An Efficient Sodium Borate-Based Electrolyte Additive to Activate Sulfur Conversion and Stabilize Calcium Anodes in Calcium−Sulfur Batteries

Abstract: Calcium−sulfur (Ca−S) batteries are considered promising candidates for large-scale energy storage owing to their high theoretical energy density, the natural abundance of both calcium and sulfur, and their inherent cost-effectiveness and sustainability. A room-temperature, reversible Ca–S battery has previously been demonstrated as a proof of concept using the calcium tetrakis(hexafluoroisopropyloxy)borate (Ca[B(hfip)4]2) electrolyte; however, it still suffers from limited cycling stability and severe Ca anode passivation. Herein, we report the improvement of the Ca−S battery performance using a sodium borate-modified electrolyte. We demonstrate that the introduction of Na+ can enhance the reversibility of sulfur redox chemistry and the Ca plating/stripping at the anode simultaneously. The Ca−S coin cells with a S/Ketjenblack/Polyaniline cathode and Ca metal anode deliver a high initial discharge capacity of 867 mAh g−1 at 0.1C (1C = 1672 mA g−1) and maintain a stable discharge voltage of around 2.2 V and a capacity of 96 mAh g−1 after 80 cycles. The modified electrolyte also enables stable operation of a Ca−S pouch cell for 50 cycles with a discharge capacity of 80 mAh g−1 at 0.1C. This work presents an effective electrolyte modification approach for advancing Ca batteries.