Electrochromism of LixFePO4 Induced by Intervalence Charge Transfer Transition

The lithium-ion battery is the most advanced energy storage system, which utilizes an electrode reaction involving reversible lithium intercalation into a solid matrix. The structural and transport properties of these battery materials have been extensively studied as a function of lithium content, and structural/electronic phase diagrams have been revealed for a wide variety of lithium intercalation compounds. Here, we focused on the electrochromic response upon lithium intercalation and discovered distinctive color changes of LixFePO4, which is recognized as a promising cathode material for large-scale lithium-ion batteries. The emergence of a broad optical absorption band over the visible spectrum with coloring from pale gray to dark green was observed in accordance with the increase of the Fe3+/Fe2+ mixed-valence state and hence the solid–solution compositional domain in the phase diagram of LixFePO4. The color changes were analyzed using ab initio computational methods and rationalized to the intervalence charge transfer (IVCT) transition and its kinetic activation energy based on Marcus–Hush theory.