Co-doped MnO₂ with abundant oxygen vacancies as a cathode for superior aqueous magnesium ion storage

Rapid capacity degradation caused by poor structural stability and slow reaction kinetics is the main obstacle faced by the cathode materials of current aqueous magnesium ion hybrid supercapacitors (MHSs). Herein, we propose and evaluate Co-doped MnO₂ (Co-MnO₂) with abundant oxygen vacancies as the cathode material for MHSs. Comprehensive studies show that the decrease in the combined valence of Mn caused by Co doping leads to more oxygen vacancies, which improves the electronic conductivity, exposes more active sites, and promotes the adsorption/desorption behavior of Mg²⁺. In addition, Co intercalating between the MnO₂ layers improves the stability of the electrode material by enhancing the strength of the interlayer framework. Consequently, an MHS based on the Co-MnO₂ cathode and AC anode exhibits an excellent energy density of 79.6 W h kg⁻¹ at a power density of 360 W kg⁻¹ and amazing long-term cycling life with 94.8% capacity retention after 15 000 cycles. This work demonstrates that the Co doping strategy can enhance the electrochemical performance of MnO₂ and opens up a new horizon for developing high-performance candidate cathodes in aqueous MHSs.