Calcium-Ion Intercalation Chemistry of the Hewettite for Rechargeable Calcium-Ion Batteries

Abstract

Calcium-based battery is a very attractive alternative to Li-ion batteries due to theabundance of calcium. The hewettite (CaV6O16·xH2O) has recevied an attention due to its typical two-dimensional layered structure consisting of V3O8 layers and the hydrated calcium ions occupied between those layers (Fig1.a). The hewettite was known to be synthesized by sol-gel, hydrothermal or ion-exchange methods with a high yield, but taking a long reaction time (several days) to synthesize. In this work, we have developed a quick synthetic method to prepare CaV6O16·xH2O from Na1.2V3O8, utilizing a relatively simple electrochemical reaction. The electrodes of Na1.2V3O8 was first prepared, consisting of the active material (Na1.2V3O8), conducting carbon (super P) and binder (PVdF) with the weight ratio of 8:1:1, coated on stainless foil. Activated carbon (AC) was served as the counter and reference electrodes. The electrode was oxidized electrochemically by holding the voltage at 0.7V vs. AC for 24hr in 1M Ca(NO3)2 aqueous electrolyte, which resulted in formation of the hewettite (CaV6O16·xH2O) that was confirmed by powder XRD and elemental analyses (ICP, EDAX). The electrochemical properties of the obtained CaV6O16·xH2O electrodes were characterized with potentiostatic cyclic voltammogram (CV) (Fig1.b), galvanostatic discharge/charge cycles, powder XRD and elemental analyses. Reversible redox reactions were observed in CVs as well as the discharge/charge curves, which were responsible for the calcium-ion intercalation and evidenced by EQCM, elemental and structural analyses. The bond valence sum mapping (Fig1.c) showed the calcium-ion pathways in the crystal structure. Figure 1