Metal-organic framework-derived NiCo 2.5 S 4 microrods wrapped in reduced graphene oxide (NCS@RGO) were synthesized for potassium-ion storage.Upon coordination with organic potassium salts,N CS@RGO exhibits an ultrahigh initial reversible specific capacity (602 mAh g À1 at 50 mA g À1 ) and ultralong cycle life (a reversible specific capacity of 495 mAh g À1 at 200 mA g À1 after 1900 cycles over 314 days). Furthermore,t he battery demonstrates ah igh initial Coulombic efficiency of 78 %, outperforming most sulfides reported previously.A dvanced ex situ characterization techniques,i ncluding atomic force microscopy, were used for evaluation and the results indicate that the organic potassium salt-containing electrolyte helps to form thin and robust solid electrolyte interphase layers,w hichr educe the formation of byproducts during the potassiation-depotassiation process and enhance the mechanical stability of electrodes.T he excellent conductivity of the RGO in the composites,a nd the robust interface between the electrodes and electrolytes,i mbue the electrode with useful properties;i ncluding,u ltrafast potassium-ion storage with areversible specific capacity of 402 mAh g À1 even at 2Ag À1 .