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

A method for preparing a positive electrode slurry, the method comprising:

• producing a composite particle, the composite particle comprising a positive electrode active material particle and a coating film, wherein producing the composite particle comprises:
• (a) preparing a mixture by mixing a coating solution and the positive electrode active material particle;
• and
• (b) producing the composite particle by drying the mixture by a spray drying method, wherein the coating solution contains a solute and a solvent, wherein the solute comprises a phosphorus compound comprising metaphosphoric acid,
• wherein:
• the coating film covers at least a part of a surface of the positive electrode active material particle;
• a glass-transition temperature of the coating film is 300° C. or less;
• and
• dispersing the composite particle and a sulfide solid electrolyte in a dispersion medium.