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We report on the reversible, electrochemical (de)fluorination of the defect fluoride pyrochlore CsMnFeF6 at room temperature using a liquid electrolyte. CsMnFeF6 was synthesized via three different methods (hydrothermal, ceramic, and mechanochemical), each of which yield products of varying particle size and phase purity. Using galvanostatic cycling, we found that after three oxidative/ reductive cycles, approximately one fluoride ion can be reversibly inserted and removed from mechanochemically synthesized CsMnFeF6 for multiple cycles. Ex-situ X-ray absorption spectroscopy confirmed that both the Mn2+ and Fe3+ in this composition are redox active during cycling. Electrochemical impedance spectroscopy and ex-situ synchrotron powder diffraction were utilized to investigate the delayed onset of significant fluoride (de)insertion. We observed decreased impedance after one full cycle and subtle expansion and contraction of the CsMnFeF6 cubic lattice on oxidation (insertion) and reduction (removal), respectively, over the first two cycles. Our results suggest the formation of fluoride vacancies in early cycles generates mixed-valent Fe that enhances the conductivity and improves the reversibility in later cycles.

Lithium-ion batteries (LIBs) featuring high energy density have received particular attention in recent years due to their promise as electrical energy storage devices for electric vehicles and portable electronic devices. However, most of the commercial LIBs use graphite anode which suffer from the formation of solid-electrolyte interface (SEI) layer. Moreover, the poor performance of graphite anodes at high C-rates limits their application for fast-charging LIBs. To address the challenges associated with graphite anodes, niobium-containing Wadsley-Roth shear phase materials have been suggested. Thanks to the redox potential of Nb4+ (above 1 V versus Li/Li+), the niobium containing Wadsley-Roth shear phase materials avoid SEI formation. Furthermore, a combination of edge-shared and corner-shared octahedra in this type of shear phase materials forms many tunnel-like regions where Li-ions can rapidly diffuse during lithiation and delithiation. In fact, niobium-containing Wadsley-Roth shear phase materials show impressive cycling performance as fast-charging battery materials and maintain more than 50 % of their theoretical capacity at C-rate of 10 C. This study aims to understand the fast-charging capability of niobium containing Wadsley-Roth shear phase materials accompanied by ion ordering in the application of LIBs. In addition, it aims to provide experimental evidence to elucidate the occurrence of Li-ion ordering in this type of shear phase materials. To do so, galvanostatic cycling and cyclic voltammetry were performed on selected niobium-containing Wadsley-Roth shear phase materials including TiNb2O7, PNb9O25, VNb9O25, and NaNb13O33. In addition, open circuit voltage and entropic potential of those materials were measured. The results show that ion ordering characterized by a tilde-shaped fluctuation in entropic potential was observed in all tested niobium containing Wadsley-Roth shear phase materials. Furthermore, the results indicate that the b-value approaching 0.5 corresponded to the first order transition accompanied by a two-phase coexistence while the b-value approaching 1 corresponded to the ordering of Li-ions. This analysis implies that the ion ordering phenomena can enhance the fast-charging capability of the material. This study provides valuable insights for fast-charging battery materials.