Eliminating the Mn 3d Orbital Degeneracy to Suppress the Jahn–Teller Distortion for Stable MnO2 Cathode

Abstract The cycle stability of manganese‐based oxides cathode for sodium‐ion supercapacitor is impeded by severe structural collapse due to the Jahn–Teller (J–T) distortion of [MnO 6 ] octahedra as Mn 4+ is reduced to Mn 3+ . Herein, MnO 2 cathode is stabilized by Mg 2+ ions confinement between transition metal slabs. The Mg 2+ confinement induces the transformation from original cubic [MnO 6 ] octahedra to compressed configuration (com‐MnO 2 ), resulting in the elimination of Mn 3d orbital degeneracy, thereby alleviating the structural deformation of [Mn 3+ O 6 ] octahedra by suppressing J–T distortion during the reduction of Mn 4+ . As a result, the prepared com‐MnO 2 exhibits greatly improved cycling stability with 99.1% retention of its initial capacity after 20 000 cycles due to inhibited J–T distortion. Additionally, an enhanced capacity of 434 F g −1 at 1 A g −1 , a rate capacity of 275 F g −1 at 20 A g −1 can be achieved by com‐MnO 2 . The in situ XRD, Raman spectra reveal the reversible structural transformation during a cycle. It is concluded that the elimination of Mn 3d orbital degeneracy plays a crucial role in suppressing J–T distortion, which is conducive to in‐depth understanding of electron structure of transition metal oxides toward cycling performance.