Practical Aqueous Calcium-Ion Battery Full-Cells for Future Stationary Storage

There is a pressing need for high-rate cycling and cost-effective stationary energy storage systems in concomitance with the fast development of solar, wind, and other types of renewable sources of energy. Aqueous rechargeable Ca-ion batteries have the potential to meet the growing demands of stationary energy storage devices because they are abundant and safe; they can also be manufactured at a low-cost and have a higher volumetric capacity. In this study, we have demonstrated a low-cost, safe, aqueous Ca-ion battery that is based on a low potential, lower specific weight, in situ polymerized polyaniline as an anode, and a high redox-potential open-framework structured potassium copper hexacyanoferrate as a cathode. The charge-discharge mechanism of this battery includes doping/dedoping of NO₃^- at the anode, and intercalation and deintercalation of Ca-ion at the cathode. This Ca-ion battery works successfully in a 2.5 M Ca(NO₃)₂ aqueous electrolyte that exhibits 70 Wh kg^-1 specific energy at 250 W kg^-1 and even maintains a high energy density of 53 Wh kg^-1 at a higher rate of 950 W kg^-1; this indicates a good rate capability (calculation based on anode active mass). At 0.8 A g^-1, the battery provides an average specific capacity of 130 mA h g^-1, exhibiting high Coulombic efficiency (∼96%), with 95% capacity retention of over 200 cycles across its life span, which is a new achievement in the electrochemical performance of aqueous Ca-ion batteries. Furthermore, the calcium-ion storage mechanism is investigated using high-end X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. Thus, this significant electrochemical performance of the anode and the cathode renders the battery a promising candidate in grid-scale storage applications.