An electrochemical cell and its method of operation includes an electrolyte having a binary salt system of an alkali hydroxide and a second alkali salt. The anode, cathode, and electrolyte may be in the molten phase. The cell is operational for both storing electrical energy and as a source of electrical energy as part of an uninterruptible power system. The cell is particularly suited to store electrical energy produced by a renewable energy source.

An electrochemical cell and its method of operation includes an electrolyte having a binary salt system of an alkali hydroxide and a second alkali salt. The anode, cathode, and electrolyte may be in the molten phase. The cell is operational for both storing electrical energy and as a source of electrical energy as part of an uninterruptible power system. The cell is particularly suited to store electrical energy produced by a renewable energy source.

The aqueous secondary battery of the present disclosure comprises: a positive electrode active material layer, an aqueous electrolyte, and a negative electrode active material layer, wherein the aqueous electrolyte contains an aqueous solvent and potassium polyphosphate which has two or more elemental phosphorus, and any one of potassium ion, proton, hydroxide ion, and polyphosphoric anion is a carrier ion, a positive electrode active material contained in the positive electrode active material layer has a monoclinic crystal structure represented by space group C2/m, is represented by composition formula MnO.sub.2.Math.A.sub.xB.sub.y(H.sub.2O).sub.z, and the A, B, and H.sub.2O are present in a space interposed between layers composed of MnO octahedrons, A is an alkali metal or alkali rare earth metal, B is an anion, 0<x<1, 0<y<1, and 0<z<2.