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

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.