Advances in Mg-based spinel oxides for supercapacitor applications

The increasing requirement for sustainable as well as high-performance energy storage systems has accelerated the research on advanced electrode materials for supercapacitors. Mg-based spinel oxides (MgB 2 O 4 ; B = Co, Fe, Mn, Al, etc.) are considered promising candidates among various transition-metal oxides owing to their structural stability, low cost, environmental friendliness, tunable electronic properties, and multiple redox-active centres. However, their practical application is still limited by poor electrical conductivity, slow charge transfer kinetics, insufficient utilization of electroactive sites, and structural degradation during long-term cycling. In this review, the crystal chemistry, charge-storage behavior, and electrochemical performance of Mg-based spinel oxides for supercapacitor applications are overviewed. Special attention is given to the structure-property-performance relationship controlling the electrochemical behavior. Recent advances in nanostructure engineering, doping strategies, and composite design are critically discussed as practical approaches to enhance ion/electron transport, increase electroactive surface area, and improve cycling stability. In addition, the functions of carbon-based composites, transition-metal oxide hybrids, and core–shell architectures for achieving synergistic electrochemical performance are systematically analysed. Comparison of specific capacitance, rate capability, and long-term durability of representative Mg-based systems such as MgCo 2 O 4 , MgFe 2 O 4 , MgMn 2 O 4, and MgAl 2 O 4 and device level studies, were also reported. Existing challenges in terms of conductivity limitations, scalable synthesis, practical device implementation, and performance standardization are also highlighted. Finally, future perspectives on rational interface engineering, advanced heterostructure design, hierarchical nanoarchitecture fabrication, and scalable fabrication strategies for next-generation high-performance supercapacitors.