Temperature-switchable electrolyte with desirable phase transition behavior for thermal protection of lithium-ion batteries

Abstract

Thermal runaway is a primary safety concern for lithium-ion batteries (LIBs). To alleviate this concern, we propose here a temperature-switchable electrolyte (TSE) for reversible thermal protection of LIBs, based on the low critical solution temperature (LCST) behavior of poly (phenethyl methacrylate) in imidazolium-based ionic liquids. Our study reveals that the LCST of this electrolyte strongly depends on the ion-dipole interactions between polymer and ionic liquid. To enable a more reliable thermal control, we regulate the ion-dipole interactions by tailoring the electrolyte composition including alkyl chain length on the imidazolium cation, mixing ratio of two different ionic liquids, as well as polymer and lithium salt concentrations. Consequently, the as-obtained TSE demonstrates an optimal LCST (85 °C) and a rapid phase transition speed (within 5 seconds at 85 °C). Once the temperature exceeds 85 °C, the polymer rapidly precipitates from the TSE and deposits onto the electrode and separator surfaces due to the LCST-type phase transition, forming a blocking layer to interrupt ion transport between the two electrodes and thereby to halt electrode reactions. When the temperature drops below 85 °C, the polymer re-dissolves in the electrolyte to resume ion transport and electrode reactions, thus providing reversible thermal protection and preventing the battery from thermal runaway. This study offers new insights for developing reversible temperature-responsive electrolytes and therefore thermally self-protected LIBs.