Transition Metal Oxyfluorides for Next-Generation Rechargeable Batteries

Transition metal oxyfluorides are attracting much attention for next-generation rechargeable batteries, including lithium-ion batteries and those beyond lithium-ion batteries. Mixed-anion transition metal oxyfluorides offer the combined advantages of fluorides and the beneficial effects of oxides achieving improved capacity, high voltage, good conductivity and good cycling stability. Oxygen-fluorine substitution can be employed to manipulate the physiochemical properties of those corresponding transition metal oxides and/or fluorides for rechargeable batteries, as cathode and/or anode materials, achieving improved electrochemical performances. However, it is still a challenging task to develop facile procedures to produce transition metal oxyfluorides, particularly difficult on a large scale and with high purity. Various methods and approaches have been developed over the years, typically based on solid state reactions. Recently, liquid-based approaches under mild conditions for the preparation of transition metal oxyfluorides are attracting much attention. In this review, a number of transition metal oxyfluorides reported for rechargeable batteries, including VO2F, BiOF, FeOF, TiOF2, NbO2F, are discussed. Their synthetic ap-proaches, limitations and electrochemical performances are reviewed. Transition metal oxyfluorides with the presence of strong electronegativity of fluorides are often suitable as positive electrode materials. For cathode applications, the author suggests that lithium-free cathodes of transition metal oxyfluorides can be coupled with lithiated anodes to make as-assembled charged-state lithium-ion batteries. The same concept can be employed to prepare charged state sodium-ion batteries and other batteries using transition metal oxyfluorides as cathodes. The author suggests that asassembled batteries in charged state based on transition metal oxyfluorides (e.g., FeOF) as cathodes coupled with lithiated anodes will eventually be commercialized. The development of next-generation lithium-ion batteries and those so-called beyond lithium-ion batteries will depend on the capability to synthesize and produce high-quality transition metal oxyfluorides on a large scale. Those transition metal oxyfluorides not only can find practical applications in batteries, but also can be employed as model electrode systems for fundamental mechanism studies.