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 below, wherein the lithium mixed transition metal oxide is prepared by a solid-state reaction of Li 2 CO 3 and a mixed transition metal precursor under an oxygen-deficient atmosphere, and having a Li 2 CO 3 content of less than 0.07% by weight of the cathode active material as determined by pH titration:
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.
2 . The cathode active material according to claim 1 , wherein Li 2 CO 3 is contained in an amount such that less than 20 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, wherein the ratio of Li 2 CO 3 :mixed transition metal precursor is 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 wherein M is as defined in Formula I.
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 solid-state reaction includes a sintering process at 600 to 1100° C. for 3 to 20 hours.
12 . The cathode active material according to claim 11 , wherein an amount of air exceeding 2 m 3 /kg LiMO 2 during the sintering process is supplied to a reaction vessel equipped with a heat exchanger for pre-warming of the air.
13 . 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.
14 . The cathode active material according to claim 13 , wherein for the high rate of air circulation during the sintering process, at least 2 m 3 of air at room temperature per 1 kg of the final lithium mixed transition metal oxide is pumped into or out of the reaction vessel.
15 . The cathode active material according to claim 14 , wherein at least 10 m 3 of air at room temperature per 1 kg of the final lithium mixed transition metal oxide is pumped into or out of the reaction vessel.
16 . The cathode active material according to claim 12 , wherein the heat exchanger pre-warms the in-flowing air before the in-flowing air enters the reaction vessel, while cooling the out-flowing air.
17 . A lithium secondary battery comprising the cathode active material of claim 1 .
18 . A method for determining an amount of water-soluble base contained in a cathode active material comprises determining an amount of a 0.1 M HCl titrant solution neutralized by pH titration of a solution containing the water-soluble base until the pH of the solution reaches a value of less than 5, wherein the solution containing the water-soluble base is prepared by repeatedly soaking and decanting 10 g of the cathode active material with water until the resulting solution contains all of the water-soluble base in the cathode active material, and the total volume of the solution of water-soluble base is 200 mL.
19 . The method according to claim 18 , wherein pH titration with an addition of 0.1M HCl is carried out for 5 hours or less.
20 . The cathode active material of claim 1 , wherein a comparative lithium transition metal oxide that is not prepared by the solid-state reaction of Li 2 CO 3 and a mixed transition metal precursor under an oxygen-deficient atmosphere, has a Li 2 CO 3 content of greater than or equal to 0.07% as determined by pH titration.