Ti₃C₂T_<i>x</i> MXenes as Anodes for Sodium-IonBatteries: the In Situ Comprehensionof the Electrode Reaction

Since their appearance on the scene, MXenes have been recognized as promising anode materials for rechargeable batteries, thanks to the combination of structural and electronic features. The layered structure with a suitable interlayer distance, good electronic conductivity, and moldability in composition makes MXenes exploitable both as active and support materials for the fabrication of nanocomposites providing both capacitive and Faradaic contributions to the final capacity. Although a variety of possibilities has been explored, the fundamental mechanism of the electrode reaction is still hazy. We herein report the investigation of Ti₃C₂T_<i>x</i> MXenes, the benchmark composition for application in energy storage, through the combined operando X-ray absorption spectroscopy (XAS) and Raman analysis supported by density functional theory (DFT) calculations with the aim of clarifying the origin and nature of capacity when the material was cycled vs Na. The electrode reaction determined was Ti₃C₂X₂ + 1Na → Na₁Ti₃C₂X₂, defining the theoretical capacity.