Molecular Design of a Highly Stable Single-Ion Conducting Polymer Gel Electrolyte

Single-ion conducting (SIC) polymer electrolytes with a high Li transference number (t_Li⁺) have shown the capability to enable enhanced battery performance and safety by avoiding liquid-electrolyte leakage and suppressing Li dendrite formation. However, issues of insufficient ionic conductivity, low electrochemical stability, and poor polymer/electrode interfacial contact have greatly hindered their commercial use. Here, a Li-containing boron-centered fluorinated SIC polymer gel electrolyte (LiBFSIE) was rationally designed to achieve a high t_Li⁺ and high electrochemical stability. Owing to the low dissociation energy of the boron-centered anion and Li⁺, the as-prepared LiBFSIE exhibited an ionic conductivity of 2 × 10^-4 S/cm at 35 °C, which is exclusively contributed by Li ions owing to a high t_Li⁺ of 0.93. Both simulation and experimental approaches were applied to investigate the ion diffusion and concentration gradient in the LiBFSIE and non-cross-linked dual-ion systems. Typical rectangular Li stripping/plating voltage profiles demonstrated the uniform Li deposition assisted by LiBFSIE. The interfacial contact and electrolyte infiltration were further optimized with an in situ UV-vis-initiated polymerization method together with the electrode materials. By virtue of the high electrochemical stability of LiBFSIE, the cells achieved a promising average Coulombic efficiency of 99.95% over 200 cycles, which is higher than that of liquid-electrolyte-based cells. No obvious capacity fading was observed, indicating the long-term stability of LiBFSIE for lithium metal batteries.