Metallic Mo2C Quantum Dots Confined in Functional Carbon Nanofiber Films toward Efficient Sodium Storage: Heterogeneous Interface Engineering and Charge-Storage Mechanism

Sodium ion capacitors (SICs) have drawn enormous interest due to their cost efficiency, superb power/energy densities, and long-span service life. Nevertheless, the imbalance of two involved electrodes in both kinetics and stability, mainly originating from battery-type anodes, restricts their practical application. Herein, we first propose a heterointerface engineering strategy to design a flexible self-supporting hybrid film anode, where metallic Mo2C quantum dots (QDs, ∼41.1 wt %) self-encapsulated in N-doped carbon nanofibers (N-CNFs) thanks to the interfacial interactions, toward advanced SICs. The synergistic effect of structural/compositional merits is highlighted with the induced interface coupling Mo–N–C toward enhanced electrochemical kinetics/stability and reinforced electrode structural integrity. The accelerating mechanism of electron migration at the heterogeneous interfaces is unveiled with density functional theory calculations. The obtained Mo2C QDs@N-CNFs film electrode is rendered with a competitive capacity of ∼160.9 mAh g–1 at 5.0 A g–1, robust pseudocapacitive contribution, and long-duration cycling stability. Besides, the Mo2C QDs@N-CNFs-based SICs exhibit exceptional electrochemical properties. More significantly, the in-depth insights into the unique Na+-(de)intercalation mechanism of Mo2C QDs@N-CNFs are rationally proposed with in situ X-ray diffraction and electrochemical techniques. This promises the enormous potential of our designed carbon-matrix-confined Mo2C QDs nanohybrid for SICs and beyond.