CuF2 as Reversible Cathode for Fluoride Ion Batteries

In the search for novel battery systems with high energy density and low cost, fluoride ion batteries have recently emerged as a further option to store electricity with very high volumetric energy densities. Among metal fluorides, CuF₂ is an intriguing candidate for cathode materials due to its high specific capacity and high theoretical conversion potential. Here, the reversibility of CuF₂ as a cathode material in the fluoride ion battery system employing a high F^? conducting tysonite‐type La_0.9Ba_0.1F_2.9 as an electrolyte and a metallic La as an anode is investigated. For the first time, the reversible conversion mechanism of CuF₂ with the corresponding variation in fluorine content is reported on the basis of X‐ray photoelectron spectroscopy measurements and cathode/electrolyte interfacial studies by transmission electron microscopy. Investigation of the anode/electrolyte interface reveals structural variation upon cycling with the formation of intermediate layers consisting of i) hexagonal LaF₃ and monoclinic La₂O₃ phases in the pristine interface; ii) two main phases of distorted orthorhombic LaF₃ and monoclinic La₂O₃ after discharging; and iii) a tetragonal lanthanum oxyfluoride (LaOF) phase after charging. The fading mechanism of the cell capacity upon cycling can be explained by Cu diffusion into the electrolyte and side reactions due to the formation of the LaOF compound.