KxCy phase induced expanded interlayer in ultra‐thin carbon toward full potassium‐ion capacitors

Abstract Carbonaceous materials have been regarded as highly promising anode candidates for potassium storage with their cost‐effectiveness and environmental benignity. However, low specific capacity and difficulty in large‐scale synthesis largely hinder their further development. Herein, a thermal‐induced potassium–carbon alloy phase (K x C y ) with the expanded interlayer spacing strategy is first put forward. Through in situ high‐temperature X‐ray diffraction, a K 2 C 2 phase is evoked by thermal energy during the in‐situ carbonization process of carbon quantum dots intermediate derived from potassium‐containing precursors, whereas no lithium or sodium–carbon alloy phase is observed from lithium/sodium‐containing precursors. The as‐obtained ultra‐thin carbon nanosheets achieve adjustable layer spacing, preparation in bulk, delivering reversible potassium storage of 403.4 mAh g −1 at 100 mA g −1 and 161.2 mAh g −1 even at 5.0 A g −1 , which is one of the most impressive K‐storage performances reported so far with great potential application. Furthermore, the assembled potassium‐ion hybrid capacitor by combining the impressive CFMs‐900 anode with the three‐dimensional framework‐activated carbon delivers a high energy‐power density of 251.7 Wh kg −1 at 250 W kg −1 with long‐term stability. This study opens a scalable avenue to realize the expanded interlayer spacing, which can be extended to other multicarboxyl potassium salts and can provide approach for the design of high‐performance carbon anode materials for potassium storage.