Composite particle, positive electrode, all-solid-state battery, and method for producing composite particle

A composite particle comprising a positive electrode active material particle and a coating film, wherein:

• the positive electrode active material particle contains a lithium-containing composite oxide having a layered rock salt structure;
• the coating film covers at least a part of a surface of the positive electrode active material particle;
• the coating film contains a phosphate compound;
• the composite particle comprises a coverage rate of 80% or more, wherein the coverage rate is measured by X-ray photoelectron spectroscopy;
• the composite particle satisfies a relationship of the following formula (1):
• XP/T≥0.04 (1), where in the above formula (1),
• XP indicates a mass fraction of phosphorus contained in the composite particle measured by inductively coupled plasma atomic emission spectroscopy, and
• T indicates a film thickness of the coating film measured by a scanning electron microscope;
• and
• the composite particle satisfies a relationship of the following formula (2):
• CLi/CP≤2.5 (2), where in the above formula (2),
• CLi indicates elemental concentration of lithium obtained from a peak area of a Li1s spectrum measured by X-ray photoelectron spectroscopy, and
• CP indicates elemental concentration of phosphorus obtained from a peak area of a P2p spectrum measured by the X-ray photoelectron spectroscopy.