Molten salt battery, charging/discharging method and charging/discharging system

A molten salt battery which comprises: a positive electrode containing a positive electrode active material that reversibly absorbs and desorbs sodium; a negative electrode containing a negative electrode active material that reversibly absorbs and desorbs sodium; a separator that is interposed between the positive electrode and the negative electrode; and a molten salt electrolyte. The molten salt electrolyte contains 90% by mass or more of an ionic liquid, and the ionic liquid contains a first salt and a second salt. The first salt comprises sodium ions and first anions, and the second salt comprises organic cations and second anions. The positive electrode active material contains a composite oxide that has a layered O3-type crystal structure containing Na, Fe and Co. The amount of Co relative to the total amount of Fe and Co contained in the composite oxide is 40-60% by atom.