K-Birnessite Electrode Obtained by Ion Exchange for Potassium-Ion Batteries: Insight into the Concerted Ionic Diffusion and K Storage Mechanism

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 mechanism is further revealed to be the concerted K + ion diffusion with quite low activation energies by first-principle simulations. These new findings provide new insights into electrode process kinetics, and lay a solid foundation for material design and optimization of potassium-ion batteries for large-scale energy storage.