Positive electrode active material for nonaqueous electrolyte secondary battery, method for producing same, and nonaqueous electrolyte secondary battery using said positive electrode active material
Provided is a method for producing a positive electrode active material for nonaqueous electrolyte secondary batteries, including: a water-washing step of mixing, with water, Li—Ni composite oxide particles represented by the formula: Li z Ni 1-x-y Co x M y O 2 and composed of primary particles and secondary particles formed by aggregation of the primary particles to water-wash it, and performing solid-liquid separation to obtain a washed cake; a mixing step of mixing a W compound powder free from Li with the washed cake to obtain a W-containing mixture; and a heat treatment step of heating the W-containing mixture, the heat treatment step including: a first heat treatment step of heating the W-containing mixture to disperse W on the surface of the primary particles; and subsequently, a second heat treatment step of heating it at a higher temperature than in the first heat treatment step to form a lithium tungstate compound on the surface of the primary particles.
1. A positive electrode active material for nonaqueous electrolyte secondary batteries, comprising a lithium-nickel composite oxide powder having a layered crystal structure composed of primary particles and secondary particles formed by aggregation of the primary particles, the positive electrode active material being represented by a general formula: Li z Ni 1-x-y Co x M y W a O 2+α , where 0≤x≤0.35, 0≤y≤0.35, 0.95≤z≤1.30, 0<a≤0.03, and 0≤α≤0.15 are satisfied, and M is at least one element selected from Mn, V, Mg, Mo, Nb, Ti, and Al, wherein
lithium tungstate and a lithium compound other than lithium tungstate are present on a surface of the primary particles of the lithium-nickel composite oxide,
an amount of lithium contained in the lithium compound other than the lithium tungstate present on the surface of the primary particles of the lithium-nickel composite oxide is 0.01 to 0.05 mass % with respect to the total amount of the positive electrode active material, and
a content of sulfate radical in the positive electrode active material is 0.001 to 0.05 mass %.
2. The positive electrode active material for nonaqueous electrolyte secondary batteries according to claim 1 , wherein
an amount of tungsten contained in the positive electrode active material is 0.05 to 2.0 at % with respect to the total number of atoms of Ni, Co, and M contained in the lithium-nickel composite oxide powder.
3. The positive electrode active material for nonaqueous electrolyte secondary batteries according to claim 1 , wherein
the lithium tungstate is present on the surface of the primary particles of the lithium-nickel composite oxide as fine particles having a particle size of 1 to 500 nm.
4. The positive electrode active material for nonaqueous electrolyte secondary batteries according to claim 1 , wherein
the lithium tungstate is present on the surface of the primary particles of the lithium-nickel composite oxide as a coating film having a film thickness of 1 to 200 nm.
5. The positive electrode active material for nonaqueous electrolyte secondary batteries according to claim 1 , wherein
the lithium tungstate is present on the surface of the primary particles of the lithium-nickel composite oxide in both forms of fine particles having a particle size of 1 to 500 nm and a coating film having a film thickness of 1 to 200 nm.
6. The positive electrode active material for nonaqueous electrolyte secondary batteries according to claim 1 , wherein
the content of sulfate radical is 0.001 to 0.02 mass %.
7. A nonaqueous electrolyte secondary battery comprising:
a positive electrode including the positive electrode active material for nonaqueous electrolyte secondary batteries according to claim 1 .