Positive electrode active material for all-solid-state lithium secondary batteries
A positive electrode active material for an all-solid-state lithium secondary battery, which is capable of improving the rate characteristics, the cycle characteristics, and the initial charge and discharge efficiency even in the case where a sulfide-based solid electrolyte is used, wherein the surface of a lithium-containing composite oxide, referred to as present core particles, is coated with a compound, referred to as, LiAO compound, containing Li, A, where A represents one or more elements selected from the group consisting of Ti, Zr, Ta, Nb, Zn, W, and Al, and O; and a halogen is present on the surface of the present core particles.
1. A positive electrode active material for an all-solid-state lithium secondary battery comprising core particles comprising a lithium-containing composite oxide, wherein the core particles are coated with a coating compound containing Li; A, where A represents one or two or more elements selected from the group consisting of Ti, Zr, Ta, Nb, Zn, W, and Al; and O,
a halogen is present on the surface of the core particles, and
the halogen is solid-dissolved in the coating compound.
2. The positive electrode active material according to claim 1 ,
wherein a ratio, Ha − /total, of count numbers, Ha − , of secondary ion species for halogen ions with respect to the sum, total, of all count numbers of secondary ions collected from the surface of the core particles, as measured under the condition of negative secondary ion polarity using time-of-flight secondary ion mass spectroscopy (TOF-SIMS), is 0.00075 or more.
3. The positive electrode active material according to claim 1 ,
wherein a mass ratio, Ha/B, of a halogen amount, Ha in ppm, measured by ion chromatography after thermally decomposing the positive electrode active material with respect to an A element amount, B in wt %, measured by inductively coupled plasma (ICP) emission spectroscopy after completely dissolving the positive electrode active material is 0.5×10 −4 or more.
4. The positive electrode active material according to claim 1 ,
wherein a molar ratio, Ha/C×100, of the halogen, Ha, amount with respect to an A element amount, C, present on the surface of the core particles, as measured by X-ray photoelectron spectroscopy (XPS), is 1.1 or more.
5. The positive electrode active material according to claim 1 , wherein the halogen is fluorine (F).
6. The positive electrode active material according to claim 1 ,
wherein a molar ratio, Li/D, of a Li element amount with respect to the A element amount, D, present on the surface of the present core particles, as measured by X-ray photoelectron spectroscopy (XPS), is 1.5 to 33.3.
7. The positive electrode active material according to claim 1 , wherein the lithium-containing composite oxide is a lithium-containing composite oxide having a layered rock-salt structure, which is represented by a general formula: LiMO 2 , where M represents a metal element.
8. The positive electrode active material according to claim 1 , wherein the coating compound is an amorphous compound.
9. An all-solid-state lithium secondary battery, comprising the positive electrode active material according to claim 1 , a solid electrolyte, and a negative electrode active material.
10. The all-solid-state lithium secondary battery according to claim 9 , wherein the solid electrolyte is a sulfide-based solid electrolyte containing sulfur (S).
11. The positive electrode active material according to claim 2 ,
wherein a mass ratio, Ha/E, of a halogen amount, Ha in ppm measured by ion chromatography after thermally decomposing the positive electrode active material with respect to an A element amount, E in wt %, measured by inductively coupled plasma (ICP) emission spectroscopy after completely dissolving the positive electrode active material is 0.5×10 −4 or more.
12. The positive electrode active material according to claim 2 ,
wherein a molar ratio, Ha/C×100, of the halogen, Ha, amount with respect to an A element amount, C, present on the surface of the core particles, as measured by X-ray photoelectron spectroscopy (XPS), is 1.1 or more.
13. The positive electrode active material according to claim 3 ,
wherein a molar ratio, Ha/C×100, of the halogen, Ha, amount with respect to an A element amount, C, present on the surface of the core particles, as measured by X-ray photoelectron spectroscopy (XPS), is 1.1 or more.
14. The positive electrode active material for an all-solid-state lithium secondary battery according to claim 1 , the coating compound is represented by Li x AO y where x satisfies 1<x and y ranges according to the valences of the elements.