Sequential K ⁺ and Cs ⁺ Intercalation into Ti ₃ C ₂ T _X -Reduced Graphene Oxide Hydrogel for Enhanced Performance of Zinc-Ion Hybrid Supercapacitors

Preintercalation engineering is an effective strategy for regulating MXene interlayer structures. However, the competitive behavior among different cations is rarely reported. Herein, we elucidate the competitive intercalation mechanism of Cs + in K + -preintercalated Ti 3 C 2 T X and its impact on the performance of zinc-ion hybrid supercapacitors (ZHSCs). K + preintercalation expands the interlayer galleries and lowers the diffusion barrier for the subsequent Cs + insertion. Because of its larger ionic radius and stronger electrostatic interaction, Cs + competitively replaces most of the preintercalated K +, resulting in a further enlarged and stabilized interlayer spacing. When the resultant K + /Cs + -Ti 3 C 2 T X is integrated into a 3D conductive hydrogel, restacking is effectively suppressed, and ion/electron transport is facilitated. For a ZHSC constructed with a zinc plate and an electrode based on reduced graphene oxide-supported K + /Cs + -Ti 3 C 2 T X hydrogel, a high specific capacitance of 255 F g –1 is achieved at 1 A g –1 with excellent cycling stability. Results from this work reveal the intrinsic significance of competitive cation intercalation in enhancing the performance of MXenes in energy-storage devices.