Abstract
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 Ti(3)C(2)T(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 Ti(3)C(2)T(x) accompanied by Ti redox reactions; (ii) partially reversible formation and decomposition of Li(2)O on the Ti(3)C(2)T(x) surface; and (iii) electrochemical decomposition of the sulfide solid electrolyte. The Li-intercalation behavior is strongly governed by the surface terminations of Ti(3)C(2)T(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.