The Advance and Perspective on Electrode Materials for Metal–Ion Hybrid Capacitors

Metal–ion hybrid capacitors (MHC), which provide both high energy and high power density, play a key role as a bridge between the two energy storage methods of batteries and supercapacitors. The improvement of the electrochemical performance in these devices depends critically on the exploration of electrode materials which perform increased levels of energy storage. However, high-capacity capacitive materials are much needed to match high-energy battery-type electrodes. The kinetics between the capacitor-type electrodes and battery-type electrodes should be balanced. Benefiting from nanoscience and nanotechnology, their performance level will meet the requirements of practical applications. Although many researchers have reviewed various types of MHCs, available articles in recent years still lack a comprehensive and systematic overview of electrode materials in MHCs. Herein, the progress made in the field of hybrid ion capacitors is systematically reviewed, focusing on lithium, sodium, potassium, zinc, magnesium, calcium, and aluminum–ion hybrid capacitors. Initially, the device assembly modes and the charge storage mechanisms of MHCs are described. Subsequently, recent advances in electrode materials and pretreatment techniques for MHCs are highlighted. Finally, the challenges associated with MHCs are discussed and the continuing research directions are highlighted.