Carbon Quantum Dots-Derived Carbon Nanosphere Coating on Ti3C2 MXene as a Superior Anode for High-Performance Potassium-Ion Batteries

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.