Electrochemical Stability and Ionic Conductivity of AlF₃ Containing Lithium Borate Glasses: Fluorine Effect, Strength or Weakness?

Fluorides are commonly regarded as interfacial additives that enhance the electrochemical stability of solid-state battery electrolytes. In this study, we synthesized lithium borate glassy solid electrolytes and investigated the effect of adding aluminum fluoride (AlF₃) on its stability against lithium metal electrodes. Samples maintained their amorphous nature, with up to 9.20 wt.% of fluorine in the glass. Lithium borate glasses, with and without AlF₃, demonstrated an excellent electrochemical performance, sustaining a stable lithium voltage profile at current densities from 0.01 to 1 mA cm⁻² at 160°C. Notably, the lithium borate glass with the highest lithium ion content achieved the highest relative ionic conductivity and cycled stably for up to 500 h at current densities of 1 mA cm⁻² at 160°C in symmetric LiǀglassǀLi cells. However, the addition of AlF₃ to lithium borate glass significantly compromises its electrochemical stability. In long-term symmetrical cell tests, the AlF₃-containing lithium borate glass exhibited short-circuiting under 0.3 mA cm⁻², revealing unexpectedly poor stability. These findings offer valuable insights for evaluating the impact of fluorine incorporation on the performance of solid-state battery electrolytes.