Ionic and Electronic Transport in Metal Fluoride Conversion Electrodes

Abstract

Materials that undergo a lithium conversion reaction often accommodate more than one Li per transition metal, and are promising candidates for high-capacity electrodes for lithium batteries. However, little is known about the mechanisms involved in the conversion process, the origins of the large polarization during electrochemical cycling, and why some materials are reversible (e.g., FeF2) while others are not (e.g., CuF2). We investigated the conversion reaction of metal fluorides, FeF2 and CuF2, to better understand the mechanisms underlying their contrasting electrochemical behavior. Lithium conversion of FeF2 results in the formation of an interconnected network of small iron nanoparticles (< 5nm), which may provide a pathway for electron transport and a high interfacial area between the Fe and LiF phases. Conversely, lithium conversion of CuF2 results in the formation of large isolated Cu particles, which may partially explain the poor reversibility in the CuF2 system.