The deployment of solar, wind and other volatile renewable energy sources and their integration with the electric grid needs the new types of energy storage which can provide the power, energy density and cycle life. Here we report a newly developed aqueous K-ion batteries used potassium iron hexacyanoferrate dihydrate as cathode, which are safe, inexpensive, and have superior high-rate, long cycle life electrochemical performance. The as-prepared crystalline nanocubes of potassium iron hexacyanoferrate dihydrate have a low strain open framework structure. Relying on the K⁺ ions insertion/extraction into/from the structure, the cathode reacts rapidly with very little hysteresis for hundreds cycling, as revealed by the electrochemical testing, ex-situ XRD, FTIR and kinetics analyses. Most importantly, the high K⁺ ion content ensures its high discharge capacity (120 mA h g^-1). It was also revealed the materials involves two-step redox reactions at charge/discharge plateaus of +0.92/ +0.84 V and +0.3/+0.2 V which should be ascribed to the high-spin state nitrogen-coordinated Fe³⁺/Fe²⁺-[N≡ couple and low-spin state carbon-coordinated Fe^III/Fe^II-[C≡ couple, respectively. The open-framework structure enables structural integrity, leading to the fast kinetics and highly reversible properties (> 85 % capacity retention over 500 cycles at 21.4 C) as revealed by the experiments and DFT calculation. The low-cost, scalable, simple synthesis process and excellent electrode performance of potassium iron hexacyanoferrate dihydrate make it attractive for large scale energy storage systems