C using a vacuum dryer, and the cake was dried to obtain lithium-metal composite oxide in the form of a dry powder. The resulting dry powder was then annealed (heat treated) in the same manner as in Example 1, was quenched, and was then milled using a grinding mill to obtain oxide powder samples.

[0133] [Comparative Example 3]Oxide powder samples were obtained in the same manner as in Comparative Example 2, except that the lithium-metal composite oxide was not annealed (heat treated).

[0134] [Comparative Example 4]Oxide powder samples were obtained in the same manner as in Example 1, except that pure water was mixed while being sprayed, using a mixer, with the resulting lithium-metal composite oxide to obtain a mixed powder having a moisture content of 20% by mass.

[0135] [Comparative Example 5]Oxide powder samples were obtained in the same manner as in Example 1, except that the boric acid powder, measured out to a concentration of 1000 ppm relative to the oxide powder sample, was mixed as such, using a mixer, with the resulting lithium-metal composite oxide to obtain a mixed powder.

[0136] <Sample AssessmentSamples of Examples 1 through 9 and Comparative Examples 1 through 5 were assessed by the methods given below. The examples are shown along with the results.

[0137] [Composition of Precursor Composite Compounds and Lithium-Metal Composite Oxides]Samples of 0.2 g of precursor composite compounds or lithium-metal composite oxides were heated and dissolved in 25 mL of 20% hydrochloric acid solution, the solutions were cooled and then transferred to 100 mL measuring flasks, and pure water was introduced to prepare adjusted solutions. The constituent elements of the solutions that had been prepared were quantified using an ICP-AES [Optima 8300, manufactured by PerkinElmer Co., Ltd.], confirming that the proportions of the metals in the precursor composite compounds and lithium-metal composite oxides were as prescribed.

[0138] [Determination of BET specific surface area]Samples were dried and degassed for 45 minutes at 120°C in nitrogen gas, and the BET specific surface area was then determined using a BET specific surface area analyzer (MONOSORB, manufactured by Yuasa Ionics Co., Ltd.).

[0139] [Determination of Average Particle Size D50]The average particle size D50 was determined on a volume basis by a wet laser method using a laser type particle size distribution analyzer (Microtrac HRA, manufactured by Nikkiso Co., Ltd.).

[0140] [Determination by XPS]Information around the particle surface of samples was obtained by XPS (Quantera SXM, manufactured by Ulvac-PHI). Specifically, as the surface of a sample was irradiated with soft X- rays in a super-high vacuum, photoelectrons emitted from the surface were detected by an analyzer to acquire information up to a detection depth of several nm, information about elements on the surface was obtained based on the binding energy of bound electrons in the material, and the ratio between boron, etc., and lithium was quantified using the peak surface area ratio based on information about the valence or bound state from shifts in the energy of each peak.Conditions of AnalysisEquipment: Quantera SXM (Ulvac-PHI)Excitation X-ray: Monochromatic AIK 1 ,2 line (1486.6eV)X-ray diameter: 200 mPhotoelectron detection angle: 45 degrees (tilt of detector relative to sample surface)

[0141] [Determination by High-Resolution STEM]Samples were collected in a glove box (dew point: -70°C or below) and were transferred while protected against exposure to the atmosphere in a transfer vessel to a processor / analyzer. Thin section TEM analysis samples were first prepared using FIB, high-resolution STEM images were then acquired via atomic resolution electron microscopy (JEM-ARM200F Dual-X (aberration-corrected), manufactured by JEOL Ltd.) at an acceleration voltage of 200 kV, and EDX mapping analysis was performed to obtain elemental mapping images.

[0142] The Fourier transform patterns of the resulting high-resolution STEM images were checked, revealing that the samples of Examples 1 through 10 had a structure in which the surface of the lithium-metal composite oxide had been coated with two layers, consisting of first and second layers. More specifically, the lithium-metal composite oxide had a layered rock-salt structure, where the first layer was a nickel (II) oxide layer that had a NaCI structure (rock-salt structure). Figs. 2 and 3 are shown as representative figures. Fig. 2 is a high-resolution STEM image of the primary particle surface layer of a sample of the coated particles obtained in Example 10. Fig. 3 illustrates the boundary between the lithium-metal composite oxide and the first layer, as well as the boundary between the first layer and the second layer, in the high-resolution STEM image of Fig. 2. More detailed examination of the figures reveals that the layered rock-salt structure of the lithium-metal composite oxide and the rock-salt structure of the first layer have a continuous structure. As shown by the high-resolution STEM image of a coated particle lattice in Fig. 3, the oxygen atoms constituting the lithium-metal composite oxide and the oxygen atoms constitutingthe first layer (both represented by relatively white dots, a typical portion of which is surrounded by a rectangle) were disposed continuously, even across the boundary there-between.

