Zirconium Tailored Iron Fluoride Lattices and Nanocomposites for High Energy, Rate, and Long Cycle Life

Transition metal fluorides are promising alternatives to the ubiquitous intercalation-based chemistries in current secondary batteries due to their exceptional specific energy and high energy densities. Of particular interest is FeF 3 , and although much improvement in enabling FeF 3 electrochemically has been accomplished, a number of challenges remain to be addressed including poor rate capabilities, cycling stability, and electrochemical performance at room temperature. FeF 3 is an electrical insulator, where, in addition to a foundation of poor electronic conductivity, much of the impedance during the conversion reaction is related to ion transport. In this work, transport was addressed by the creation of conductive pathways through the use of a zirconium source (ZrX) to create post conversion conductive pathways through LiF. Evidence is presented that Zr was substituted within the lattice of the FeF 3 crystal lattice paralleling a systematic improvement of the conversion kinetics, cycling stability, and rate capabilities. The resulting nanocomposites consisted of a matrix of 10–23 nm zirconium enhanced iron fluoride crystallites encapsulated by X-ray amorphous MoS 2 in the original Zr modified dense FeF 3 -xMoS 2 nanocomposites. At room temperature, the composites exhibited stable cycling with capacities of up to 693 mAh g −1 , exceeding 200 cycles and good performance at 1C rate.