Mo_0.92TiTa_8.08O₂₅: Structural, Electrochemical, and Computational Investigation as the Anode for Lithium-Ion...

Wadsley-Roth (WR) materials are candidate anode materials for lithium-ion batteries due to their unique and open structure that often enables fast lithium diffusion. We report the synthesis and electrochemistry of a quaternary T[3 × 3] WR compound, Mo_0.92TiTa_8.08O₂₅. This tetragonal (I4/m) compound was synthesized in single-crystal form and has lattice parameters of a = 15.7326(4), b = 15.7326(4), and c = 3.8206(10) Å and a unit cell volume of 945.65(5) ų. For property measurements, bulk polycrystalline samples were prepared via the conventional solid-state synthesis route. The powder was used in lithium half-cells as an active material to evaluate its electrochemical properties. A reversible lithiation capacity of 846 mA h cm^-3 was obtained at a current density of 0.1C. Lithium diffusion coefficients were measured using intermittent current interruption where the parabolic trends with lithiation extent were consistent with lithium ordering. The corresponding capacity-weighted average diffusivity of 4.78 × 10^-19 m² s^-1 was 31× lower than the closely related T[3 × 3] VTa₉O₂₅, indicating hindered diffusion. Mo K edge EXAFS confirmed the T-type phase with a 4-fold Mo coordination (CN = 3.85), and Ta L₃ XANES revealed less distortion of the (Ta/Ti)O₆ octahedral sites when compared to the single-occupancy sites in VTa₉O₂₅. To explain this substantially slower diffusion observed in Mo_0.92TiTa_8.08O₂₅, calculations revealed trapping of Li in sites near the tetrahedra due to large activation barriers and also identified a restrictive multistep Li hopping path for transition from horizontal to vertical window sites when near octahedral Ti. Overall, this work demonstrates that the chemical composition strongly influences Li diffusion in a variety of ways in WR compounds.