Recent progress and prospects in anode materials for potassium-ion capacitors

Potassium-ion capacitors (PICs) are promising energy storage devices, which are competitive with lithium-ion and sodium ion capacitors. PICs combine the advantages of a battery-type anode and a capacitive cathode, resulting in a low cost, high energy density, high power density and long cycle life. However, there is still a mismatch between the anode and cathode materials for achieving the optimum specific capacity and kinetics in PICs. Early studies have shown that the careful selection of electrode materials and their optimization is an effective way to solve this problem. We focus on the development of PIC anode materials including insertion-type and conversion-type materials. The insertion-type materials include carbon materials (graphite, soft carbons, hard carbons, etc.), K-based titanates, MXenes, and dipotassium terephthalate. The conversion-type materials include metal sulfides/selenides, metal phosphides and sodium super ionic conductor-type phosphates (NASICON). Their preparation, structural characteristics, electrochemical performance as anode materials in half-cell and PIC devices are summarized and discussed. The future prospects and challenges of PICs are also considered.