SODIUM ION SECONDARY BATTERY

摘要

PROBLEM TO BE SOLVED: To provide a sodium ion secondary battery excellent in overcharge resistance.SOLUTION: A sodium ion secondary battery includes a positive electrode including a positive electrode active material, a negative electrode, a separator interposed between the positive and negative electrodes, and an electrolyte including sodium ions. The positive electrode active material includes a sodium-containing transition metal oxide reversely absorbing and desorbing sodium ions. In the sodium-containing transition metal oxide, a ratio: Na/Mof sodium atoms to transition metal atoms satisfies the relation of Na/M≤0.3 in a fully charged state in a reversible charging-discharging. A ratio: C/Cof the sum Cof reversible capacity and irreversible capacity of the negative electrode to the sum Cof reversible capacity and irreversible capacity of the positive electrode satisfies the relation of 1≤C/C.