Why Copper Fluoride Fails In Lithium-Ion Batteries

In lithium-ion battery research, sometimes promising-looking electrode materials fail inexplicably. Copper fluoride is one such material. Now, an experimental study reveals the chemical basis of CuF₂’s shortcomings, a key step toward rendering that material and related ones useful for battery applications (J. Phys. Chem. C 2014, DOI: 10.1021/jp503902z). Most Li-ion battery research focuses on Li intercalation compounds, spongelike materials into which Li ions insert themselves as the battery is used to provide power. Metal fluorides, including FeF₂ and NiF₂, are attractive nonintercalating electrode candidates because they provide high charge capacity and capacity retention. But test batteries with electrodes made of CuF₂, which is especially promising because of its high operating voltage and charge capacity, fail quickly. A team led by Xiao Hua and Clare P. Grey of Cambridge University and Stony Brook University, SUNY, now know why. On the basis of X-ray, NMR, and electrochemistry analyses, the team finds that during the first charge cycle, CuF₂ dissolves in the liquid electrolyte producing Cu⁺ species. They consume LiF (at the other electrode) and prevent CuF₂ from re-forming when the battery is recharged. Those problems might be avoided with protective electrode coatings or by using substitute electrolytes, the team suggests.