Reversible Three-Electron Redox Behaviors of FeF₃ Nanocrystals as High-Capacity Cathode-Active Materials for Li-Ion Batteries

Three types of FeF 3 nanocrystals were synthesized by different chemical routes and investigated as a cathode-active material for rechargeable lithium batteries. XRD and TEM analyses revealed that the as-synthesized FeF 3 samples have a pure ReO 3 -type structure with a uniformly distributed crystallite size of ∼10 to 20 nm. Charge−discharge experiments in combination with cyclic voltammetric and XRD evidence demonstrated that the FeF 3 in the nanocomposite electrode can realize a reversible electrochemical conversion reaction from Fe 3+ to Fe 0 and vice versa, enabling a complete utilization of its three-electron redox capacity (∼712 mAh·g −1 ). Particularly, the FeF 3 /C nanocomposites can be well cycled at very high rates of 1000−2000 mA·g −1, giving a considerably high capacity of ∼500 mAh·g −1 . These results seem to indicate that the electrochemical conversion reaction can not only give a high capacity but also proceed reversibly and rapidly at room temperature as long as the electroactive FeF 3 particles are sufficiently downsized, electrically wired, and well-protected from aggregation. The high-rate capability of the FeF 3 /C nanocomposite also suggests its potential applications for high-capacity rechargeable lithium batteries.