Bronze and pyrochlore type iron fluorides as cathode materials for Li/Na batteries

Abstract

Iron fluoride is the key candidate of cathode materials for high capacity Li/Na based batteries, owing to high ionicity of Fe–F and large theoretical capacity of fluoride. Exploring novel mineral phases of iron fluoride enables the improvement of intrinsic ion/electron conductivity, decreasing the use of electrochemical non-active components. It can lead to the activation of fluorides in terms of conversion reaction and large-sized cation (e.g., Na+) storage. This paper summarizes the recent progress on exploring novel structure prototypes of energy storage iron fluorides by module chemistry and open framework strategies, especially on hexagonal tungsten bronze (HTB), tetragonal tungsten bronze (TTB) and pyrochlore phases. These results address the important issues of fluorides on their poor conductivity, high carbon wire content, low Li-insertion power density and low Na-storage energy density. The HTB iron fluoride (FeF3 × 0.33H2O) is of tunnel structure, which is beneficial for the kinetic improvement and high rate Li (de)insertion. Complete dehydration of HTB phase is enabled by enhancing the crystallinity of HTB and removing its surface coating species. Pyrochlore iron fluoride of microporous framework (FeF3 × 0.5H2O) is endowed with interconnected three-dimensional (3D) open ion channels, enabling high reversible capacity of Na storage even under the involvement of conversion reaction. In contrast to HTB phase, dehydrating more flexible pyrochlore phase would cause serious amorphization (i.e., from topotactic densification of pyrochlore) however with the preservation of short-range ordering. The dehydration of open framework fluorides remarkably improves the conversion capacity and its retention, especially with good maintenance of higher voltage intercalation region. Ionic liquid based synthesis methods, e.g., dissolution-precipitation fluorination, ionothermal fluorination and solid-solid topotactic transformation in a top-down way, are explored to prepare the HTB and pyrochlore phases. The ionic liquid residual at fluoride grain or carbon wire surfaces allows an optimized ion/electron mixed conductive network based on the interaction with ionic liquid interlayer as binder. By using thermally more stable K-ion as channel filler instead of H2O molecule, more robust iron fluoride of TTB (K0.6FeF3) with coexistence of Fe2+ and Fe3+ is achieved by either conventional solid state reaction or mechanochemical method. TTB phase enables a near zero-strain reversible Na storage as cathode.