Moderate solvation structures of lithium ions for high-voltage lithium metal batteries at-40 °C

Lithium metal batteries (LMBs) are considered highly promising due to their high-energy-density; however, they suffer from challenges such as lithium dendrite growth at low temperatures (LT) and severe decomposition at high cut-off voltages. Here, a quasi-solid-state electrolyte (QSSE) containing a carboxylic ester solvent with an ethoxy side difluoro-substitution group (-OCH2CF2H) has been developed. By withdrawing the electron cloud of the carbonyl group (C 00000000 00000000 00000000 00000000 11111111 00000000 11111111 00000000 00000000 00000000 O) and transferring it to the fluorine atoms, the -OCH2CF2H group achieves a balanced charge dispersion between the fluorine and carbonyl oxygen atoms. Consequently, the QSSE forms a moderate solvation sheath through Li-F and Li-O coordination with the fluorinated carboxylic ester solvent, which not only promotes the de-solvation of Li+ at low temperatures but also induces the formation of a LiF-rich interphase to suppress lithium dendrite growth and detrimental side reactions on the cathodes' surfaces. As a result, the QSSE enables stable cycling of a high-voltage Li & Vert;LiNi0.6Mn0.2Co0.2O2 (NCM622) cell at 4.6 V, with a high-capacity retention of 85% and an average coulombic efficiency (CE) exceeding 99.9% over 700 cycles at -20 degrees C. Even at a lower temperature of -40 degrees C, the Li & Vert;NCM622 cell provides a high capacity retention of 87.9% after 125 cycles. Moreover, a prototype 450 W h kg-1 pouch cell (2.9 A h) operates for 75 cycles at -20 degrees C with 83.4% capacity retention using a low electrolyte/capacity (E/C) ratio of 1.5 g A h-1. This design strategy provides a promising approach for future exploration of high-voltage lithium metal batteries under low-temperature conditions.