Method for producing positive electrode active material for nonaqueous electrolyte secondary batteries, positive electrode active material for nonaqueous electrolyte secondary batteries, and nonaqueous electrolyte secondary battery using same
A method for manufacturing a positive active material for a nonaqueous electrolyte secondary battery having both thermal stability and charge-discharge capacity at a high level as well as excellent cycle characteristics. The method for manufacturing a positive active material for a nonaqueous electrolyte secondary battery includes: a step of adding a niobium salt solution and an acid simultaneously to a slurry of a nickel-containing hydroxide, and controlling the pH of the slurry at between 7 and 11 on a 25° C. basis to obtain a nickel-containing hydroxide coated with a niobium compound; a step of mixing the nickel-containing hydroxide coated with the niobium compound with a lithium compound to obtain a lithium mixture; and a step of firing the lithium mixture in an oxidizing atmosphere at 700° C. to 830° C. to obtain a lithium-transition metal composite oxide.
1. A method for manufacturing a positive active material formed of a lithium-transition metal composite oxide for a nonaqueous electrolyte secondary battery, the method comprising:
coating a nickel-containing hydroxide with a niobium compound by simultaneously adding a niobium salt solution and an acid to a slurry of a nickel-containing hydroxide, and pH of the slurry is controlled to between 7 and 11 on a 25° C. basis, thereby obtaining a nickel-containing hydroxide coated with a niobium compound;
mixing the nickel-containing hydroxide coated with the niobium compound with a lithium compound, thereby obtaining a lithium mixture; and
firing the lithium mixture at 700 to 830° C. in an oxidizing atmosphere, thereby obtaining the lithium-transition metal composite oxide,
wherein the lithium-transition metal composite oxide is represented by a General Formula Li d Ni 1-a-b-c Co a M b Nb c O 2 , where M is at least one element selected from Mn, V, Mg, Ti and Al, and a, b, c and d satisfy 0.05≦a≦0.35, 0≦b≦0.10, 0.006≦c≦0.06, and 0.95≦d≦1.20, and
the positive active material is composed of particles with a polycrystalline structure.
2. The method for manufacturing a positive active material for a nonaqueous electrolyte secondary battery according to claim 1 , wherein the step of coating niobium is preceded by a step of adding an alkaline aqueous solution to a mixed aqueous solution containing at least nickel and cobalt to crystallize the solution and obtain the nickel-containing hydroxide, the nickel-containing hydroxide being represented by a General Formula (1) Ni 1-a′-b′ Co a′ M b′ (OH) 2 , where M is at least one element selected from Mn, V, Mg, Ti and Al, and a′ and b′ satisfy 0.05≦a′≦0.35, and 0≦b′≦0.10.
3. The method for manufacturing a positive active material for a nonaqueous electrolyte secondary battery according to claim 1 , wherein c representing the niobium content of the lithium-transition metal composite oxide satisfies 0.006≦c≦0.05.
4. The method for manufacturing a positive active material for a nonaqueous electrolyte secondary battery according to claim 1 , wherein the pH of the slurry is controlled at between 7 and 10 on a 25° C. basis in the step of coating niobium.
5. The method for manufacturing a positive active material for a nonaqueous electrolyte secondary battery according to claim 1 , wherein the step of mixing is preceded by a step of heat treating the nickel-containing hydroxide coated with the niobium compound at 105° C. to 800° C.
6. The method for manufacturing a positive active material for a nonaqueous electrolyte secondary battery according to claim 1 , further comprising a step of water washing the lithium-transition metal composite oxide obtained in the firing step with water in a proportion of 100 to 2000 g/L.
7. A positive active material for a nonaqueous electrolyte secondary battery, manufactured by the method according to claim 1 , the positive active material comprising a lithium-transition metal composite oxide which is represented by the General Formula Li d Ni 1-a-b-c Co a M b Nb c O 2 , where M is at least one element selected from Mn, V, Mg, Ti and Al, and 0.05≦a≦0.35, 0≦b≦0.10, 0.006≦c≦0.06, and 0.95≦d≦1.20 are established and which is composed of particles with a polycrystalline structure, wherein the material has a specific surface area of 2.0 to 7.0 m 2 /g, and a porous structure with a ratio of (total number of voids in a cross-section of a particle)/(a first longest cross-sectional diameter of the particle) is 2/μm or more, the number of voids being obtained by counting voids, each void having a second longest cross-sectional diameter of 0.3 μm or more between any two points on an outer edge of the void in an observation plane.
8. The positive active material for a nonaqueous electrolyte secondary battery according to claim 7 , wherein the crystallite diameter of the lithium-transition metal composite oxide is 10 to 150 nm.
9. The positive active material for a nonaqueous electrolyte secondary battery according to claim 8 , wherein c representing the niobium content of the lithium-transition metal composite oxide satisfies 0.006≦c≦0.05.
10. A nonaqueous electrolyte secondary battery comprising a positive active material for a nonaqueous electrolyte secondary battery according to claim 7 in the positive electrode.
11. The method for manufacturing a positive active material for a nonaqueous electrolyte secondary battery according to claim 2 , wherein c representing the niobium content of the lithium-transition metal composite oxide satisfies 0.006≦c≦0.05.
12. A nonaqueous electrolyte secondary battery comprising a positive active material for a nonaqueous electrolyte secondary battery according to claim 8 in the positive electrode.
13. A nonaqueous electrolyte secondary battery comprising a positive active material for a nonaqueous electrolyte secondary battery according to claim 9 in the positive electrode.