A Liquid-like Quasi-solid Polymer Electrolyte for High-performance Sodium Metal Batteries

Abstract

Conventional liquid electrolytes cannot adequately ensure the safety of sodium metal batteries (SMBs), making quasi-solid polymer electrolytes (QSPEs) attractive alternatives. Herein, we introduce a QSPE that exhibits liquid-level ionic conductivity with the mechanical robustness of a solid polymer electrolyte. Optimizing the ratio of poly(vinylidene fluoride-co-hexafluoropropylene) and poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) yields a “liquid-like” electrolyte exhibiting a room-temperature ionic conductivity of 1.01 mS cm-1, a Na+ transference number of 0.78, and a glass-transition temperature of 22.5°C. Raman spectroscopy shows that the electron-donating -C-O-C- units promote the formation of aggregated ion pairs, enabling an inorganic-rich interphase. Density functional theory calculations predict that fluoride-containing reduction products form more readily than non-fluorinated counterparts, leading to the formation of a stable solid electrolyte interphase and the suppression of dendritic growth. Molecular dynamics (MD) simulations give insights into the solvation environment and coordination of Na+ ions with electron-withdrawing groups. Simulations also predict high ionic conductivity and transference numbers, confirming the high ion transport in the QSPE seen experimentally. Consistent with these insights, a Na//Na symmetric cell cycles stably for 200 h at high current density with superior charge transfer proficiency, while a Na//Prussian blue full cell delivers an initial capacity of 159 mAh g-1 at 0.5 C and retains outstanding performance over 500 cycles. These findings underscore the importance of judicious polymer blending in advancing high-performance SMBs.