Flexible Carbon Dots-Intercalated MXene Film Electrode with Outstanding Volumetric Performance for Supercapacitors

2D MXenes have emerged as promising supercapacitor electrode materials due to their metallic conductivity, pseudo-capacitive mechanism, and high density. However, layer-restacking is a bottleneck that restrains their ionic kinetics and active site exposure. Herein, a carbon dots-intercalated strategy is proposed to fabricate flexible MXene film electrodes with both large ion-accessible active surfaces and high density through gelation of calcium alginate (CA) within the MXene nanosheets followed by carbonization. The formation of CA hydrogel within the MXene nanosheets accompanied by evaporative drying endow the MXene/CA film with high density. In the carbonization process, the CA-derived carbon dots can intercalate into the MXene nanosheets, increasing the interlayer spacing and promoting the electrolytic diffusion inside the MXene film. Consequently, the carbon dots-intercalated MXene films exhibit high volumetric capacitance (1244.6 F cm⁻³ at 1 A g⁻¹), superior rate capability (662.5 F cm⁻³ at 1000 A g⁻¹), and excellent cycling stability (93.5% capacitance retention after 30 000 cycles) in 3 m H₂SO₄. Additionally, an all-solid-state symmetric supercapacitor based on the carbon dots-intercalated MXene film achieves a high volumetric energy density of 27.2 Wh L⁻¹. This study provides a simple yet efficient strategy to construct high-volumetric performance MXene film electrodes for advanced supercapacitors.