Electrochromic Corundum-like Compound Based on the Reversible (De)insertion of Lithium: Li₂Ni₂W₂O₉

The search for efficient anodic electrochromic materials is essential to the development of electrochromic devices, such as smart windows. Magnetron-sputtered lithium-nickel-tungsten mixed oxides are good candidates to tackle this issue; however, they display a complicated microstructure, making it difficult to pinpoint the origin of their electro-optical properties. Herein, by exploring the Li₂O-NiO-WO₃ phase diagram, we obtained a new phase, Li₂Ni₂W₂O₉, that crystallizes in the orthorhombic Pbcn space group. This material can reversibly uptake/release 0.75 Li⁺ (31 mA h·g^-1) when cycled between 2.5 and 5.0 V versus Li⁺/Li. Moreover, through operando optical microscopy, we show that this new phase is electrochromic, and crucial information can be accessed about the diffusion-limited insertion of lithium at the single-particle scale. This study sets the ground for future syntheses of electrochemically active materials crystallizing in the ramsayite structure and details how the electrochromic properties of battery materials can be used to shed some light on their electrochemical mechanisms.