Tuning the Nucleophilicity of Anion in Lithium Salt to Enable an Anion‐Rich Solvation Sheath for Stable Lithium Metal Batteries

Abstract Traditional lithium salts typically adhere to the designing principles of enhancing cation‐anion dissociation degree to obtain a high electrolyte conductivity. This promotes the invention of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), where the symmetric electron‐withdrawing trifluoromethanesulfonyl groups significantly delocalize the negative charge density around the nitrogen atom, thereby weakening the electrostatic interaction between Li⁺ and the anion. Herein, deviating from the general principle, lithium (methanesulfonyl)(trifluoromethanesulfonyl) imide (LiMTFSI) is deliberately designed by substituting a unilateral electron‐withdrawing trifluoromethyl (─CF₃) group of LiTFSI with an electron‐donating methyl (─CH₃) group, to tune the nucleophilicity of the anion. This modification enhances Li‐anion interaction, causing the anion to replace the solvent molecules in the Li⁺ solvation shell. Additionally, the MTFSI⁻ anion exhibits an elevated donor number to facilitate the solubility of LiNO₃ in carbonate‐based electrolytes. The synergistic effect of these changes suppresses the decomposition of solvent and helps construct a stable solid electrolyte interphase (SEI) enriched with multiple inorganic lithium salts (e.g., Li₂S, Li₃N, and LiN_xO_y) on the Li metal anode, which enables the 500 mAh Li||LiNi_0.5Co_0.2Mn_0.3O₂ pouch cell to operate steadily for 150 cycles. It is believed this work would provide new insights and another dimension for designing functional anions beyond their role as charge carriers.