Concentrated, Gradient Electrolyte Design for Superior Low-Temperature Li-Metal Batteries

Improving the low-temperature performance of lithium-ion batteries is critical for their widespread adoption in cold environments. In this study, we designed a novel LHCE featuring a solvent polarity gradient, designed to maximize both room- and low-temperature ion mobility. Extremely polar fluoroethylene carbonate (FEC) and low-freezing-point, −135 °C, non-polar nonaflurobutyl methyl ether (NONA) were supplemented by two intermediate solvents with incremental step-downs in polarity. The intermediate solvents consist of methyl (2,2,2-triflooethyl) carbonate (FEMC) and either diethylene carbonate (DEC), ethyl methyl carbonate (EMC), or dibutyl carbonate (DBC). The four solvents were combined with 1 M lithium bis(fluorosulfonyl)amide (LiFSI) salt and were able to accommodate 37.5% diluent volume, resulting in ultra-low electrolyte freezing points below −120 °C. This contrasts with our previously investigated three-solvent LHCE, which only allowed for a 14% diluent volume and a −85 °C freezing point. Localized high salt concentrations were shown by less than 3% of FSI- anions being free in solution. The gradient LHCEs also showed room-temperature ionic conductivities above 10–3 S/cm and maintained high ion mobility below −40 °C. Lithium metal coin cells with LiFePO4 (LFP) cathodes featuring the gradient LHCEs, a reference three-solvent LHCE, and commercial (1 M LiPF6 in 1:1 EC:DEC) electrolyte were constructed. All gradient LHCEs outperformed both the three-solvent and commercial electrolytes at all temperatures, with the DEC-based gradient LHCE showing the best performance of 159.7 mAh/g at 25 °C and 109.2 mAh/g at −50 °C, corresponding to a 68% capacity retention. These findings highlight the potential of LHCE systems to improve battery performance in low-temperature environments and propose a new gradient design strategy for electrolytes to yield advantages of both polar and weakly polar solvents.