Cathode active material, method for manufacturing same, and lithium secondary battery comprising same
A method for manufacturing a positive active material is provided. The method includes forming a positive active material precursor including nickel, mixing and firing the positive active material precursor and lithium salt to form a preliminary positive active material particle, forming a coating material including fluorine on the preliminary positive active material particle by dry-mixing the preliminary positive active material particle with a coating source including fluorine, and manufacturing a positive active material particle by thermally treating the preliminary positive active material particle on which the coating material is formed.
1. A method for manufacturing a positive active material, the method comprising:
forming a positive active material precursor including nickel;
mixing and firing the positive active material precursor and a lithium salt to form a preliminary positive active material particle;
forming a coating material including fluorine on the preliminary positive active material particle by dry-mixing the preliminary positive active material particle with a coating source including fluorine; and
manufacturing a positive active material particle by thermally treating the preliminary positive active material particle on which the coating material is formed, at a temperature lower than a crystal structure changing temperature of the positive active material particle, said crystal structure changing temperature being a temperature where the fluorine included in the coating source permeates into a crystal lattice of the positive active material particle,
wherein the positive active material particle comprises nickel, and
wherein the crystal structure changing temperature decreases as a content of nickel included in the positive active material particle increases.
2. The method of claim 1 , wherein the preliminary positive active material particle is dry-mixed with the coating source such that surface residual lithium of the preliminary positive active material particle is reduced.
3. The method of claim 1 , wherein the amount of surface residual lithium of the positive active material particle is less than the amount of surface residual lithium of the preliminary positive active material particle dry-mixed with the coating source.
4. The method of claim 1 , wherein the coating material reacts with surface residual lithium of the preliminary positive active material particle by the thermal treating of the preliminary positive active material particle to form a compound of lithium and fluorine.
5. The method of claim 1 , wherein the positive active material precursor further includes at least one of cobalt, manganese, or aluminum.
6. The method of claim 1 , wherein the coating source includes NH 4 F or NH 4 HF 2 .
7. The method of claim 1 , wherein the positive active material precursor comprises:
a first portion having a metal concentration gradient; and
a second portion having a metal concentration gradient of which a magnitude is different from that of the metal concentration gradient of the first portion.
8. The method of claim 1 , wherein the positive active material precursor has a metal concentration gradient in a whole of a particle thereof.
9. The method of claim 1 , wherein the positive active material precursor comprises a core portion and a shell portion surrounding the core portion, and
wherein at least one of the core portion or the shell portion has a metal concentration gradient.