Application of Potassium Pyrophosphate Aqueous Electrolytes to Nickel Metal Hydride Batteries

A highly concentrated potassium pyrophosphate (K4P2O7) aqueous solution exhibits not only an increase in the potential window similar to conventional water-in-salt electrolytes (WISEs) but also unique properties such as high stability at low temperatures and superionic conductivity. For exploring active materials for the highly concentrated K4P2O7 aqueous electrolyte, we first conducted discharge–charge evaluations using γ-type KxNiO2·nH2O, which has a large interlayer distance that could potentially allow the migration of potassium ions (K+). However, our study reveals that the discharge–charge reaction in the electrode material could be progressed by the insertion and extraction of protons as charge-compensating ions rather than K+, even though the electrolyte is near-neutral (weakly basic). Based on the result, our study also reveals that layered nickel hydroxide materials such as Ni(OH)2 and metal hydride (MH) of hydrogen storage alloys, which are the electrode materials of a nickel–metal hydride (NiMH) battery, are active in the K4P2O7 aqueous electrolyte. In contrast to hydroxide-based alkaline solutions, this is the first NiMH cell that functions in near-neutral electrolytes (pH 11), which is advantageous in terms of material corrosion and safety. Although protons act as charge-compensating ions, it seems that K+ also acts as a carrier ion supporting proton transfer in the near-neutral K4P2O7 aqueous electrolyte owing to its high transference number, especially at high currents. Our findings have the potential to pave the way for developing electrolytes not only for WISEs but also for proton batteries.