[0143] The average thickness of the first layer and the second layer was determined via high- resolution STEM at 10 000 OOOx magnification. Specifically, the boundaries of the lithium-metal composite oxide, the first layer, and the second layer were identified as noted above, and 22 points (equally dividing the boundary length into 23 equal parts) were then established on the boundary between the lithium-metal composite oxide and the first layer to determine the thickness of the first layer. The shortest distance from each of 20 points (minus the 2 points at both ends) to the boundary between the first layer and second layer was used as the thickness at each point, and the average of the 20 points was used as the average thickness of the first layer. For the second layer, 22 points (equally dividing the boundary length into 23 equal parts) were then established on the boundary between the first layer and the second layer. The shortest distance from each of 20 points (minus the 2 points at both ends) to the surface of the second layer was used as the thickness at each point, and the average of the 20 points was used as the average thickness of the second layer.

[0144] High-resolution STEM observation of the surface layer of the particles revealed that the average thickness of the first layer was 10 nm, and the thickness of the second layer was 10 nm. The results show that, in the present invention, both the first layer and second layer had coatings of several tens of nanometers in thickness.

[0145] Comparative Examples 1 through 4 do not have first and second layers, and Comparative Example 5 does not have the first layer.

[0146] [Determination of Moisture Content]The moisture content (ppm) of the samples was determined as the amount of moisture that had been produced up to 300°C based on the Karl Fischer method (coulometric titration).

[0147] [Determination of Residual Lithium Hydroxide]The amount of residual lithium hydroxide in samples was determined and calculated based on the Warder method during neutralizing titration. Specifically, the amount was determined by adding 20 g of a particle powder sample to 100 mL of water, stirring the contents for 20 minutes at room temperature, filtering off the solids, and titrating the resulting supernatant using 0.2 N hydrochloric acid. Two points where the gradient was greatest on a pH curve drawn by plotting the titrated amount (mL) on the horizontal axis and supernatant pH on the vertical axis were used as the first titration point and second titration point, starting from the point where the titrated amount was smaller, and the amount of residual lithium was the value obtained from the titrated amounts at these points, as calculated using a calculation formula.

[0148] [Gelation Test]Gelation tests were conducted as follows. 20 g of positive electrode active material and 2.2 g of polyvinylidene fluoride (as the binder) were measured out and introduced into a pot, 9.6 mL of N-methylpyrrolidone were added, and the contents were mixed for 10 minutes (5 minutes of mixing, twice) using a non-bubbling kneader. The resulting slurry was introduced into DuranBottles (registered trademark) and stored without being sealed with a lid at 18°C and 40% RH. After 12 hours, the slurry was taken out and was added drop-wise onto aluminum foil, and assessments were based on the following criteria.A: Fluid, and could be printed on aluminum foil to produce a smooth, compact coated surface. B: Fluid, but when printed on aluminum foil, the coated surface was marred by scratches, cissing, and bleeding.C: Not fluid and could not be printed.

[0149] [Nonaqueous Electrolyte Secondary Battery Characteristics](Production of Coin Cells Using Positive Electrode Active Material)2032-type coin cells employing the positive electrode active material were produced using a positive electrode, negative electrode, and electrolytic solution that had been produced as follows.

[0150] • Positive ElectrodeAcetylene black and graphite were used as conductive agents in an acetylene black:graphite ratio (weight ratio) of 1 :1 , and polyvinylidene fluoride was used as the binder, where the samples serving as the positive electrode active material, the conductive agents, and the binder were blended to a positive electrode active materiakconductive agentbinder ratio (weight ratio) of 90:6:4, and a slurry obtained by mixing these materials with N-methylpyrrolidone was applied onto aluminum foil. The coated aluminum foil was dried at 110°C to prepare a sheet which was punched to a diameter of 15 mm and then rolled to a composite material density of 3.0 g / cm3, and this was used as the positive electrode.

[0151] • Negative ElectrodeLithium foil having a thickness of 500 pm punched to a diameter of 16 mm was used as the negative electrode.

[0152] • Electrolytic SolutionA mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) was prepared at a volume ratio of EC:DMC=1 :2, and a solution obtained by mixing a 1M LiPFe electrolyte therewith was used as the electrolytic solution.

[0153] (Initial Charging Capacity and Initial Charging / Discharging Efficiency)Coin cells produced by the method above were charged (constant current) at a current density of 18 mA / g (corresponding to a C rating of 0.1 C) to 4.30 V (upper limit voltage) at 25°C, and were then charged at a constant voltage to a current of 1.8 mA / g. The capacity at this time was used as the initial charging capacity (mAh / g).

[0154] After a 5-minute break, constant current discharging was then performed at a current density of 18 mA / g to 3.00 V under the same conditions, and the initial discharging capacity (mAh / g) was determined after a 5-minute break. It should be noted that the series of steps up to measurement of the initial discharging capacity was considered to be one charging / discharging cycle according to Condition A.

