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.