Tailored Iron Fluoride Lattices and Nanocomposites for High Energy, Rate, and Long Cycle Life
Authors: Harrop Ben , Halajko Anna , Amatucci Glenn G 
Published: December 2024

Due to the rapidly growing energy demands of portable electronics, electric vehicles and grid storage, the need for efficient, energy dense secondary batteries is more important than ever before. Current commercial lithium batteries generally use a layered transition metal oxide, such as LiCoO2 (LCO), which is an intercalation positive electrode that has a theoretical capacity of 274mAh/g, but a practical capacity of only 140-160 mAh/g (3). Current graphite negative electrode capacities are approximately 300mAh/g (4), lithium metal has a capacity of 3860mAh/g (5), and newly incorporated Si alloys enable a range of capacities between the two end points. Considering this, it is clear the limiting factor for lithium battery capacities is the positive electrode. Further, LCO as with many of the Ni based layered compounds is toxic, expensive, and requires mining which is both detrimental to the environment and generally done in unethical working conditions (2). Conversion materials offer a solution to this problem having significantly higher capacity than their intercalation counterparts and many of which are sourced from abundant elements. Of particular interest is iron(III) fluoride (FeF3) because of its high theoretical capacity (712mAh/g), low toxicity, and low cost and environmental impact due to the abundance of iron (1). Although much improvement has been accomplished in the community, a number of challenges remain to be addressed before conversion materials can become a competitive alternative. These limitations include poor rate capabilities, cycling stability and electrochemical performance at room temperature. FeF3 is an electrical insulator, which makes it a poor electrode material at room temperature. Our group has also shown that the majority of the impedance is related to ion transport (6). To work around the transport challenged nature of FeF3 and enable it electrochemically, molybdenum disulfide (MoS2) was used to create conductive pathways in the form of a dense FeF3:xMoS2 nanocomposites. The resulting nanocomposites created a matrix of 10-23nm iron fluoride crystallites encapsulated by quasi 2D MoS2 nanosheets. This nanocomposite also led to modification of the core FeF3 crystal lattice and voltage profile paralleling a systematic improvement of the conversion kinetics, performance, and rate capabilities. At room temperature, the new dense composites exhibited very stable cycling with capacities of up to 693mAh/g (1303 Wh/kg) and good performance at 1C rate; both a significant improvement when compared to benchmark FeF3: C nanocomposites (Fig. 1). Details regarding mechanism, enabling electrolytes, comprehensive physical characterization, data in Li-ion cells and pathways for future enhancement will be discussed.

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Figure 1