Material for lithium secondary battery of high performance
Provided is a cathode active material containing a Ni-based lithium mixed transition metal oxide. More specifically, the cathode active material comprises the lithium mixed transition metal oxide having a composition represented by Formula I of Li x M y O 2 wherein M, x and y are as defined in the specification, which is prepared by a solid-state reaction of Li 2 CO 3 with a mixed transition metal precursor under an oxygen-deficient atmosphere, and has a Li 2 CO 3 content of less than 0.07% by weight of the cathode active material as determined by pH titration. The cathode active material in accordance with the present invention and substantially free of water-soluble bases such as lithium carbonates and lithium sulfates and therefore has excellent high-temperature and storage stabilities and a stable crystal structure. A secondary battery comprising such a cathode active material exhibits a high capacity and excellent characteristics, and can be produced by an environmentally friendly method with low production costs and high production efficiency.
1. A cathode active material comprising a lithium mixed transition metal oxide having a composition represented by Formula I:
Li x M y O 2 (I)
wherein:
M=M′ 1−k A k , wherein M′ is Ni 1−a−b (Ni 1/2 Mn 1/2 ) a Co b , 0.65≦a+b≦0.85 and 0.1≦b≦0.4;
A is a dopant;
0≦k<0.05; and
x+y=2 and 0.95≦x≦1.05,
wherein the lithium mixed transition metal oxide has a Li 2 CO 3 content of less than 0.07% by weight of the cathode active material as determined by pH titration, and is prepared by a solid-state reaction of Li 2 CO 3 and a mixed transition metal precursor under an oxygen-deficient atmosphere,
wherein the solid-state reaction includes a sintering process at 600° C. to 1100° C. for 3 to 20 hours, and
wherein an amount of air exceeding 2 m 3 /kg of Li x M y O 2 during the sintering process is supplied to a reaction vessel equipped with a heat exchanger for pre-warming of the air.
2. The cathode active material according to claim 1 , wherein Li 2 CO 3 is contained in an amount such that less than mL of a 0.1M HCl titrant solution is added during pH titration of a solution of water-soluble bases extracted from the cathode active material to reach a value of less than 5, wherein the solution of water-soluble bases is prepared by repeatedly soaking and decanting 10 g of the cathode active material with water such that the resulting solution contains all of the water-soluble bases in the cathode active material, and wherein the total volume of solution of water-soluble base is 200 mL.
3. The cathode active material according to claim 2 , wherein the amount of the 0.1M HCl solution added during pH titration to reach the pH of less than 5 is less than 10 mL.
4. The cathode active material according to claim 1 , wherein the oxygen-deficient atmosphere has an oxygen concentration of 10 to 50% by volume.
5. The cathode active material according to claim 4 , wherein the oxygen concentration is from 10% to 30% by volume.
6. The cathode active material according to claim 4 , wherein the oxygen-deficient atmosphere is an air atmosphere.
7. The cathode active material according to claim 1 , wherein a mixing ratio of Li 2 CO 3 and the mixed transition metal precursor for the solid-state reaction is from 0.95 to 1.04:1 by w/w.
8. The cathode active material according to claim 1 , wherein the mixed transition metal precursor is one or more selected from the group consisting of M(OH) 2 and MOOH.
9. The cathode active material according to claim 8 , wherein the mixed transition metal precursor is MOOH, and is prepared by an ammonia-free process.
10. The cathode active material according to claim 1 , wherein the mixed transition metal precursor has a tap density of 1.1 to 1.6 g/cm 3 .
11. The cathode active material according to claim 1 , wherein the lithium mixed transition metal oxide is prepared by a large-scale process under a high rate of air circulation.
12. The cathode active material according to claim 11 , wherein at least 2 m 3 of air at room temperature per 1 kg of the lithium mixed transition metal oxide is pumped into or out of a reaction vessel.
13. The cathode active material according to claim 12 , wherein at least 10 m 3 of air at room temperature per 1 kg of the lithium mixed transition metal oxide is pumped into or out of the reaction vessel.
14. The cathode active material according to claim 1 , wherein the heat exchanger pre-warms in-flowing air before the in-flowing air enters the reaction vessel, while cooling out-flowing air.
15. A lithium secondary battery comprising the cathode active material of claim 1 .