Transition metal fluorides are promising for use as high-capacity cathodes in rechargeable lithium batteries for large-scale applications, such as electric vehicles and grid-scale storage, but energy-efficiency and kinetics related issues remain a major hurdle to their commercial use. Cu based fluorides are particularly attractive due to the intrinsic high redox potential and extraordinarily high specific energy density. Here we report on the synthesis, structural and electrochemical properties of new nanostructured ternary metal fluorides, which may overcome some of these issues. By substituting Cu into the Fe lattice, forming the solid solution Cu_yFe_1-yF₂, reversible Cu and Fe redox reactions were achieved with surprisingly small hysteresis (<150 mV). This finding indicates that cation substitution may provide a new pathway for tailoring electrochemical properties of conversion electrodes [1]. The Li storage/release mechanisms and limits to cycling stability of Cu_yFe_1-yF₂ were also investigated by combining electrochemical measurement with comprehensive structural and chemical analysis using in-situ X-ray absorption spectroscopy, X-ray diffraction, and transmission electron microscopy-electron energy loss spectroscopy (TEM-EELS). Detailed lithium reaction mechanisms, Cu-loss related issues along with possible remedy solutions in the Cu_yFe_1-yF₂ system, will be discussed.   The work was supported as part of the NorthEastern Center for Chemical Energy Storage, an EFRC Center funded by the U.S. DOE-BES, under Award Number DESC0001294, and by DOE-EERE under the Advanced Battery Materials Research program, under Contract No. DE-SC0012704.   [1] Wang et al., “Ternary Metal Fluorides as High-Energy Cathodes with Low Cycling Hysteresis”, Nat. Commun. 6: 6668 (2015).