Li _6−x Fe _1−x Al _x Cl ₈ Solid Electrolytes for Cost‐Effective All‐Solid‐State LiFePO ₄ Batteries

Bulk‐type all‐solid‐Li batteries based on inorganic solid electrolytes (SEs) are considered promising candidates for next‐generation energy storage systems for their potential to overcome the limitations of current lithium‐ion batteries, such as safety concerns, narrow operating temperature. Following the intensive efforts to search for affordable sulfide SEs, recently, chloride SEs are in the spotlight for their better high‐voltage stability and potentially lower cost than the sulfide counterparts. However, many chloride SEs exhibiting appreciable ionic conductivity adopt rare‐earth metals, which arises the motivation to search for new chloride SEs based on earth‐abundant minerals. Herein, Li 6−x Fe 1−x Al x Cl 8 (0 ≤ × ≤ 0.8) SEs exhibiting significantly improved ionic conductivity of 3.9 × 10 −5 S cm −1 at 25°C for Li 5.5 Fe 0.5 Al 0.5 Cl 8 compared to Li 6 FeCl 8 (3.1 × 10 −6 S cm −1 ) are reported. The formation of a metastable spinel‐like phase was observed for Li 5.5 Fe 0.5 Al 0.5 Cl 8 prepared by mechanical ball‐milling. Furthermore, its electrochemical properties as a catholyte were examined, coupled with a LiFePO 4 cathode active material.