Monofluoro-ether electrolyte design with reduced Li+-anion coordination enables fast-charging ultrahigh-voltage lithium metal batteries

Ether-based electrolytes are particularly useful for lithium metal batteries (LMBs) and have shown improved anodic stability with the high concentration design. Nevertheless, the vaunted anion-reinforced solvation in concentrated electrolytes has non-negligible adverse effect on ion conduction and battery rate performance. Here, we propose a new solvent design strategy of ether monofluorination to tune the Li+-anion interaction for fast-charging LMBs. With the highly polar monofluoro functional groups, the ether-based electrolyte not only exhibits excellent high oxidation stability due to the strong electron-withdrawing effect of F, but also has a greatly increased ionic conductivity because of reduced anion coordination. The unique solvation structure also enables the formation of robust and highly conductive interphases at both the Li anode and NMC811 surfaces. High Li CEs (~99.4%) and stable long-term cycling with low overpotential under ultrahigh current densities (10 mA cm-2) could be achieved for Li anode. Li||NMC811 cells exhibit outstanding cycling performance under ultrahigh cut-off voltages of 4.6 V and 4.7 V, or at high current densities (5.1 mA cm-2). This work provides critical insights into the ion-solvent interactions in the solvation complex and their profound influence on the battery electrochemical performances.