Comprehensive study of the CuF₂ conversion reaction mechanism in a lithium ion battery

Conversion materials for lithium ion batteries have recently attracted considerable attention due to their exceptional specific capacities. Some metal fluorides, such as CuF₂, are promising candidates for cathode materials owing to their high operating potential, which stems from the high electronegativity of fluorine. However, the high ionicity of the metal-fluorine bond leads to a large band gap that renders these materials poor electronic conductors. Nanosizing the active material and embedding it within a conductive matrix such as carbon can greatly improve its electrochemical performance. In contrast to other fluorides, such as FeF₂ and NiF₂, good capacity retention has not, however, been achieved for CuF₂. The reaction mechanisms that occur in the first and subsequent cycles and the reasons for the poor charge performance of CuF₂ are studied in this paper via a variety of characterization methods. In situ pair distribution function analysis clearly shows CuF₂ conversion in the first discharge. However, few structural changes are seen in the following charge and subsequent cycles. Cyclic voltammetry results, in combination with in situ X-ray absorption near edge structure and ex situ nuclear magnetic resonance spectroscopy, indicate that Cu dissolution is associated with the consumption of the LiF phase, which occurs during the first charge via the formation of a Cu¹⁺ intermediate. The dissolution process consequently prevents Cu and LiF from transforming back to CuF₂. Such side reactions result in negligible capacity in subsequent cycles and make this material challenging to use in a rechargeable battery.