Molecular fluorination towards deep eutectic amide-based electrolyte for stable high voltage lithium–metal batteries

Abstract

Currently, non-flammable deep eutectic electrolytes (DEEs), typically based on N-methylacetamide (NMAC), have been deemed as high-quality electrolytes employed in lithium-metal batteries (LMBs). However, the unstable interphase chemistry derived from high reactivity of amide groups towards aggressive electrodes (Li and NCM cathode) and tight Li⁺-amide coordination still exists as the unavoidable “sore point” for DEEs innovation as yet. Herein, inspired by fluorinated solvent strategy, N-Methyl-2,2,2-trifluoroacetamide (FNMAC), is proposed to design the FNMAC-based DEE (F-DEE-1:n, n = 2 ∼ 8) solely containing lithium bis(trifluoromethanesulphonyl)imide (LiTFSI) salt. Introducing electron-withdrawing –CF₃ group is conducive to realizing excellent oxidation resistance as well as stable interphase chemistry, which impairs Li⁺-amide strong coordination bringing forth anion-rich solvation sheath and robust solid electrolyte interface (SEI) with high inorganic content, together with promoting the fast desolvation of Li⁺. Consequently, the F-DEE-1:4 endows NCM622||Li cells with excellent rate capability and outstanding long lifespan along with high capacity retention of ∼91.3 % after cycling 420 times, much superior to those using NMAC-based DEE (N-DEE-1:4). This work is instructive for high-quality DEEs innovation and emphasizes the close correlation between Li⁺ coordination environment and stable interphase chemistry within LMBs.