Potassium-ion Intercalation Mechanism in Layered Na2Mn3O7

Potassium-ion batteries (KIBs) are emerging as promising candidates for large scale applications due to their low cost, elemental abundance, and smaller Stokes radius of K+ relative to Li+ and Na+ ions. However, the large size of K+ ion remains the bottleneck for the development of KIBs. Herein, layered sodium manganese oxide Na2Mn3O7 is demonstrated as a promising intercalation host for K ion for the first time. Prepared by facile one-step solid-state synthesis, Na2Mn3O7 compound crystallized in a triclinic structure (space group P1̅). An initial discharge capacity of 152 mA h g–1 at a current rate of C/20 was realized with a nominal voltage of 2.1 V vs K/K+. The mechanistic aspects of K+ ion (de)intercalation was investigated using X-ray photoelectron spectroscopy (XPS), ex situ X-ray diffraction (XRD), and potentiostatic intermittent titration technique (PITT). Similar to the Na+-ion insertion, the K+-ion insertion into Na2Mn3O7 involves a two-phase redox mechanism. The layered Na2Mn3O7 forms a potential insertion material for low cost K-ion batteries.