High‐Performance Rechargeable Aluminum‐Ion Batteries Enabled by Composite FeF3 @ Expanded Graphite Cathode and Carbon Nanotube‐Modified Separator

Abstract Rechargeable aluminum ion batteries (AIBs) are one of the most promising battery technologies for future large‐scale energy storage due to their high theoretical volumetric capacity, low‐cost, and high safety. However, the low capacity of the intercalation‐type cathode materials reduces the competitiveness of AIBs in practical applications. Herein, a conversion‐type FeF 3 ‐expanded graphite (EG) composite is synthesized as a novel cathode material for AIBs with good conductivity and cycle stability. Combined with the introduction of a single‐wall carbon nanotube modified separator, the shuttle effect of the intermediate product, FeCl 2 , is significantly restrained. Moreover, enhanced coulombic efficiency and reversible capacity are achieved. The AIB exhibits a satisfying reversible specific capacity of 266 mAh g −1 at 60 mA g −1 after 200 cycles, and good Coulombic efficiency of nearly 100% after 400 cycles at a current density of 100 mA g −1 . Ex situ X‐ray diffraction and X‐ray photoelectron spectroscopy are applied to explore the energy storage mechanism of FeF 3 in AIBs. The results reveal that the intercalation of Al 3+ species and the reduction of Fe 3+ species occurrs in the discharge process. These findings are meaningful for the fundamental understanding of the FeF 3 cathode for AIBs and provide unprecedented insight into novel conversion type cathode materials for AIBs.