Real-Time Imaging of Intercalation-Conversion Li Storage in MXenes for Solid-State Batteries

2D transition metal carbides and nitrides (MXenes) are promising electrode materials for next-generation energy storage devices. However, their charge-storage mechanisms in solid-state systems remain poorly understood, hindering further performance optimization. Here, Li-ion intercalation and conversion reactions in Ti₃C₂T_x are directly visualized in sulfide-based solid-state Li batteries using operando scanning transmission electron microscopy combined with electron energy-loss spectroscopy. The real-time observations reveal three distinct reaction pathways: (i) Li (de)intercalation within the interlayer spacings of Ti₃C₂T_x accompanied by Ti redox reactions; (ii) partially reversible formation and decomposition of Li₂O on the Ti₃C₂T_x surface; and (iii) electrochemical decomposition of the sulfide solid electrolyte. The Li-intercalation behavior is strongly governed by the surface terminations of Ti₃C₂T_x: O-terminated MXenes enable efficient Li accommodation and redox activity at room temperature, whereas F- and Cl-terminated MXenes require elevated temperatures for sufficient Li penetration. These findings provide direct nanoscale evidence of intercalation and conversion processes and highlight surface-termination engineering as an effective strategy to enhance Li accommodation and improve their redox utilization in all-solid-state battery systems.