Development of Iron Fluoride Based Nanocomposite Materials to Enable High Performance Aluminium-Ion Batteries
Authors: Zhang Juyan , Zhang Lan , Zhao Yunlong , Meng Jiashen , Wen Bohua , Zhang Suojiang , Cai Qiong 
Published: February 2024

Developing grid-scale energy storage is important for the penetration of intermittent renewable energies such as wind and solar, but remains one of the biggest challenges in the field of electrochemical energy storage [1]. The application of mature lithium-ion batteries (LIBs) to grid energy storage is controversial due to the limited Li resources and geographical distribution, high cost of materials (e.g., Co, Li), limited lifetime, and safety concerns [2]. Rechargeable aluminium ion batteries (AIBs), which normally utilises aluminium (Al) metal as anode, are one of the most promising battery technologies for future large-scale energy storage, due to the high theoretical volumetric capacity (8046 mAh cm−3), high safety, and low cost and high abundance of aluminium (the third most abundant metal in the earth crust) [3].
AIBs have achieved long cycle life (>7500 cycles) when using graphite and graphene as cathode materials [4-5]. Nevertheless, the reported graphite-based cathodes have intrinsically low storage capacities (60–200 mAh g-1) due to the intercalation mechanism of the solvated ions rather than the multivalent Al3+ transformation. Extensive efforts have been made to develop new cathode materials to promote the specific/volumetric capacity of AIBs, including transition metal oxides [6], sulfides [7], selenides [8] and others. These AIBs based on non-graphite cathodes usually demonstrate either low discharge voltage, or high initial capacity but significant capacity decay and poor cycle life. To further improve the performance of AIBs, new cathode materials with high storage capacity and long cycle life needs to be developed.
In this paper, we report the development of a nanoscale FeF3@expaned graphite (EG) composite as a novel conversion-type cathode material for AIBs [9]. AIB coin cells were assembled using high-purity Al foil as the anode, the ionic liquid [EMIm]Cl/AlCl3 as the electrolyte, and the FeF3@EG composite as cathode. The conversion reaction between the Al3+ ions and FeF3 through transferring three electrons for per Al3+ ion reacted could boost the storage capacity of AIBs. A single-wall carbon nanotube-modified separator was introduced into the system, to significantly restrict the shuttle effect of the intermediate product of FeF3. The assembled AIBs exhibited a satisfactory reversible specific capacity of 266 mAh g-1 at a current density of 60 mA g-1 after 200 cycles, and a good Coulombic efficiency approaching 100% after 400 cycles at a current density of 100 mA g-1 [9]. Ex-situ X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) have been applied to explore the energy storage mechanism of FeF3 in AIBs for the first time [9].
To further overcome the corrosion issue of ionic liquid, a gel polymer electrolyte (GPE) has been successfully synthesised via an innovative method where no solvent or initiator was utilised in the polymerisation process. The application of GPE significantly reduced the corrosivity and enhances the moisture sensitivity of EMIC ionic liquid, as well as improving the reversible ability of the AIBs. The FeF3@EG-based AIB with 0.8g-EMIC-gel electrolyte exhibits a reversible capacity of 204.5 mAh g-1after 1000 cycles at a current density of 100 mA g-1 and stable rate performance for 600 cycles with a Coulombic efficiency of approximately 95%.
This work provides unprecedented insight into novel conversion type cathode materials for AIBs. The findings in this work can serve as guidance for the successful design of low cost and high discharge capacity AIBs for large-scale energy storage and are also meaningful for the fundamental understanding of the metal fluorides cathodes for AIBs.
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