Cathode active material, method of preparing the cathode active material, and all-solid-state battery including the same

1. A cathode active material comprising:

a first composite oxide represented by Formula (1):

x V 2 O 5 .Li 3 PO 4   (1)

wherein, in Formula (1), x satisfies 2<x≤10.

2. The cathode active material of claim 1 , further comprising a compound represented by Formula (1A):

Li a M b O c   (1A)

wherein, in Formula (1A), M is a Group 3 to Group 15 element or a combination comprising at least one of a Group 3 to Group 15 element, and a, b, and c respectively satisfy 1≤a≤2, 0<b≤1, and 1<c≤4.

3. The cathode active material of claim 1 , wherein the cathode active material further comprises a compound of the formula Li a M b O c , a content of which is represented by Formula (2):

y ( x V 2 O 5 .Li 3 PO 4 ).(100− y )Li a M b O c   (2)

wherein, in Formula (2), M is a Group 3 to Group 15 element or a combination comprising at least one of a Group 3 to Group 15 element, and x, y, a, b, and c respectively satisfy 2<x≤10, 95≤y<100, 1≤a≤2, 0<b≤1, and 1<c≤4.

4. The cathode active material of claim 3 , wherein in Formula (2), M comprises B, Zr, Nb, Mo, W, or a combination thereof.

5. The cathode active material of claim 3 , wherein Formula (2) is y(xV 2 O 5 .Li 3 PO 4 ).(100−y)LiNbO 3 , y(xV 2 O 5 .Li 3 PO 4 ).(100−y)Li 2 MoO 4 , y(xV 2 O 5 .Li 3 PO 4 ).(100−y)LiBO 2 , y(xV 2 O 5 .Li 3 PO 4 ).(100−y)Li 2 WO 4 , or y(xV 2 O 5 .Li 3 PO 4 ).(100−y)Li 2 ZrO 3 , wherein x and y respectively satisfy 2<x≤10 and 95≤y<100.

6. The cathode active material of claim 3 , wherein Formula (2) is 95(4V 2 O 5 .Li 3 PO 4 ). 5LiNbO 3 , 95(4V 2 O 5 .Li 3 PO 4 ).5Li 2 MoO 4 , 95(4V 2 O 5 .Li 3 PO 4 ).5LiBO 2 , 95(4V 2 O 5 .Li 3 PO 4 ).5Li 2 WO 4 , 99(4V 2 O 5 .Li 3 PO 4 ).1Li 2 MoO 4 , or 99(4V 2 O 5 .Li 3 PO 4 ).1Li 2 ZrO 3 .

7. The cathode active material of claim 1 , further comprising a third composite oxide, the third composite oxide comprising:

a core; and

a coating layer disposed on the core,

wherein the core comprises the first composite oxide represented by Formula (1), and

wherein the coating layer comprises a Group 3 to Group 15 element.

8. The cathode active material of claim 7 , wherein the coating layer comprises B, Zr, Nb, Mo, W, or a combination thereof.

9. The cathode active material of claim 7 , wherein an amount of the coating layer is in a range of about 0.1 weight percent to about 10 weight percent, based on a total weight of the third composite oxide.

10. The cathode active material of claim 7 , wherein the coating layer comprises a compound represented by Formula (1B):

Li a M b O c   (1B)

wherein, in Formula (1B), M is a Group 3 to Group 15 element or a combination comprising at least one of a Group 3 to Group 15 element, and a, b, and c respectively satisfy 1≤a≤2, 0<b≤1, and 1<c≤4.

11. The cathode active material of claim 10 , wherein M comprises B, Zr, Nb, Mo, W, or a combination thereof.

12. The cathode active material of claim 10 , wherein the coating layer comprises LiNbO 3 , Li 2 MoO 4 , LiBO 2 , Li 2 WO 4 , Li 2 ZrO 3 , or a combination thereof.

13. The cathode active material of claim 1 , wherein an average particle diameter of the first composite oxide is in a range of about 0.1 micrometer to about 10 micrometers.

14. The cathode active material of claim 1 , wherein a discharge capacity of the first composite oxide is 270 milliamperes per gram or greater, when determined in a lithium battery on a second cycle.

15. An all-solid-state secondary battery comprising:

a cathode comprising the cathode active material of claim 1 ;

an anode; and

a solid electrolyte layer disposed between the cathode and the anode.

16. The all-solid-state secondary battery of claim 15 , wherein the solid electrolyte layer comprises a solid electrolyte,

wherein the solid electrolyte comprises S, Li, and Si, P, B, or a combination thereof.

17. A method of preparing the cathode active material of claim 1 , the method comprising:

contacting vanadium oxide and lithium phosphoric acid to prepare a mixture; and

milling the mixture to obtain a composite oxide to prepare the cathode active material.

18. The method of claim 17 , wherein the vanadium oxide and lithium phosphoric acid are contacted at a molar ratio in a range of greater than about 2:1 to about 10:1 or less.

19. The method of claim 17 , wherein the mixture further comprises a compound represented by Formula (1A):

Li a M b O c   (1A)

wherein, in Formula (1A), M is a Group 3 to Group 15 element or a combination comprising at least one of a Group 3 to Group 15 element, and a, b, and c respectively satisfy 1≤a≤2, 0<b≤1, and 1<c≤4.

20. The method of claim 17 , further comprising:

contacting a coating solution and the composite oxide, wherein the coating solution comprises a compound of Formula (1B):

Li a M b O c   (1B),

wherein, in Formula (1B), M is a Group 3 to Group 15 element or a combination comprising at least one of a Group 3 to Group 15 element, and a, b, and c respectively satisfy 1≤a≤2, 0<b≤1, and 1<c≤4, to obtain a mixed solution; and

drying and heat-treating the mixed solution under oxidative conditions to obtain a Li a M b O c —coated composite oxide.