Understanding the Lithium Storage Mechanism of Ti₃C₂Tₓ MXene

MXenes, as an emerging family of conductive two-dimensional materials, hold promise for late-model electrode materials in Li-ion batteries. A primary challenge hindering the development of MXenes as electrode materials is that a complete understanding of the intrinsic storage mechanism underlying the charge/discharge behavior remains elusive. This article presents two key discoveries: first, the characteristics of the Ti₃C₂Tₓ structure can be modified systematically by calcination in various atmospheres, and second, these structural changes greatly affect Li-ion storage behavior, which reveals the mechanism for lithium storage in Ti₃C₂Tₓ MXene. Specifically, via ammonization, the interlayer spacing gets dilated and uniform, giving rise to only one redox couple. In stark contrast, there are two well-recognized redox couples corresponding to two interlayer spacings in pristine Ti₃C₂Tₓ MXene, in which Li-ion (de)intercalation occurs between interlayers in a sequential manner as evidenced by ex situ X-ray diffraction (XRD). Notably, the XRD diffraction peaks shift hardly in the whole range of charge/discharge voltage, indicating a zero-strain feature upon Li-ion (de)intercalation. Moreover, the diffusion-controlled contribution percentage to capacity inversely depends on the scan rate. The understanding suggests a new design principle of the MXene anode: reduced lateral size to shorten the diffusion path and dilated interlayer spacing.