[0155] The initial charging / discharging efficiency was calculated on the basis of the following equation using the measured value of the initial charging capacity and the measured value of the initial discharging capacity.Initial charging / discharging efficiency (%) = (initial discharging capacity / initial charging capacity) * 100

[0156] (Cycle Retention Rate)The coin cells that had been produced were charged / discharged for 104 cycles at 60°C under the conditions indicated below. It should be noted that 1 C was considered to be a current density of 180 mA / g. When charged, the coin cells were charged at a constant current to the cut-off voltage, and were then charged at a constant voltage to a current of 1.8 mA / g.• 1st, 2nd and 104th cycles:Constant current charging at 0.2 C to 4.3 V, then constant voltage charging to 0.01 C 5-minute breakConstant current discharging at 0.2 C to 3.0 V5-minute break• 3rd to 103rd cycles:Constant current charging at 0.5 C to 4.3 V, then constant voltage charging to 0.01 C 5-minute breakConstant current discharging at 1 C to 3.0 V5-minute breakThe cycle retention rate at 60°C was calculated based on the following equation using the measured value of the discharging capacity in the 2nd cycle and the measured value of the discharging capacity in the 104th cycle.Cycle retention rate (%)= (discharging capacity in the 104th cycle / discharging capacity in the 2nd cycle) * 100

[0157] (Initial Reaction Resistance)Using the coin cells produced by the method above, the initial reaction resistance was determined at 25°C under the conditions indicated below.• 1st cycle:Constant current charging at 0.1 C to 4.3 V, then constant voltage charging to 0.01 C 5-minute breakConstant current discharging at 0.1 C to 2.5 V5-minute break• 2nd cycle:Constant current charging at 0.1 C to 4.3 V, then constant voltage charging to 0.01 CThe impedance was determined at 25°C under the conditions indicated below using cells for which the 2nd cycle of charging had been completed. The result was used as the impedance measurement result for the 2nd cycle (reaction resistance at the 2nd cycle (initial reaction resistance)).Frequency range: 300 k-0.01 Hz (76 points)Amplitude: 10 mV

[0158] The manufacturing conditions and assessment results of Examples 1 through 10 and Comparative Examples 1 through 5 are shown in Table 1 below.

[0159] [Key to Symbols]

[0160] 1 : Coated particle2: Lithium-metal composite oxide 3: First layer4: Second layer

Claims

CLAIMS

1. An oxide powder comprising coated particles that have: a lithium-metal composite oxide, in the form of primary particles or secondary particles, that comprises at least lithium and nickel; a first layer comprising nickel (II) oxide on at least part of the surface of the lithium-metal composite oxide; and a second layer comprising an oxide, including lithium and one or more selected from the group consisting of boron, phosphorus, and sulfur, on at least part of the surface of the first layer, wherein the BET specific surface area is 0.3 m2 / g or more, and the LiOH content is 0.35% by mass or less relative to the oxide powder.

2. The oxide powder according to claim 1 , wherein the lithium-metal composite oxide has a layered rock-salt structure and is represented by the general formula LiaNii-b-cMnbMcO2 (in the formula, M is one or more elements other than Li, Ni, Mn, and O; 0.95<a<1.15; and 0<b+c<0.70).

3. The oxide powder according to claim 1 or 2, wherein the average thickness of the first layer is 3 nm to 100 nm.

4. The oxide powder according to claim 1 or 2, wherein the lithium-metal composite oxide and the first layer have a continuous structure.

5. A positive electrode active material for a nonaqueous electrolyte secondary battery, comprising the oxide powder according to claim 1 or 2.

6. A nonaqueous electrolyte secondary battery, comprising the positive electrode active material for a nonaqueous electrolyte secondary battery according to claim 5.

7. A method for producing an oxide powder, wherein to a lithium-metal composite oxide, in the form of primary particles or secondary particles, that contains at least lithium and nickel, is added or sprayed a compound including one or more selected from the group consisting of boron, phosphorus, and sulfur, along with at least 5% by mass of water relative to the lithium-metal composite oxide, and heat treatment is then carried out to obtain the oxide powder according to claim 1.

8. The method for producing an oxide powder, wherein a compound including one or more selected from the group consisting of boron, phosphorus, and sulfur is added to or sprayed onto a lithium-metal composite oxide that comprises at least lithium and nickel, that is in the form of primary particles or secondary particles, and that has anLiOH content of 0.4% by mass or less relative to the lithium-metal composite oxide, and heat treatment is then carried out to obtain the oxide powder according to claim 1.

9. The method for producing an oxide powder according to claim 7 or 8, wherein the ratio of the BET specific surface area of the oxide powder after the heat treatment relative to the BET specific surface area of the lithium-metal composite oxide before the heat treatment is 1.2 or more.

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