Synergic Coordination Effect in Nonflammable Deep Eutectic Electrolyte for High-Performance Sodium-Ion Batteries

Deep eutectic electrolytes (DEEs) are emerging as a highly promising class of next-generation electrolytes, poised to revolutionize the performance of safe sodium-ion batteries (SIBs). Nevertheless, a significant challenge hindering the progress of SIBs is the scarcity of suitable DEEs capable of harmonizing electrolyte conductivity, interfacial compatibility, and safety. In this study, we have engineered a DEE leveraging the synergistic interplay between Na-bonds and hydrogen bonds, achieved through the integration of N-methylacetamide (NMA) and sodium-difluoro(oxalato)borate (NaDFOB). The findings reveal that the distinctive interaction between the polar C═O group in NMA and Na⁺ ions, coupled with hydrogen bonding with DFOB^- anions, facilitates the formation of the DEE. The designed DEE demonstrates exceptional ionic conductivity (4.03 mS cm^-1 at 25 °C), a high oxidation voltage (4.63 V versus Na⁺/Na), and nonflammability. The unique solvation structure markedly enhances the compatibility of the electrode-electrolyte interface, with the NVP||Na cell exhibiting outstanding cycling performance (86.8% retention after 7000 cycles at 5 C) and superior rate capability. Moreover, the DEEs exhibit robust performance at elevated temperatures (94.3% retention after 120 cycles with NVP||Na cell), and the full cell demonstrates significant enhancements in both cycle stability (90.7% retention after 300 cycles) and rate performance. The incorporation of amide-based eutectic electrolytes into SIBs infuses fresh vitality into electrolyte design, propelling the advancement of high-performance SIBs.