Unraveling the Dual Functionality of High‐Donor‐Number Anion in Lean‐Electrolyte Lithium‐Sulfur Batteries

Minimizing electrolyte use is essential to achieve high practical energy density of lithium–sulfur (Li–S) batteries. However, the sulfur cathode is more readily passivated under a lean electrolyte condition, resulting in low sulfur utilization. In addition, continuous electrolyte decomposition on the Li metal anode aggravates the problem, provoking rapid capacity decay. In this work, the dual functionalities of NO3− as a high‐donor‐number (DN) salt anion is presented, which improves the sulfur utilization and cycling stability of lean‐electrolyte Li–S batteries. The NO3− anion elevates the solubility of the sulfur species based on its high electron donating ability, achieving a high sulfur utilization of above 1200 mA h g−1. Furthermore, the anion suppresses electrolyte decomposition on the Li metal by regulating the lithium ion (Li+) solvation sheath, enhancing the cycle performance of the lean electrolyte cell. By understanding the anionic effects, this work demonstrates the potential of the high‐DN electrolyte, which is beneficial for both the cathode and anode of Li–S batteries. The dual functionalities of nitrate (NO3−) anions, beneficial for both electrodes, are eludicated in a lithium–sulfur (Li–S) battery system. The strong solvating power of the high‐donor‐number NO3− anion enables 3D lithium sulfide growth on the cathode and suppresses solvent decomposition on the anode. The addition of LiNO3 enables high sulfur utilization and long cycling stability of lean‐electrolyte Li–S batteries.