Fluorination-Enhanced Ionic Conductivity in Cost-Effective Halide Solid Electrolytes

Advancing all-solid-state batteries requires solid-state ionic conductors that exhibit both high ionic conductivity and excellent electrochemical stability to ensure compatibility with high-energy-density electrodes. Fluorination has been an effective strategy for enhancing the electrochemical stability, but it often comes at the cost of decreased ionic conductivity. Herein, we report that substituting fluorine for chlorine in a cost-effective crystalline Li–Al–Cl solid electrolyte system can unexpectedly enhance the ionic conductivity. The optimized fluorinated compound, 0.7LiF–AlCl3 (LACF0.7), exhibits an ionic conductivity 60 times higher than that of LiAlCl4 (LAC) synthesized via the same solid-state method and 6 times higher than that of mechanochemically synthesized LAC. Through multimodal structural characterizations, we reveal that fluorination leads to a Li-deficient and Al-excess composition with a new octahedral Al environment while maintaining the monoclinic structural framework akin to LAC. The enhanced ionic conductivity is attributed to two factors: the creation of more distorted and diverse local Li environments through fluorination, and the weakening of Li–F interactions due to strong Al–F bonding in the vicinity. This work highlights how fluorination can be beneficial to ionic conductivity, opening new opportunities to design halide-based solid electrolytes within fluorinated compositional spaces.