Rechargeable Calcium-Sulfur Batteries Enabled by an Efficient Borate-Based Electrolyte

Rechargeable metal-sulfur batteries show great promise for energy storage applications because of their potentially high energy and low cost. The multivalent-metal based electrochemical system exhibits the particular advantage of the feasibility of dendrite-free metal anode. Calcium (Ca) represents a promising anode material owing to the low reductive potential, high capacity, and abundant natural resources. However, calcium-sulfur (Ca-S) battery technology is in an early R&D stage, facing the fundamental challenge to develop a suitable electrolyte enabling reversible electrochemical Ca deposition, and at the same time, sulfur redox reactions in the system. Herein, a study of a room-temperature Ca-S battery by employing a stable and efficient calcium tetrakis(hexafluoroisopropyloxy) borate Ca[B(hfip)₄ ]₂ electrolyte is presented. The Ca-S batteries exhibit a cell voltage of ≈2.1 V (close to its thermodynamic value) and good reversibility. The mechanistic studies hint at a redox chemistry of sulfur with polysulfide/sulfide species involved in the Ca-based system.