Plating and stripping calcium in an organic electrolyte

There is considerable interest in multivalent cation batteries, such as those based on Mg, Ca or Al1-11. Most attention has focused on Mg. In all cases the metal anode represents a significant challenge. Recent work has shown that calcium can be plated and stripped, but only at elevated temperatures, 75 to 100 °C, with small capacities, typically 0.165 mAh cm-2 , and accompanied by significant side reactions7. Here we demonstrate that calcium can be plated and stripped at room temperature with capacities of 1 mAh cm-2 at a rate of 1 mA cm-2, with low polarization (~ 100 mV) and in excess of 50 cycles. The dominant product is Ca, accompanied by a small amount of CaH2 thatforms by reaction between the deposited Ca and the electrolyte, Ca(BH4)2 in tetrahydrofuran (THF). This occurs in preference to the reactions which take place in most electrolyte solutions forming CaCO3, Ca(OH)2 and calcium alkoxides and normally terminate the electrochemistry. The CaH2 protects the Ca metal at open circuit. While this work does not solve all the problems of calcium as an anode in calcium-ion batteries, it does demonstrate that significant quantities of calcium can be plated and stripped at room temperature with low polarization