Insight into the unusual intercalation/deintercalation phenomena of alkali cations in the layered manganese oxide for electrochemical capacitors

The unusual intercalation phenomena of alkali cations (Li+, Na+, K+, Rb+, and Cs+) in the sulfate-based electrolyte on the electrochemical behavior of birnessite-type layered-manganese oxide nanosheets with Li-intercalated cation (Li-MnO2) were studied by in situ electrochemical synchrotron X-ray Absorption spectroscopy. Li-MnO2 exhibits the highest charge storage capacity in Na2SO4(aq), followed by Li2SO4, K2SO4, and Cs2SO4, respectively. Whilst, it does not exhibit the charge-storage characteristics in Rb2SO4 as the solvated Rb+ could not intercalate into the lamellar structure of birnessite. Understanding unusual intercalation phenomena can be useful for further development of the electrochemical capacitors.