ADDITIVE FOR SODIUM ION SECONDARY BATTERIES, AND SODIUM ION SECONDARY BATTERY

An additive for a sodium ion secondary battery of the present invention includes a compound of at least one of a saturated cyclic carbonate having a fluoro group and a chain carbonate having a fluoro group. A sodium ion secondary battery (1) of the present invention includes: a non-aqueous electrolytic solution including the additive for a sodium ion secondary battery and a non-aqueous solvent containing a saturated cyclic carbonate or a non-aqueous solvent containing a saturated cyclic carbonate and a chain carbonate; a positive electrode (11); and a negative electrode (12) that includes a coating formed in a surface of the negative electrode, the coating containing a composite material having carbon, oxygen, fluorine and sodium in the surface and includes a negative-electrode active material containing a hard carbon.