Modulating the hydration number of calcium ions by varying the electrolyte concentration: Electrochemical performance in a Prussian blue electrode/aqueous electrolyte system for calcium-ion batteries

We report on the effects of the changed hydration number of calcium ions due to the variation in the electrolyte concentration in a Prussian blue analogue electrode, CuHCF, and aqueous electrolyte system on its performance, for calcium-ion batteries (CIBs). As determined by Raman spectroscopy, the coordination number of the calcium ions, that is, the number of nitrate anions surrounding it in the aqueous electrolyte gradually increased upon increasing the electrolyte concentration from 1.0 to 8.4 mol dm−3. In other words, the hydration number of calcium ions gradually decreased. Additionally, the activation energy, Ea, for the interfacial charge transfer during the process of calcium ion insertion into the CuHCF electrode was estimated from the temperature dependence of the charge transfer resistance in these aqueous electrolytes via electrochemical impedance spectroscopy. The Ea value gradually decreased with increasing electrolyte concentration. These results are directly related to the electrochemical performance such as the discharge/charge capacities, rate capability, and cyclability in this system for CIBs.