Recent advances in potassium-ion hybrid capacitors: Electrode materials, storage mechanisms and performance evaluation

For the purpose of bridging the gap between traditional batteries with high energy density and supercapacitors with high power output and long lifespan, hybrid capacitors whose dual electrodes are composed of a battery-type electrode and a capacitor-type electrode are gradually being valued. In recent years, with the unceasing progress of potassium-ion batteries (PIBs), the corresponding potassium-ion hybrid capacitors (PIHCs) have also attracted widespread attention. However, PIHCs still face quite a few problems such as poor cycle life, inferior rate performance and undesirable energy density and power density. It is generally believed that these unsatisfactory properties ascribe to the large radius of K+. For this reason, a lot of works focusing on the design of materials’ structures have been made. In fact, in addition to electrode materials, electrolytes, binders and separators will all have an important impact on the performances of PIHCs. In this review, we not only summarize various battery-type materials used in PIHCs, but also introduce capacitor-type materials, electrolytes, separators, binders, etc. that have been used in PIHCs. Besides, the charge storage mechanisms of PIHCs are discussed as well. In order to reflect the development status of PIHCs and look forward to their prospects, the comparison of PIHCs with other metal-ion hybrid capacitors is also discussed. Finally, some opinions and suggestions on PIHCs are presented. Hope this review can be helpful to readers.