Revisiting the Electronic Structure of α-MnO2 for Energy Storage Application

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Herein, we report combined experimental and theoretical electronic structure analyses of cryptomelane-type α-MnO 2 with K intercalation to identify the ground-state electronic structure of the electrode for electrochemical energy storage application. Intercalation of K into the tunnels of α-MnO 2 provides enhancement in the lattice constants and dense agglomerated microstructure, which was observed from X-ray diffraction and scanning electron microscopy analysis, respectively. Mn 3+ and Mn 4+ mixed valence states are observed in the electrodes, which mediate the redox reaction observed from the in-operando Mn K -edge XANES and XAFS measurements. Oxygen vacancies are introduced in the electrode material with K intercalation, which align the valence band and conduction band to increase the band gap energy ( E g ) of the electrode. The density of states calculation using generalized gradient approximation (GGA) suggest that the Mn 3d, O 2p, and K 4s dominant spectral weight spread below the Fermi level from 2.0 to 8.0 eV, 2.5 to 8.0 eV, and 0.5 to 6.0 eV, respectively. A detailed comparison of the calculated density of states and measured valence band suggest that the Mn 3d - K 4s hybridization-induced mid-gap states appeared across the band gap and associated with charge-switching state, which accelerate the electrochemical redox reaction. Electrochemical performance is therefore improved with K intercalation into the tunnels of α-MnO 2, suggesting that the mid-gap-induced charge-switching state promotes the redox reaction.

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