We propose potassium ferrous ferricyanide (KFe II [Fe III (CN) 6 ]) nanoparticles with a 3D open framework structure as a cathode for nonaqueous K-ion batteries. Electrochemical reaction mechanism analyses identify that two redox-active sites based on C and N coordinated Fe II /Fe III redox couples play a role in K-ion storage, and no phase change occurs in the different states of the initial and second charge-discharge processes. Thus, the KFe II [Fe III (CN) 6 ] electrode exhibits a high discharge capacity of 118.7 mA h g -1 at an operating voltage of 3.34 V and extremely excellent cycling stability with a capacity value of 111.3 mA h g -1 after 100 cycles at 10 mA g -1 . Moreover, an ultralong cycling lifespan of 1000 cycles with a high capacity retention of 80.49% and extraordinary voltage stability at 100 mA g -1 can be acquired. Ex situ characterizations verify that the outstanding electrochemical performance of KFe II [Fe III (CN) 6 ] is attributed to superior structural stability and electrochemical reversibility upon long-term cycling. Therefore, the KFe II [Fe III (CN) 6 ] material can make KIBs competitive in EES applications.