Poly(Ionic Liquid)-in-Salt Electrolytes: Unlocking the Potential of Extreme Salt Concentrations for Enhanced Battery Performance

Polymer-in-salt electrolytes (PISEs) were introduced three decades ago as a potential solution to the inherently low Li-ion conductivity in solvent-free solid polymer electrolytes.1 Despite significant progress, this approach still faces considerable challenges, ranging from a fundamental understanding to the development of suitable polymers and salts. A critical issue is maintaining both the stability and high conductivity of molten salts within a polymer matrix, which has constrained their further exploration. In this study,2 we propose a promising solution by integrating cationic poly(ionic liquids) (polyIL) with a crystallization-resistive salt consisting of asymmetric anions. This approach enabled the development of a stable molten-salt electrolyte with an exceptionally high Li-salt content of up to 90 mol %, offering a valuable opportunity for the in-depth understanding of PISEs at an extremely high salt concentration. A combination of molecular dynamics (MD) simulations with experimental techniques, including DSC, PFG-NMR, BDS, and rheology measurements, was employed to elucidate the ion coordination and coupling/decoupling ion transport from structural dynamics over an unprecedented salt range. A notable transition in the coordination environment, from polycation−anion−Li+ co-coordination to an anion−Li+ aggregation-dominated coordination, was clarified as the polycations-to-Li ratio increases from 1:2 to 1:8. This transition significantly impacts the electrolyte glass transition temperature (T g), ion transport, and Li-ion transference number of the electrolytes. Our findings illustrate how the ion transport behavior shifts between decoupling and coupling with the structural dynamics. At a 1:2 ratio, a decoupling from structural dynamics dominated by the polymer is observed, transitioning to coupling with structural dynamics at a 1:8 ratio, due to the formation of anion−Li+ aggregates alongside the polycation−anion−Li+ co-coordination structure at increased salt concentrations. The Li-ion transference number significantly increases above 0.8. Nevertheless, ion motion is intricately affected by the structural dynamics and the T g. The T g changes nonmonotonically with an increased salt concentration in the mixed anion system and can be further lowered at the highest salt concentration by modifying the ratio of two anions. This strategy facilitates structural motion and enhances ionic conductivity at the 1:8 salt system, thus maintaining a high Li-ion transference number. The presence of anion−Li+ aggregation domains was also shown to have a positive impact on improving battery performance compared to the previously reported 1:1.5 PDADMAFSI/LiFSI system,3with enhanced Li conductivity, lowered interfacial resistance and polarization, and a long stable Li deposition/dissolution. A detailed discussion will include the future design and optimization of polymer-in-salt electrolytes. References: 1) A. Angell et al., Nature, 1993, 362, 137. 2) S. Kondou, M. Forsyth, and F. Chen et al., J. Am. Chem. Soc., 2024, 146, 33169-33178. 3) X. Wang, F. Chen, and M. Forsyth et al., Joule, 2019, 3, 2687-2702.