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

ABSTRACT 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 (AlF 3 ) 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 AlF 3 , 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 AlF 3 to lithium borate glass significantly compromises its electrochemical stability. In long‐term symmetrical cell tests, the AlF 3 ‐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.