Metal selenide is contemplated as expeditious anode material for potassiumion batteries (KIBs). Here, nickel selenide (NiSe 2 -Ni 0.85 Se) was investigated as an anode material for KIBs. The NiSe x transformed into K 2 Se and K 2 Se 3 (K 2 Se x ) and reversibly returned to NiSe x after the discharge and charge processes (17NiSe 2 + 3Ni 0.85 Se + 38 K + + 38e À $ 19.55Ni + 10K 2 Se + 9K 2 Se 3 ). Furthermore, to enhance the electrochemical properties of nickel selenide, yolk-shell-structured nickel selenide and hollow carbon nanosphere composites, in which nickel selenides intensively existed in the core and were partially formed into nanocrystals in the shell, were produced by simple salt-infiltration and a post-treatment. NiSe x @C exhibited improved cycle and rate properties compared to bare NiSe x because of its structural merits that led to stable cycle performance and high electrochemical kinetics. NiSe x @C exhibited 373 mA h g À1 for the 100th cycle at 0.05 A g À1 and 234 mA h g À1 at 1.0 A g À1 .