Advanced cathode materials for metal ion hybrid capacitors: Structure and mechanisms

Metal ion hybrid capacitors (MIHCs) consist of both capacitance-type electrodes and battery-type electrodes, thereby combining the advantages of metal ion batteries and supercapacitors. Nevertheless, the disparity between the specific capacity and rate performance of the cathode and anode materials suggests that the current performance of MIHCs does not meet practical requirements. The performance of MIHCs is affected by the type, composition, and morphology of the cathode material, affirming their essential role as critical components. Therefore, there is a need to concentrate on and discuss the electrochemical properties of electrode materials, as well as strategies aimed at enhancing their electrochemical performance. This article introduces the composition, types, and energy storage mechanisms of MIHCs. Subsequently, a review is conducted on the cathode materials used in lithium, sodium, potassium, and zinc ion capacitors, with emphasis on their physical and chemical properties and the resulting electrochemical performance. Moreover, a comprehensive investigation is conducted to explore effective strategies for enhancing the electrochemical properties of materials, incorporating a substantial number of theoretical calculations and experimental results. Finally, this review provides valuable suggestions for the development and large-scale production of cathode electrode materials for MIHCs (Li+, Na+, K+, and Zn2+), addressing current issues and proposing potential research directions.