Copper(II) fluoride (CuF₂) is a promising cathode material for the next generation of lithium-ion batteries due to its high theoretical gravimetric (1645 Wh·kg^-1) and volumetric (3797 Wh·L^-1) energy density. In theory, it can reversibly store two incoming lithium ions in what is referred to a conversion reaction (2 Li⁺ + CuF₂ + 2 e^- → 2 LiF + Cu⁰). Although near-theoretical capacity (~515 mAh·g^-1) has been achieved for the first discharge, capacity greatly decreases with every cycle thereafter. This is surprising as materials such as iron(II) fluoride, which undergo a similar reaction (2 Li⁺ + FeF₂ + 2 e^- → 2 LiF + Fe⁰) can easily achieve 10 cycles without any major capacity degradation. The lack of reversible capacity in CuF₂ may be due to irreversible crystal structure changes inside of cathode which inhibit delithiation upon charging or due to the loss of copper through the electrolyte. Evidence for the latter for this can be seen through the copper plating of the lithium metal anode after cycling, detected by energy dispersive x-ray spectroscopy (EDS) analysis, or through the decrease in copper peak intensity in X-ray diffraction (XRD) and EDS spectra of the cathodes upon recharging. Correlation of copper content to capacity suggests that copper loss from the cathode is not the only factor in capacity loss and scanning electron microscopy (SEM) imaging shows large morphological differences between a pristine and charged sample. Decoupling these two intrinsic challenges for CuF₂ batteries will enable the design of more reversible conversion reaction cathode materials in the future. For now, application of CuF₂ in primary lithium-ion batteries remains promising, and continues to be expanded upon today.

This work is supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) program under Award No. DE-EE0006852.