Moderate solvation structure of lithium ions for high-voltage lithium metal batteries at −40 ºC

Abstract

Lithium metal batteries (LMBs) are considered highly promising due to their high-energy-density, however, 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 ethoxy side difluoro-substitution group (−OCH2CF2H) has been developed. By withdrawing the electron cloud of the carbonyl group (C=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||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 ºC. Even under a lower temperature of –40 ºC, the Li||NCM622 cell provides a high a high capacity retention of 87.9% after 125 cycles. Moreover, a prototype 450 Wh kg–1 pouch cell (2.9 Ah) operates for 75 cycles at −20 ºC with 83.4 % capacity retention using a low electrolyte/capacity (E/C) ratio of 1.5 g Ah–1. This design strategy provides a promising approach for future exploration of high-voltage lithium metal batteries under low-temperature conditions.