SODIUM ION SECONDARY BATTERY

Provided is a sodium-ion secondary battery having a high capacity and excellent safety. A sodium-ion secondary battery includes: a positive electrode containing a positive-electrode active material made of a crystallized glass containing crystals represented by a general formula NaMPO(where 1≤x≤2.8, 0.95≤y≤1.6, 6.5≤z≤8, and M is at least one selected from among Fe, Ni, Co, Mn, and Cr); a negative electrode containing a negative-electrode active material made of hard carbon; and a non-aqueous electrolyte.