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 [MnO₆] octahedra as Mn⁴⁺ is reduced to Mn³⁺. Herein, MnO₂ cathode is stabilized by Mg²⁺ ions confinement between transition metal slabs. The Mg²⁺ confinement induces the transformation from original cubic [MnO₆] octahedra to compressed configuration (com‐MnO₂), resulting in the elimination of Mn 3d orbital degeneracy, thereby alleviating the structural deformation of [Mn³⁺O₆] octahedra by suppressing J–T distortion during the reduction of Mn⁴⁺. As a result, the prepared com‐MnO₂ 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‐MnO₂. 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.