Soft Alkali Metal Nitrate Additive for Lean Electrolyte Li‐S Batteries

To develop high‐energy‐dense lithium–sulfur batteries (LSBs), improving cycle performance by suppressing electrolyte depletion and stabilizing the anode–electrolyte interphase under lean‐electrolyte conditions is essential. Lithium nitrate (LiNO 3 ) is regarded as an effective additive that reduces electrolyte decomposition and stabilizes the interphases of the cathode and anode in LSBs, however, it also increases cell overvoltage, limiting rate performance. Herein, by exploring various metal nitrates, we found that cesium nitrate (CsNO 3 ) improves both rate and cycle performance in lean electrolyte LSBs. Cs + , a soft acid, stabilizes polysulfides (a soft base) in the form of solid precipitate, while NO 3 − suppress electrolyte decomposition by forming a sulfate‐based solid electrolyte interphase. As a result, the CsNO 3 additive enables stable LSB operation over 300 cycles at an electrolyte/sulfur (E/S) ratio of 5 µL mg s −1 and a current rate of 0.3C due to the suppressed electrolyte depletion. A pouch‐type LSB with CsNO 3 delivered an energy density of 408 Wh kg − 1 at a challenging E/S ratio of 2.0 µL mg s − 1 . Our work inspires the development of additives with dual functionalities—polysulfide stabilization and interphase stabilization to enable high‐energy‐density LSBs with extended lifespans.