New iron-based intercalation host for lithium-ion batteries

The discovery of high-performance cathode materials is imperative for advances in current lithium-ion battery technology. Although extensive efforts have been focused on developing novel cathode materials, it has been a grand challenge to find candidates that can outperform state-of-the-art cathode materials such as layered, olivine, and spine] lithium transition-metal oxides. This issue arises because there are only a limited number of intercalation hosts with appropriate redox potential and lithium solubility. Here, we present a new iron-based intercalation host for lithium ion by exploring the "host formation reaction" from the nano composite of lithium and transition-metal compound hosts (i.e., LiF and FeO). Fluoride ions released from LiF decomposition during charging induce an unexpected phase transition of FeO to a new host structure of cubic-FeOF, a new polymorphic structure of FeOF that is different from the well-known rutile-FeOF. Cubic-FeOF electrode exhibits a higher redox potential (3.2 V) including lower voltage hysteresis and more extended stability of the structure than those of the rutile-FeOF undergoing partial lithiation followed by the conversion reaction. The discovery of new intercalation host from the host formation reaction of the nanocomposite suggests a new unexplored avenue in the development of novel cathode materials for lithium-ion batteries.