Positive-electrode active material powder, positive electrode containing positive-electrode active material powder, and secondary battery
Provided is cathode active material powder or a cathode that is capable of improving the cycle characteristics of nonaqueous electrolyte rechargeable batteries at high voltage and exhibiting excellent capacity retention. The powder has a composition of: Li x−w Na w Co 1−y M y O 2+z (0.950≤x≤1.100, 0<y≤0.050, −0.1≤z≤0.1, 0≤w≤0.020; M: rare earth elements, Ti, Zr, Mo, Mn, Ni, Cu, Al, Ga, etc), and a core-shell structure composed of a lithium-containing composite oxide region as core and a Li-ion-conductive oxide region as shell. The core contains Li, Co, and oxygen, and has Co concentration of not higher than 90% as calculated by formula: ((Co concentration on shell external surface)/(Co concentration at shell boundary of core/shell structure))×100. The shell is an amorphous layer containing La, Ti, Co, and oxygen, covering the core at least partly.
1. A cathode active material powder for a nonaqueous electrolyte rechargeable battery, said cathode active material powder having the following formula (I):
Li x−w Na w Co 1−y M y O 2+z (I)
wherein x, y, z, and w satisfy 0.950≤x≤1.100, 0<y≤0.050, −0.1≤z≤0.1, and 0≤w≤0.020, respectively; M contains Ti and La, and optionally contains one or more elements selected from the group consisting of Mg, Zr, and Al,
wherein said cathode active material powder has a core-shell structure consisting of a lithium-containing composite oxide region as a core and a Li-ion-conductive oxide region as a shell,
wherein said cathode active material powder is produced by a method containing the steps of coating core precursor particles at least partly with a raw shell material and calcining the resultant at a temperature of not lower than 550° C. and not higher than 700° C. for 1 to 24 hours,
wherein said core comprises Li, Co, and oxygen, and has a Co concentration ratio of not lower than 22% and not higher than 90% as calculated by formula (II):
((Co concentration on external surface of shell)/(Co concentration at boundary between core and shell in core-shell structure))×100 (II)
and
wherein said shell is an amorphous layer comprising La, Ti, Co, and oxygen, covering said core at least partly.
2. The cathode active material powder according to claim 1 , wherein said amorphous layer is 5 to 2000 nm in thickness.
3. The cathode active material powder according to claim 1 , wherein a La concentration ratio as calculated by formula (III):
(La concentration at boundary between core and shell in core-shell structure/La concentration on external surface of shell)×100 (III)
is not higher than 5%.
4. The cathode active material powder according to claim 3 , wherein said La concentration ratio is 0.
5. The cathode active material powder according to claim 1 , wherein a Ti concentration as calculated by formula (IV):
(Ti concentration at boundary between core and shell in core-shell structure/Ti concentration on external surface of shell)×100 (IV)
is not higher than 5%.
6. The cathode active material powder according to claim 1 , wherein a ratio of Co concentration A in the core to Co concentration B in the shell (AB) is 1.1 to 5.0.
7. The cathode active material powder according to claim 1 , wherein said shell further comprises Li, Na, or K.
8. The cathode active material powder according to claim 1 , wherein said core comprises Na.
9. The cathode active material powder according to claim 1 , wherein said core is free of rare earth elements.
10. The cathode active material powder according to claim 1 , wherein said cathode active material powder has a specific surface area of 0.1 to 1.0 m 2 /g.
11. A cathode for a nonaqueous electrolyte rechargeable battery comprising a cathode active material powder of claim 1 , an electrically conductive material, and a binder.
12. A rechargeable battery comprising a cathode of claim 11 , an anode, a nonaqueous electrolyte, and a separator.