kind of secondary batteries will find promising applications in large-scale energy storage. [3] Owing to the low-cost [4] and low standard potential [5] of potassium (K), KIBs could in principle offer cost-effectiveness and higher energy density, which would lead to a substantial advance in energy storage technology. The fast K + ion diffusion property provides the possibility of saving the charging time of KIBs as the next generation energy storage and transport devices. [6] Then compared with Na-ion battery (NIB), a huge advantage lies in the application of graphite anode for KIB, [7] which is not feasible for Na + insertion into the graphite. [8] And also, the weaker Lewis acidity of K + ions ensures smaller solvated ions, leading to the higher mobility and larger transference number in electrolytes and lower desolvation energy, which offers faster K + ion diffusion kinetics with regard to that of Li + or Na + ions. [4,9] Based on these advantages, many investigations on the available anode materials for KIBs, such as carbonaceous materials, [8a,10] oxides, [11] sulfides, [12] organic materials, [13] alloy, [5] prussian blue analogues, [14] etc., have been reported continuously in the past few decades, [2a,15] however, fewer studies are devoted to the development of cathode. Present cathodes could be summarized as the layered transition metal oxides, [16] polyanions and pyrophosphates, [17] prussian blue analogs, [18] nanostructured iron composite, [19] organic materials, [20] and other potential materials. Up to now, most of the available studies are working on screening for new electrode materials with enhanced electrochemical performance, while the underlying K storage and transport mechanism in the electrode materials is lack of exploration and comprehension. [2a,21] Therefore, studying the electrochemical behavior of potassium ions is very important for building a better KIBs system.In specific, alkali transition metal oxides (AMO 2 : A = Li, Na, K; M = Ni, Co, Mn, Fe, etc.) with layered structure have been widely studied as promising cathodes for rechargeable batteries, [22] because of the topotactic de/intercalation feature. However, the accommodation of the bulky K + ion is less favorable than that of Li + and Na + ion in such compact structures, [2a] which will plausibly hinder the K + ion diffusion kinetics. [23] Previous reports [16e,24] have shown the sluggish diffusion performance of K + ions in KIBs with alkali transition Novel and low-cost rechargeable batteries are of considerable interest for application in large-scale energy storage systems. In this context, K-Birnessite is synthesized using a facile solid-state reaction as a promising cathode for potassium-ion batteries. During synthesis, an ion exchange protocol is applied to increase K content in the K-Birnessite electrode, which results in a reversible capacity as high as 125 mAh g −1 at 0.2 C. Upon K + exchange the reversible phase transitions are verified by in situ X-ray diffraction (XRD) characterization. The underlying mechani...