Process for producing carbonate compound and cathode active material

1. A process for producing a carbonate compound, which comprises mixing the following sulfate (A) and the following carbonate (B) in the form of aqueous solutions and controlling the proportion of Mn to the total of Ni, Co and Mn contained in the sulfate (A) to be higher than 65 mol % at the initiation of the mixing, to precipitate a carbonate compound having a proportion of Mn of from 33.3 to 65 mol %, a proportion of Ni of from 17.5 to 50 mol % and a proportion of Co of from 0 to 33.3 mol % to the total of Ni, Co and Mn in the total average composition:

sulfate (A): a sulfate comprising a sulfate of Mn and a sulfate of Ni, or a sulfate comprising a sulfate of Mn, a sulfate of Ni and a sulfate of Co, and

carbonate (B): at least one carbonate selected from the group consisting of sodium carbonate and potassium carbonate.

2. The process for producing a carbonate compound according to claim 1 , wherein at the initiation of mixing the sulfate (A) and the carbonate (B), a pH of the mixed solution in a mixing tank is maintained at from 7 to 12.

3. The process for producing a carbonate compound according to claim 1 , wherein a concentration of transitional metal element (X) comprising Mn, Ni and Co in the aqueous solution of the sulfate (A) is from 0.1 to 3 mol/kg.

4. The process for producing a carbonate compound according to claim 1 , wherein a concentration of the carbonate (B) in the aqueous solution of the carbonate (B) is from 0.1 to 2 mol/kg.

5. The process for producing a carbonate compound according to claim 1 , which comprises mixing the sulfate (A) and the carbonate (B) in the form of aqueous solutions, and controlling in a first precipitation step, the proportion of Mn to the total of Ni, Co and Mn contained in the sulfate (A) to be higher than 65 mol % and controlling in a second precipitation step, the proportion of Mn to the total of Ni, Co and Mn contained in the sulfate (A) to be less than 65 mol %, to precipitate a carbonate compound having the proportion of Mn of from 33.3 to 65 mol %, the proportion of Ni of from 17.5 to 50 mol % and the proportion of Co of from 0 to 33.3 mol % to the total of Ni, Co and Mn in the total average composition.

6. The process for producing a carbonate compound according to claim 5 , wherein in the first precipitation step, the proportion of Mn in the sulfate (A) is higher than 65 mol % and at most 100 mol %, the proportion of Ni is from 0 to 35 mol %, and the proportion of Co is from 0 to 33.3 mol %.

7. A process for producing a cathode active material, which comprises mixing the carbonate compound obtained by the process as defined in claim 1 and lithium carbonate, followed by firing at from 500 to 1,000° C.

8. The process for producing a cathode active material according to claim 7 , wherein a number of moles of Li contained in the lithium carbonate is from 1.1 to 1.4 times a number of moles of the total of Ni, Co and Mn contained in the carbonate compound.

9. The process for producing a cathode active material according to claim 7 , wherein the cathode active material to be obtained is a compound (1) represented by the following formula (1):

Li 1+a Ni b Co c Mn d M′ e O 2+f   (1)

wherein M′ is at least one member selected from the group consisting of Mg, Ca, Sr, Ba and Al; a to e are respectively 0.1≦a≦0.4, 0.175≦b≦0.5, 0≦c≦0.333, 0.333≦d≦0.65, 0≦e≦0.05, 0.9≦b+c+d≦1.05; and f is a numerical value to be determined depending on a valence of Li, Ni, Co, Mn and M′.

10. A cathode for a lithium ion secondary battery, comprising a cathode current collector and a cathode active material layer formed on the cathode current collector, wherein the cathode active material layer contains the cathode active material obtained by the process as defined in claim 7 , an electrically conductive material and a binder.

11. A lithium ion secondary battery comprising the cathode for a lithium ion secondary battery as defined in claim 10 , an anode and a non-aqueous electrolyte.