Regulating Coordination Chemistry in Deep Eutectic Electrolytes via Electron-Withdrawing Effect Within Hydrogen Bond...

The deployment of high-voltage cathodes for lithium metal batteries (LMBs) imposes stricter requirements on the electrolyte stability. Deep eutectic electrolyte (DEE) is one of the most promising candidates. In DEEs, the strong interaction between the hydrogen bond donor (HBD) and Li⁺ causes sluggish desolvation, and HBD-anion coordination structure induces severe oxidation decomposition. In this work, electron-withdrawing groups were introduced onto benzonitrile to synergistically tune multiple coordination interactions. Leveraging the stronger electron-withdrawing and steric hindrance effects of trifluoromethyl (-CF₃) group, the optimized HBD, 4-Fluoro-2-(trifluoromethyl)benzonitrile (FTFBN), exhibits weaker coordination ability with Li⁺ and a low HBD-anion coordination number. It constructs an anion-dominated solvation structure and inorganic-rich cathode-electrolyte interphase (CEI) on lithium cobalt oxide (LCO), effectively suppressesing irreversible phase transitions and interfacial side reactions. As a result, the FTFBN-based DEE demonstrates a high Li⁺ transference number (t_Li ⁺) of 0.74 and stable symmetric cell cycling for over 900 h (0.5 mA cm^-2 and 0.5 mAh cm^-2). Moreover, it supports long-term cycling stability in 4.5 V Li‖LCO cells, retaining 84% capacity after 400 cycles. This work highlights the vital role of HBD molecular engineering in optimizing the solvation structure and interfacial chemistry for LMBs.