Achieving ultra-long cycling life for MnO₂ cathode: Modulating Mn³⁺ spin state to suppress Jahn–Teller distortion and manganese dissolution

MnO₂ is emerging as an electrode material for sodium-ion capacitors due to its high specific capacity and low cost. However, Jahn-teller distortion and manganese dissolution pose a formidable challenge in practical applications. Herein, the Mn³⁺ spin state, causing J-T distortion, is modulated to address these issues, which is achieved through strategically incorporating Co into the MnO₂ lattice to increase electron occupancy in the t_2g orbital. The transition from a high-spin Mn³⁺ to a low-spin state leads to electron movement from the d_x²_-y² orbitals to the d_xy orbitals, which effectively lowers the energy level of the e_g orbitals, reduces orbital degeneracy, and enhances the stability of the Mn-O bond. DFT calculations, In situ Raman and inductively coupled plasma optical emission spectroscopy (ICP-OES) demonstrate that the distortion of [MnO₆] and Mn dissolution of Co-MnO₂ are reduced by 69 % and 80 % respectively compared with MnO₂ during the charge-discharge cycle. Consequently, Co-MnO₂ exhibits approximately 98 % capacity retention after 40,000 cycles at 10 A/g, achieving exceptional long-term stability. This study provides new insights into the relationships among J-T distortion, manganese dissolution and spin state, provides a novel approach to enhance the stability of MnO₂ for electrochemistry applications.