(a) Schematic representation of the working principle behind a complete cycle of the mixing entropy battery, showing how energy extraction can be accomplished: step 1, charge in river water; step 2, exchange to seawater; step 3, discharge in seawater; step 4, exchange to river water. (b) Typical form of a cycle of battery cell voltage (ΔE) vs charge (q) in a mixing entropy battery, demonstrating the extractable energy.