MXenes are well known for their potential application in supercapacitors due to their high-rate intercalation pseudocapacitance and long cyclability.However,the reported low capacity of pristine MXenes hinders their practical application in lithium-ion batteries.In this work,a robust strategy is developed to control the functional groups of Nb2CTx MXene.The capacity of pristine Nb2CTx MXene can be significantly increased by Li+ intercalation and surface modification.The specific capacity of the treated Nb2CTx is up to 448 mAh g-1 at 0.05 A g-1,and at a large current density of 2 A g-1 remains a high reversible capacity retention rate of 75％ after an ultra-long cycle of 2000 cycles.These values exceed most of the reported pristine MXenes (including the most studied Ti3C2Tx) and carbon-based materials.It demonstrates that this strategy has great help to improve the electrochemical performance of pristine MXene,and the results enhance the promise of MXenes in the application of lithium-ion batteries.