Mixed-Metal Fluoride Pyrochlore Synthesis and Investigation as an Alkali-Ion Battery Cathode

Recent studies have identified iron-fluoride pyrochlores to be a promising cathode for lithium- and sodium-ion batteries as they demonstrate high specific capacities and rapid alkali-ion diffusion using earth-abundant materials. However, there has been limited research into mixed-metal fluoride pyrochlores as intercalation cathodes. Recent work^[1] has identified a mechanochemical route to the production of the NH₄Mn²⁺Fe³⁺F₆ pyrochlore, but required fluorine gas to remove the ammonium ions and oxidize the Mn²⁺ species. <br/>In our work, NH₄Mn²⁺Fe³⁺F₆ was synthesized by the same mechanochemical method, oxidized to MnFeF₆ by several nontoxic alternatives, and electrochemically investigated. These novel oxidation methods allow access to the Mn^2+/3+ redox, increasing the specific capacity of the material. Removal of ammonium frees the channels, which decreases the barrier to intercalation by alkali ions and increases the rate of ion diffusion. Thus, the mixed-metal pyrochlore can be competitive as a cathode material against other metal-fluoride cathodes. These methods are the first to avoid use of toxic fluorine agents in production of the iron-fluoride pyrochlore type. The obtained pyrochlore was characterized by XRD, XPS, TGA, and IR. The efficacy of this material was then evaluated for use as an alkali battery cathode in an electrochemical cell and is reported for the first time. The high performance of the mixed-metal fluoride pyrochlore in lithium- and sodium-ion batteries demonstrates that an industrially favorable, environmentally friendly pyrochlore synthesis leads to an effective alkali-ion cathode.<br/><br/>[1] Lemoine, K.; Moury, R.; Durand, E.; Arroyo-de Dompablo, E.; Morán, E.; Leblanc, M.; Hémon-Ribaud, A.; Grenèche, J.-M.; Galven, C.; Gunes, V. First Mixed-Metal Fluoride Pyrochlores Obtained by Topotactic Oxidation of Ammonium Fluorides under F2 Gas. Cryst. Growth Des 2021, 21, 935, DOI: 10.1021/acs.cgd.0c01279