Toward a Reversible Calcium-Sulfur Battery with a Lithium-Ion Mediation Approach

Calcium represents a promising anode for the development of high-energy-density, low-cost batteries. However, a lack of suitable electrolytes has restricted the development of rechargeable batteries with a Ca anode. Furthermore, to achieve a high energy density system, sulfur would be an ideal cathode to couple with the Ca anode. Unfortunately, a reversible calcium-sulfur (Ca-S) battery has not yet been reported. Herein, a basic study of a reversible nonaqueous room-temperature Ca-S battery is presented. The reversibility of the Ca-S chemistry and high utilization of the sulfur cathode are enabled by employing a Li ⁺ -ion-mediated calcium-based electrolyte. Mechanistic insights pursued by spectroscopic, electrochemical, microscopic, and theoretical simulation (density functional theory) investigations imply that the Li ⁺ -ions in the Ca-electrolyte stimulate the reactivation of polysulfide/sulfide species. The coordination of lithium to sulfur reduces the formation of sturdy Ca-S ionic bonds, thus boosting the reversibility of the Ca-S chemistry. In addition, the presence of Li ⁺ -ions facilitates the ionic charge transfer both in the electrolyte and across the solid electrolyte interphase layer, consequently reducing the interfacial and bulk impedance of Ca-S batteries. As a result, both the utilization of active sulfur in the cathode and the discharge voltage of Ca-S batteries are significantly improved.