Cation-size-directed interfacial and electronic engineering of Ti₃C₂T_x MXenes via ionic liquid intercalation for high-rate proton storage

High-rate supercapacitors deliver rapid charge and discharge capabilities crucial for stabilizing intermittent renewable power, providing backup energy in microgrid disruptions, and mitigating energy surges in AI data centers. However, achieving both high-rate capability and structural stability—particularly in emerging MXene materials—remains a formidable challenge. Here, we report a cation-size-directed synthesis strategy using ionic liquids as both etchants and structure-directing agents to produce Ti₃C₂T_x MXenes with tunable interlayer spacing. By precisely matching the interlayer distance to the alkyl-chain length of the ionic liquid cations, we enable optimized ion accessibility and efficient surface utilization, thereby maximizing charge storage. In situ mechanistic investigations reveal that rotating cations within the MXene–electrolyte interface dynamically stabilize the layered microenvironment, mitigating restacking and accelerating ion transport. This work establishes a direct correlation among cation size, MXene interfacial structure, and electrochemical kinetics, highlighting the dual function of ionic liquids in material synthesis and electrolyte engineering. Collectively, these insights introduce a versatile design principle for constructing compact, high-rate, and durable energy storage devices.