Salt Anion's Donor Number Strategy Achieving Stable NCM622 Cathode at 4.7 V

The donor number (DN) has emerged as an important descriptor for optimizing lithium metal battery (LMB) performance, especially in regulating solvation structures and constructing high-quality electrode/electrolyte interphases. However, high DN solvents can compromise the intrinsic high-voltage stability (>5 V) of conventional electrolytes due to their limited electrochemical stability. In this study, a novel strategy is presented that utilizes the anion's DN for non-destructive regulation of ionic liquids (IL) to achieve advanced electrolytes at 5.3 V. It is demonstrated that introducing high DN salt anions competes with EMIM⁺ in EMIM-TFSI, forming strong interactions with EMIM and enhancing the stability of the IL electrolyte. The expelled TFSI⁻ ions tend to coordinate with Li⁺, facilitating the formation of high-quality solid/cathode electrolyte interphases. Consequently, the Li//NCM622 cells with high DN salt anions (LiClO₄-IL and LiOTF-IL) show remarkable capacity retention rates of 93.5% and 94.6%, respectively, after 100 cycles over a voltage range of 2.8–4.7 V. Moreover, the Li//NCM622 cells using LiClO₄-IL maintain a capacity retention of 81.6% and an average Coulombic efficiency of 99.4% after 350 cycles at 2.8–4.6 V. The proposed DN tuning mechanism is believed to offers new insights for designing high-energy-density LMBs.