Potassium-ion batteries (PIBs) are receiving increasing attention at present because of their cheap and lithium-like charge/discharge processes. Nevertheless, the large potassium-ion radius leads to poor potassium intercalation/depotassium kinetics and unstable structure, hindering their development. Here, we synthesized a novel carbon quantum dot-derived carbon nanosphere-encapsulated Ti3C2 MXene (CNS@Ti3C2) composite by polymer pyrolysis, while carbon nanospheres were derived from carbon quantum dots. The composites can suppress the layer stacking of Ti3C2 and prevent oxidation, thereby stabilizing the layered structure of Ti3C2 MXene and improving the cycle life. Besides, carbon nanospheres can increase the specific surface area and active sites, and then more potassium ions can enter the electrode material and boost the reversible capacity. Further, carbon nanospheres are embedded between the Ti3C2 layers, which can increase the interlayer spacing, and the potassium ions are more easily inserted and extracted, thereby improving the potassium storage power and rate performance. The CNS@Ti3C2 composite possesses an excellent synergy, resulting in a high reversible capacity of 229 mAh g–1 at 100 mA g–1 after 200 repeated cycles and a long cycle life of 205 mAh g–1 at 500 mA g–1 after 1000 repeated cycles with high coulombic efficiency (above 99%). This work offers a novel strategy to utilize carbon with MXene in energy storage.

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Potassium-ion batteries (PIBs) are of great importance in energy storage due to their natural abundance, low cost, and high operating potential compared to lithium and sodium. Herein, we first propose carbon quantum dot coating on VSe 2 nanosheets (VSe 2 @CQDs) as anodes for PIBs with the solvothermal and annealing methods. In the composite, carbon quantum dots are embedded in VSe 2 nanosheets, which inhibit the aggregation and volume expansion of the VSe 2 nanosheets during cycling, resulting in stabilizing the structure and long cycling stability. Besides, with abundant edge positions and surface oxygencontaining functional groups, small-sized carbon quantum dots have strong absorption of K + , enhancing the capacity of the composite. As a result, for PIBs, the composite exhibits a superior reversible capacity of 362 mA h g −1 after 200 cycles at 100 mA g −1 with a high capacity retention of 76.7%, high rate capacity (255 mA h g −1 at 1000 mA g −1 ), and a long-term cycling life of 162 mA h g −1 after 3000 cycles at 500 mA g −1 with a high coulombic efficiency (above 99%). The VSe 2 @CQD composite shows the best performances reported to date for long-term cycling stability, excellent reversible capacity, and rate capacity in TMD-based PIB anodic materials. The excellent performance and simple synthesis indicate the promising VSe 2 @CQD composite as a potential anode material for PIBs.

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