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

Single-ion conducting polymer electrolyte with its high Li transference number (tLi+) has showed the capability in enabling high battery performance and safety by avoiding liquid electrolyte leakeage and suppressing the Li dendrite formation. However, the insufficient ionic conductivity, low electrochemical stability, and poor polymer/electrode interfacial issues greatly hindered its commecial use. Here, a Li-containing boron centered fluorinated single ion conducting (SIC) polymer gel electrolyte (LiBFSIE) was rationally designed in targeting a high tLi+ and high electrochemical stability. Owing to the low dissociation energy of the boron-center anion and Li+, the as-prepared LiBFSIE exhibited an ionic conductivity of 2 × 10-4 S/cm at 35 ºC, which is exclusively contributed by Li ions due to the high tLi+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. Benefited from the high electrochemical stability of LiBFSIE, a promising average coulombic efficiency of 99.95% over 200 cycles was achieved, which is higher than that of the liquid electrolyte based cells. No obvious capacity fading was observed, indicating the long term-stability of LiBFSIE for lithium metal batteries.