Fluorine-Like BH₄-Doped Li₆PS₅Cl with Improved Ionic Conductivity and Electrochemical Stability

Sulfide-based solid electrolytes with high ionic conductivity have attracted a lot of attention. However, the incompatibility and interfacial instability of sulfides with the lithium metal anode have emerged as pivotal constraints on their development. To address this challenge, we proposed and successfully synthesized the BH₄^- doped argyrodite-type electrolyte Li₆PS₅Cl_0.9(BH₄)_0.1 by mechanical ball milling and annealing. This electrolyte not only exhibits an exceptionally high ionic conductivity of 2.83 × 10^-3 S cm^-1 at 25 °C but also demonstrates outstanding electrochemical stability. The Li/Li₆PS₅Cl_0.9(BH₄)_0.1/Li symmetric cell can stably run for more than 400 h at a current density of 0.2 mA cm^-2. In sharp contrast, although the F^- doped sample, Li₆PS₅Cl_0.3F_0.7, can highly improve Li₆PS₅Cl's electrochemical stability, the ionic conductivity will reduce dramatically to 6.63 × 10^-4 S cm^-1. The stepwise current method reveals a critical current density of 3.5 mA cm^-2 for Li₆PS₅Cl_0.9(BH₄)_0.1, which makes it a competitive sulfide-based solid electrolyte. This research offers valuable insights for designing new borohydride-containing solid electrolytes.