Mn3+ eg Configuration and Electron Transfer in Na-Incorporating α-MnO2 to Improve Electrochemical Supercapacitor: An In Situ and Ex Situ X-ray Absorption Spectroscopic Investigation

Cryptomelane-type α-MnO2 has a high specific capacitance and cyclic stability because of its combination of 1 × 1 and 2 × 2 tunnel structures that are formed from the edge- and corner-sharing octahedra. It, therefore, has the potential to host various cations, which improve its electrochemical properties. The Na incorporation provides an opportunity for charge compensation by forming mixed-valence Mn3+ and Mn4+ states in α-MnO2. This study shows that the incorporation of Na results in superior electrochemical properties by enhancing the specific capacitance and capacity retention of α-MnO2 to 234.4 F g–1 and ∼96%, respectively. The electrochemical performance is improved following Na incorporation by the induced phase competition between α-MnO2 and the induced β-MnO2, while the charge neutrality between Jahn–Teller (JT) active Mn3+ and Mn4+ ions is maintained. The formed Mn3+/Mn4+ mixed-valence species increase the strength of the redox reaction at the electrode–electrolyte interface, as observed from the in-situ Mn K-edge X-ray absorption near-edge structure (XANES) and the extended X-ray absorption fine structure. Ex-situ Mn L3,2-edge XANES analysis further reveals that the introduction of eg from the Mn3+ 3d orbitals causes electron transfer via O 2p states by establishing the double-exchange (DE) Mn3+–O–Mn4+ interaction. This study suggests that the electron exchange also involves the super-exchange (SE) Mn4+–O–Mn4+ interaction, based on field-dependent magnetization (M–H) measurement, which accelerates the redox reaction in α-MnO2. This work demonstrates that JT active Mn3+ eg configuration is responsible for the great improvement of electrochemical properties that are induced by the incorporation and insertion/extraction of Na+ ions with the charge/discharge process, which causes electron hopping between Mn3+/Mn4+ and Mn4+ through oxygen ligand by DE/SE in the α-MnO2.