Mn3+ e(g) Configuration and Electron Transfer in Na-Incorporating alpha-MnO2 to Improve Electrochemical Supercapacitor: An In Situ and Ex Situ X-ray Absorption Spectroscopic Investigation

Cryptomelane-type alpha-MnO2 has a highspecific capacitanceand cyclic stability because of its combination of 1 x 1 and2 x 2 tunnel structures that are formed from the edge- and corner-sharingoctahedra. It, therefore, has the potential to host various cations,which improve its electrochemical properties. The Na incorporationprovides an opportunity for charge compensation by forming mixed-valenceMn(3+) and Mn4+ states in alpha-MnO2. This study shows that the incorporation of Na results in superiorelectrochemical properties by enhancing the specific capacitance andcapacity retention of alpha-MnO2 to 234.4 F g(-1) and similar to 96%, respectively. The electrochemical performance isimproved following Na incorporation by the induced phase competitionbetween alpha-MnO2 and the induced beta-MnO2, while the charge neutrality between Jahn-Teller (JT) activeMn(3+) and Mn4+ ions is maintained. The formedMn(3+)/Mn4+ mixed-valence species increase thestrength of the redox reaction at the electrode-electrolyteinterface, as observed from the in-situ Mn K-edge X-ray absorption near-edge structure (XANES) andthe extended X-ray absorption fine structure. Ex-situ Mn L (3,2)-edge XANES analysis furtherreveals that the introduction of e(g) from the Mn3+ 3d orbitals causes electron transfer via O 2p states by establishingthe double-exchange (DE) Mn3+-O-Mn4+ interaction. This study suggests that the electron exchange alsoinvolves the super-exchange (SE) Mn4+-O-Mn4+ interaction, based on field-dependent magnetization (M-H) measurement, which acceleratesthe redox reaction in alpha-MnO2. This work demonstratesthat JT active Mn3+ e(g) configuration is responsiblefor the great improvement of electrochemical properties that are inducedby the incorporation and insertion/extraction of Na+ ionswith the charge/discharge process, which causes electron hopping betweenMn(3+)/Mn4+ and Mn4+ through oxygenligand by DE/SE in the alpha-MnO2.