Phase transition-induced optimization approach for magnesium-ion storage in MnO2: Enabling high-performance in aqueous hybrid and asymmetric supercapacitors

The development of high-performance magnesium-ion-based hybrid capacitors and asymmetric supercapacitors is hindered by the limitations of cathode materials, particularly in terms of specific surface area and the availability of active sites. This study presents a phase transition approach to overcome these challenges by enhancing the magnesium-ion (Mg 2+ ) storage capacity of MnO 2 through a general, single-step method that is applicable to various MnO 2 phases. The method involves low-temperature treatment, which engenders a considerable increase in the specific surface area and the transformation of the material's morphology into a three-dimensional hierarchical structure. This structural change, along with the introduction of oxygen vacancies and fluoride anion doping, results in a noticeable improvement in the specific capacitance of the resulting material (denoted as MnO 2-x F y ). Utilizing K 1.33 Mn 8 O 16 as a case study, the specific surface area was significantly enhanced from an initial 39.6 to an impressive 255.5 m 2  g −1 following the treatment process, and the specific capacitance enhanced from 101.8 to 491.5 F g −1 at 0.5 A g −1 . Similar enhancements were achieved for other MnO 2 phases, including alpha-MnO 2 , Na 0.55 MnO 2 , and beta-MnO 2 . Furthermore, the as-obtained MnO 2-x F y materials were then used as the cathode to construct hybrid capacitors and asymmetric supercapacitors, which exhibited high specific capacitance and good cycling stability when paired with 3,4,9,10-perylenetetracarboxylic diimide (PTCDI) or activated carbon anodes. This work highlights a universal strategy for cathode material design that integrates structural and defect engineering, providing a foundation for the development of advanced magnesium-ion-based hybrid capacitors and asymmetric supercapacitors.