METHOD FOR PRODUCING COMPOSITE PARTICLE, POSITIVE ELECTRODE, AND ALL-SOLID-STATE BATTERY, AND COMPOSITE PARTICLE, POSITIVE ELECTRODE, AND ALL-SOLID-STATE BATTERY

A composite particle includes a positive electrode active material particle and a coating film. In a method for producing the composite particle, the coating film covers at least a part of a surface of the positive electrode active material particle, the coating film contains a phosphorus compound, and a glass-transition temperature of the coating film is 300° C. or less.