Synergistic Cosolvent/Additive Strategies Enable Stable Cycling of Eutectic Electrolytes in Lithium Metal Batteries over a Broad Temperature Range

Amide-based deep eutectic electrolytes derived from deep eutectic solvents (DESs) are promising for lithium metal batteries (LMBs), yet they are hindered by high viscosity, low conductivity, and poor Li compatibility. We report an innovative dual-modifier strategy utilizing γ-valerolactone (GVL) as a cosolvent and fluoroethylene carbonate (FEC) as a film-forming additive, within an amide-based DES. Spectroscopic and computational analyses reveal that the concerted incorporation of GVL and FEC reconfigures the first Li⁺solvation shell, enriching anions therein to orchestrate the formation of an inorganic-rich solid electrolyte interphase while simultaneously diminishing amide content and suppressing its detrimental decomposition. The optimized electrolyte exhibits high ionic conductivity (1.77 mS cm^–1at 25 °C), a Li⁺transference number of 0.55, robust interfacial stability, and nonflammability, synergistically enabling the enhanced battery performance. The Li||LiFePO₄cell utilizing such an electrolyte exhibits a capacity retention of 82.4% after 400 cycles at 1C and 30 °C, outperforming the baseline DES (3.9%). These cells also exhibit superior performance across extended temperatures (0 and 50 °C). In conclusion, this dual-modifier strategy enhances DES electrolyte performance in LMBs by concurrently regulating Li⁺solvation chemistry and stabilizing interfaces.