Investigation of key electronic states in layered mixed chalcogenides with a d0 transition metal as Li-ion cathodes

Lithium-rich transition metal chalogenides are witnessing a revival as candidates for Li-ion cathode materials, spurred by the boost in their capacities from transcending conventional redox processes based on cationic states and tapping into additional chalcogenide states. A particularly striking case is Li(2)TiS(3-y)Se(y), which feature a d0 metal. While the end members are expectedly inactive, substantial capacities are measured when both Se and S are present. Using X-ray absorption spectroscopy, we confirm that, in Li(2)TiS(2.4)Se(0.6), this behavior is underpinned by concurrent and reversible redox of only S and Se, and identify key electronic states. Moreover, wavelet transforms of the extended X-ray absorption fine structure provide direct evidence of the formation of short Se-Se units upon charging. The study uncovers the underpinnings of this intriguing reactivity and highlights the richness of redox chemistry in complex solids.