Unlocking the Potential of Fluoride-Based Solid Electrolytes for Solid-State Lithium Batteries

The development of high energy density and sustainable all-solid-state lithium batteries relies on the development of suitable Li+ transporting solid electrolytes with high chemical and electrochemical stability, good interfacial compatibility, and high ionic conductivity. Ceramic-based electrolytes show high bulk Li+ conductivity and stability but exhibit poor mechanical properties. In contrast, a few sulfide-based electrolytes show high total Li+ conductivity and better mechanical properties but poor chemical and electrochemical stability. Moreover, both types of electrolytes exhibit interfacial compatibility issues with several electrode materials. Here, we reveal the potential of Li-containing metal fluorides as Li+ conducting solid electrolytes for solid-state lithium batteries, demonstrating their viability with a case study on β-Li3AlF6. We have synthesized β-Li3AlF6 by mechanical milling and investigated its properties as a solid electrolyte. An ionic conductivity of 3.9 × 10–6 S cm–1 was observed at 100 °C, which was increased to 1.8 × 10–5 S cm–1 by compositing with nanocrystalline alumina (γ-Al2O3). Furthermore, the performance of β-Li3AlF6 as a solid electrolyte was successfully tested in an all-solid-state lithium battery by using LiMn2O4 as a cathode and Li metal as an anode. Finally, we have used density functional theory to shed light on the Li diffusion pathways and associated activation barriers in β-Li3AlF6. Overall, our studies reveal the hidden potential of Li-containing metal fluorides as solid electrolytes for all-solid-state lithium batteries.