Synthesis, characterization, and electrochemical, optical and magnetic properties of new members of the Li2M2W2O9 (M = Mn, Fe, Co) family

Abstract

The relentless quest for new electrode materials for energy storage and electrochromic devices remains essential to improve current technology. Building on our previous discovery of Li₂Ni₂W₂O₉, a corundum-like compound with reversible Li⁺ intercalation and electrochromic behavior, we report three new phases – Li₂Mn₂W₂O₉, Li₂Fe₂W₂O₉ and Li₂Co₂W₂O₉ – synthesized via ceramic and carbothermal reduction methods. The three phases crystallize in either the orthorhombic Pbcn or the trigonal Pc1 space groups and feature cationic mixing between the 3d-transition metal (Mn, Fe or Co) and Li at varying levels. These materials were characterized to investigate their unique structural features, electrochemical behavior, optical response and magnetic properties. Operando optical reflection microscopy revealed distinct light-matter interactions: Li₂Fe₂W₂O₉ and Li₂Mn₂W₂O₉ showed contrast changes due to volume change during Li⁺ (de)intercalation, while Li₂Co₂W₂O₉ exhibited dual optical responses dominated by either light absorption or volume change. This approach enabled us to probe and comparatively rank electrochromic efficiency across the series as: Li₂Ni₂W₂O₉ > Li₂Co₂W₂O₉ > Li₂Mn₂W₂O₉ ≫ Li₂Fe₂W₂O₉. Magnetic characterization uncovers long-range antiferromagnetic ordering in both Li₂Fe₂W₂O₉ and Li₂Co₂W₂O₉, with magnetic structures proposed in the Pbc′n and refined in the Pb′c′n Shubnikov space groups, respectively. This comprehensive study reveals how transition-metal chemistry governs the interplay between electrochemical, optical, and magnetic properties in the Li₂M₂W₂O₉ family, offering a tunable platform for future multifunctional energy materials.