For the first time, an ionic liquid was grafted onto Ti3C2Tx MXene interlayers (MXene-g-IL) using a radiation technique. The IL was tightly immobilized on the surface of MXene nanosheets via chemical linkage, which exhibited excellent specific capacitance (160 F g-1 at 5 mV s-1) and improved structural stability (maintaining the sheet-like structure for 180 days). The facile, efficient, and scalable synthetic strategy derived from the radiation technique can open a new avenue for covalent functionalization of MXene-based materials and promote their further application.