Investigating the Mechanism of Reversible Lithium Insertion into Anti-NASICON Fe₂(WO₄)₃

The gram-scale preparation of Fe₂(WO₄)₃ by a new solution-based route and detailed characterization of the material are presented. The resulting Fe₂(WO₄)₃ undergoes a reversible electrochemical reaction against lithium centered around 3.0 V with capacities near 93% of the theoretical maximum. Evolution of the Fe₂(WO₄)₃ structure upon lithium insertion and deinsertion is probed using a battery of characterization techniques, including in situ X-ray diffraction, neutron total scattering, and X-ray absorption spectroscopy (XAS). A structural transformation from monoclinic to orthorhombic phases is confirmed during lithium intercalation. XAS and neutron total scattering measurements verify that Fe₂(WO₄)₃ consists of trivalent iron and hexavalent tungsten ions. As lithium ions are inserted into the framework, iron ions are reduced to the divalent state, while the tungsten ions are electrochemically inactive and remain in the hexavalent state. Lithium insertion occurs via a concerted rotation of the rigid polyhedra in the host lattice driven by electrostatic interactions with the Li⁺ ions; the magnitude of these polyhedral rotations was found to be slightly larger for Fe₂(WO₄)₃ than for the Fe₂(MoO₄)₃ analog.