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

Abstract

Cryptomelane-type α-MnO 2 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 Mn 3+ and Mn 4+ states in α-MnO 2 . This study shows that the incorporation of Na results in superior electrochemical properties by enhancing the specific capacitance and capacity retention of α-MnO 2 to 234.4 F g –1 and ∼96%, respectively. The electrochemical performance is improved following Na incorporation by the induced phase competition between α-MnO 2 and the induced β-MnO 2, while the charge neutrality between Jahn–Teller (JT) active Mn 3+ and Mn 4+ ions is maintained. The formed Mn 3+ /Mn 4+ 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 L 3,2 -edge XANES analysis further reveals that the introduction of e g from the Mn 3+ 3d orbitals causes electron transfer via O 2p states by establishing the double-exchange (DE) Mn 3+ –O–Mn 4+ interaction. This study suggests that the electron exchange also involves the super-exchange (SE) Mn 4+ –O–Mn 4+ interaction, based on field-dependent magnetization ( M – H ) measurement, which accelerates the redox reaction in α-MnO 2 . This work demonstrates that JT active Mn 3+ e g 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 Mn 3+ /Mn 4+ and Mn 4+ through oxygen ligand by DE/SE in the α-MnO 2 .