Coated nickel hydroxide powder for alkali secondary battery positive electrode active material and method of producing same

1. A coated nickel hydroxide powder which has been washed with water until an amount of ammonium ions eluted into a suspension obtained by adding 10 mL of water to 10 g of the coated nickel hydroxide powder becomes 0.35 mmol/L or less and which can be used as a paste for an alkaline secondary battery positive electrode active material, comprising:

core particles constituting of nickel hydroxide; and

a coating layer constituting of a cobalt compound including cobalt hydroxide, cobalt oxyhydroxide, or a mixture thereof, formed on a surface of the core particles,

wherein the coated nickel hydroxide powder has a total carbon content of 800 ppm by mass or less and has a median diameter at 50% cumulative volume of 5 to 15 μm as measured by a laser diffraction scattering method,

wherein when 10 mL of water is added to 10 g of the coated nickel hydroxide powder to prepare a suspension for evaluation, the suspension has a pH of 10.2 to 13 as measured at 25° C. and a total amount of eluted nitrate ions and nitrite ions in the suspension is 0.60 mmol/L or less,

and wherein when 10 mL of water is added to 1 g of the coated nickel hydroxide powder to prepare a second suspension and then the second suspension is allowed to stand for 10 minutes to obtain a supernatant, the supernatant has a turbidity specified in JIS K0101 of 350 degrees or more.

2. The coated nickel hydroxide powder for an alkaline secondary battery positive electrode active material according to claim 1 , wherein an amount of eluted nitrate ions in the suspension is 0.52 mmol/L or less.

3. A method for producing a coated nickel hydroxide powder which has a median diameter at 50% cumulative volume of 5 to 15 μm as measured by a laser diffraction scattering method and is used as a paste for an alkaline secondary battery positive electrode active material, the method comprising:

a crystallization step in which an ammonium ion supplier-containing aqueous solution in which the ammonium ion supplier has a capability to form a nickel ammine complex in an aqueous reaction solution and an aqueous alkali solution are supplied to a nickel-containing aqueous solution of an aqueous nickel sulfate solution to obtain nickel hydroxide particles crystallized out by neutralization as core particles;

a coating step in which an aqueous cobalt salt solution and an aqueous alkali solution are supplied to a stirred slurry of the nickel hydroxide particles as core particles while the pH of the slurry is maintained in a range of 8 to 11.5 as measured at 25° C. and the temperature of the slurry is maintained in a range between 30 to 60° C. so as to form a coating layer made of a cobalt compound on a surface of the core particles to obtain a coated nickel hydroxide powder;

a washing step in which the coated nickel hydroxide powder is washed with water in an amount of 4 times by mass or more an amount of the coated nickel hydroxide powder; and

a drying step in which the washed coated nickel hydroxide powder is dried in a non-oxidizing atmosphere of either vacuum or a nitrogen gas atmosphere, the non-oxidizing atmosphere having a partial pressure of a carbon-containing gas being 15 Pa or less such that the coated nickel hydroxide powder has a total carbon content of 800 ppm by mass or less,

wherein in the washing step, the coated nickel hydroxide powder is washed until an amount of ammonium ions eluted into a suspension obtained by adding 10 mL of water to 10 g of the coated nickel hydroxide powder before the drying step becomes 0.35 mmol/L or less,

and wherein after the drying step, a total amount of eluted nitrate ions and nitrite ions in a suspension obtained by adding 10 mL of water to 10 g of the coated nickel hydroxide powder after the drying step is 0.60 mmol/L or less.

4. The method for producing a coated nickel hydroxide powder for an alkaline secondary battery positive electrode active material according to claim 3 , further comprising an oxidizing step after the coating step, in which at least a part of any cobalt hydroxide constituting the coating layer is oxidized to cobalt oxyhydroxide by supplying air or oxygen to the stirred slurry or adding an oxidizer.