Positive active material for lithium-ion secondary battery, positive electrode for lithium-ion secondary battery, and lithium-ion secondary battery

1. A positive active material for a lithium-ion secondary battery, the positive active material comprising:

a lithium composite oxide particle represented by the following general formula (I):

Li x (Mn 2−(a+b+c+d) Ni a Ti b Fe c M d )(O 4−y F y )  (I)

wherein

0.4<a<0.5,

0.01≤b<0.2,

0.01≤c<0.1,

0≤d<0.2,

0.9<x<1.3,

0.05≤y≤0.2, and

when 0<d is satisfied, M is at least one element selected from the group consisting of aluminum, magnesium, calcium, barium, strontium, scandium, vanadium, chromium, cobalt, copper, zinc, gallium, yttrium, ruthenium, rhodium, lead, indium, tin, antimony, lanthanum, cerium, samarium, zirconium, niobium, tantalum, molybdenum, and tungsten,

the lithium composite oxide particle contains a spinel crystal phase as a first oxide phase and a layered crystal phase as a second oxide phase,

the lithium composite oxide particle includes a particle center portion and a surface layer portion that is closer to a surface of the lithium composite oxide particle than the particle center portion is, and

a fluorine atom concentration Fc (at %) of the particle center portion measured by energy dispersive X-ray spectroscopy is lower than a fluorine atom concentration Fs (at %) of the surface layer portion.

2. The positive active material according to claim 1 , wherein

a portion of oxygen atoms in the lithium composite oxide particle is substituted with the fluorine atoms.

3. The positive active material according to claim 2 , wherein

the fluorine atoms are present in the particle center portion in a proportion of 10 at % or lower.

4. The positive active material according to claim 1 , wherein

the Fc is 0 at % or higher and 10 at % or lower.

5. The positive active material according to claim 1 , wherein

the Fs is 30 at % or higher.

6. The positive active material according to claim 1 , wherein

a difference obtained by subtracting the Fc from the Fs is 20 at % or greater.

7. The positive active material according to claim 1 , wherein

at least one of a portion of the nickel atoms or a portion of the manganese atoms is substituted with iron atoms, and

at least one of a portion of the nickel atoms or a portion of the manganese atoms is substituted with titanium atoms.

8. The positive active material according to claim 1 , wherein

the particle center portion is a region extending, toward a center of the lithium composite oxide particle, from a position of 100 nm or greater inward from the surface of the lithium composite oxide particle.

9. The positive active material according to claim 8 , wherein

the surface layer portion is a region of 20 nm or smaller from the surface toward the center.

10. The positive active material according to claim 1 , wherein

the particle center portion is a portion of the lithium composite oxide particle containing no fluorine atoms and extending from a center of the lithium composite oxide particle toward the surface of the lithium composite oxide particle, and

the surface layer portion is a portion of the lithium composite oxide particle containing the fluorine atoms and extending from the surface toward the center.

11. A positive electrode for a lithium-ion secondary battery, the positive electrode comprising

the positive active material according to claim 1 .

12. A lithium-ion secondary battery comprising

the positive electrode according to claim 11 .