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 [MnO6] octahedra as Mn(4+ )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 [Mn(3+)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.