Among the post‐lithium battery technologies, potassium‐ion batteries are promising for cost‐effective large‐scale energy storage, as potash is an abundant resource. However, a major challenge is to understand the structure‐performance relationships of carbon anodes for potassium‐ion storage. In this study, we have designed a variety of carbon composite materials from 100 % graphite to 100 % soft carbon and in between, with tunable structural features to fundamentally understand the roles of different carbon structural features in potassium ion insertion. We have found that the graphite‐soft carbon composites (G‐SC) show a high charge capacity of 280.2 mAh g−1 with an increased initial coulombic efficiency, representing the best reversibility among different carbon composites. Electrochemical impedance spectroscopy, cyclic voltammetry, and ex‐situ structural characterizations have been applied to substantiate that the presence of soft carbon in G‐SC inhibits the solid electrolyte interface layer formation and provides structural protection to the graphitic layers.
