Fluorination of Lithium-Excess Transition Metal Oxide Cathode Materials

Fluorination of Li-ion cathode materials is of significant interest as it is claimed to lead to significant improvements in long-term reversible capacity. However, the mechanism by which LiF incorporates and improves performance remains uncertain. Indeed, recent evidence suggests that fluorine is often present as a coating layer rather than incorporated into the bulk of the material. In this work, first-principles calculations are used to investigate the thermodynamics of fluorination in transition metal oxide cathodes to determine the conditions under which bulk fluorination is possible. It is found that unlike classic well-ordered cathodes, which cannot incorporate fluorine, disordered rock salt-structured materials achieve significant fluorination levels due to the presence of locally metal-poor, lithium-rich environments that are highly preferred for fluorine. As well as explaining the fluorination process in known materials, this finding is encouraging for the development of new disordered rock salt lithium-excess transition metal oxides, a promising new class of Li-ion battery cathode materials that offer superior practical capacity to traditional layered oxides. In particular, it is found that bulk fluorination may serve as an alternative source of Li-excess in these compounds that can replace the conventional substitution of a heavy redox-inactive element on the transition metal sublattice.