Long-Cycle-Life Calcium Battery with a High-Capacity Conversion Cathode Enabled by a Ca/Li Hybrid Electrolyte.

Calcium (Ca) batteries represent an attractive option for electrochemical energy storage due to physicochemical and economic reasons. The standard reduction potential of Ca (-2.87 V) is close to Li and promises a wide voltage window for Ca full batteries, while the high abundance of Ca in the earth's crust implicates low material costs. However, the development of Ca batteries is currently hindered by technical issues such as the lack of compatible electrolytes for reversible Ca plating/stripping and high-capacity cathodes with fast kinetics. Herein, we employed FeS as a conversion cathode material and combined it with a Li/Ca hybrid electrolyte for Ca batteries. We demonstrate that Li ions ensured reversible Ca plating/stripping on the Ca metal anode with a small overpotential. At the same time, they enable the conversion of FeS, offering high discharge capacity. As a result, the Ca/FeS cell demonstrated an excellent long-term cycling performance with a high discharge capacity of 303 mAh g over 200 cycles. Even though the practical application of such an approach is questionable due to the high quantity of electrolytes, we believe that our scientific findings still provide new directions for studying Ca batteries with long-term cycling.