Carbon dot enhanced micro-explosion for ion-accessible and highly conductive MXene films via fast Joule heating

Thermal treatment is commonly used to enhance the energy storage performance of MXene film by inducing structural reconstruction. However, it further densifies the intrinsically restacked nanosheets, restricting ion transport, and can induce oxidation that degrades active sites. These coupled adverse effects are frequently overlooked. Here, we present a synergistic strategy combining fast Joule heating with carbon dots (CDs) to construct dense yet ion-accessible MXene films. Electrostatic assembly anchors ultrasmall CDs onto MXene without compromising dense packing. During second-scale Joule heating, rapid gas release from CDs and MXene collectively triggers a confined micro-explosion effect, leading to localized macropore formation accompanied by intensified overall densification, which simultaneously enhances ion transport by creating accessible channels and preserves fast electron conduction through densified regions. Simultaneously, CDs suppress MXene oxidation during thermal treatment by anchoring at defect-rich sites and forming a carbon-rich interfacial layer that passivates active sites. The optimally treated CDs/MXene films deliver a ∼49% increase in specific capacitance (486 F/g at 0.1 A/g), a 549% enhancement in electrical conductivity, and outstanding capacitance retention of 75.4% at 10 A/g. This work establishes CDs as active thermal-structural modulators that fundamentally break the intrinsic trade-off between conductivity enhancement and ion accessibility in thermally treated MXene films.