Secondary battery

A secondary battery comprising:

• a positive electrode active material,
• wherein the positive electrode active material comprises magnesium and lithium cobalt oxide,
• wherein magnesium has a concentration gradient in a surface portion of the positive electrode active material, and
• wherein in an EDX line analysis result of magnesium contained in the positive electrode active material, when a peak width at a height which is ⅕ of a height of a maximum value of a detected amount of magnesium is divided into two parts by a perpendicular extending from the maximum value to a horizontal axis, a peak width MgWcore on an inner portion side is larger than a peak width MgWshell on a surface side.

A secondary battery comprising:

• a positive electrode active material,
• wherein the positive electrode active material comprises an additive element whose oxide can be a semiconductor, magnesium, and lithium cobalt oxide,
• wherein the positive electrode active material comprises a crack portion,
• wherein the positive electrode active material comprises a surface portion and an inner wall surface portion,
• wherein the surface portion is a region within 10 nm from a surface other than the crack portion in a direction substantially perpendicular to the surface,
• wherein the inner wall surface portion is a region within 10 nm from a surface of the crack portion in a direction substantially perpendicular to the surface and is a region other than the surface portion,
• wherein magnesium is detected from the surface portion and the crack portion,
• wherein the additive element whose oxide can be a semiconductor is detected from the surface portion and is one or two selected from nickel and titanium,
• wherein magnesium has a concentration gradient in the surface portion of the positive electrode active material, and
• wherein in an EDX line analysis result of magnesium contained in the positive electrode active material, when a peak width at a height which is ⅕ of a height of a maximum value of a detected amount of magnesium is divided into two parts by a perpendicular extending from the maximum value to a horizontal axis, a peak width MgWcore on an inner portion side is larger than a peak width MgWshell on a surface side.