Potassium-ion batteries (PIBs) are promising candidates for next-generation energy storage devices due to the earth abundance of potassium,low cost,and stable redox potentials.However,the lack of promising high-performance electrode materials for the intercalation/deintercalation of large potassium ions is a major challenge up to date.Herein,we report a novel uniform nickel selenide nanoparticles encapsulated in nitrogen-doped carbon (defined as “NiSe@NC”) as an anode for PIBs,which exhibits superior rate performance and cyclic stability.Benefiting from the unique hierarchical core-shell like nanostructure,the intrinsic properties of metal-selenium bonds,synergetic effect of different components,and a remarkable pseudocapacitance effect,the anode exhibits a very high reversible capacity of 438 mA·h·g-1 at 50 mA·g-1,an excellent rate capability,and remarkable cycling performance over 2,000 cycles.The electrochemical mechanism were investigated by the in-situ X-ray diffraction,ex-situ high-resolution transmission electron microscopy,selected area electron diffraction,and first principle calculations.In addition,NiSe@NC anode also shows high reversible capacity of 512 mA·h·g-1 at 100 mA·g-1 with 84％ initial Coulombic efficiency,remarkable rate performance,and excellent cycling life for sodium ion batteries.We believe the proposed simple approach will pave a new way to synthesize suitable anode materials for secondary ion batteries.