Water is considered the universal solvent and can dissolve large concentrations of diverse ionic compounds.  Its high dielectric constant and low viscosity enable very high ionic conductivity for aqueous electrolytes (~1 S/cm), which is two orders of magnitude higher than those achieved with conventional organic electrolytes and particularly attractive for high power density batteries, since the cation transference numbers are also higher.  Additional advantages of water based electrolytes are lower cost and non-flammability. Their main bottleneck is the limited thermodynamic electrochemical window of water (1.23V), currently being challenged, through the use of highly concentrated electrolytes (“water in salt concept”).  Time is needed to assess whether the concept is efficient using high electrode loadings and if the stability of the Solid Electrolyte Interphase (SEI) formed is enough to grant operation at slow rates without significant corrosion.

Strong research efforts are currently focused on rechargeable aqueous M-ion batteries (M = Li, Na) mimicking the organic electrolyte Li-ion or Na-ion concepts, the main challenge being the development of new negative electrodes able to operate within the water redox stability window. The use of sodium and inexpensive abundant metal based electrode materials (based on Ti or Mn for instance) would add the sustainability perspective. Given the good power rate of the current aqueous M-ion technologies, they may be competitive versus asymmetric supercapacitors in terms of performances, especially if similar cost can be achieved. A more challenging topic is divalent ion concepts (M = Zn) using a Zn metal anode which could, in principle, deliver higher energy density, but for which issues still remain related to developing appropriate positive electrode materials for reversible Zn ion insertion and side reactions involving mostly H⁺ or OH⁻ species, which are not yet mastered.