High-Performance Copper/Copper Oxide-Based Cathode Prepared by a Facile Ball-Milling Method for All-Solid-State Fluoride-Ion Batteries

Benefiting from the high theoretical volumetric energy density of the metal/metal fluoride (M/MFx) cathodes, all-solid-state fluoride-ion batteries (FIBs) are anticipated to be one of the next-generation energy storage devices. However, M/MFx electrodes have low rate capability due to the large diffusion overpotential of fluoride ions because the reaction proceeds by a two-phase reaction mechanism between the metal M phase and the metal fluoride MFx phase, which has significantly different lattice constants. To address this problem, uniformly distributed nanoparticles should be designed to shorten the diffusion pathway. Herein, we report a facile ball-milling method for preparing Cu-based cathode materials. Our findings reveal that as the ball-milling rotation speed increases, there is a significant decrease in the crystallite size of the solid electrolyte and a transformation of Cu oxides into metallic Cu, accompanied by an increase in the crystallite size. Among the as-prepared cathode composites, a fine mixture of metallic Cu and Cu oxides with intermediate rotation speed (300 rpm) exhibits superior electrochemical performance, with a reversible capacity of 400 mAh gCu2O–1 after 20 cycles. Furthermore, it exhibits excellent rate capability by combining the high capacity of Cu with the satisfactory rate performance of Cu2O, achieving a capacity of 174 mAh gCu2O–1 at a current density of 550 mA gCu2O–1, which is currently the highest reported for Cu-based cathode materials in FIBs. A charge compensation mechanism involving Cu0/Cu2+ and Cu+/Cu2+ redox reactions has been confirmed by electrochemical methods, X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and scanning transmission electron microscopy (STEM) measurements. The dominant factors affecting the impedance spectra of the as-prepared composites were also been investigated. It is believed that the cathode composites prepared by a facile ball-milling method in this work will lead to a significant step in the application of all-solid-state FIBs.