Intercalation-type batteries based on the alkali metal K, an earth-abundant element, are attracting increasing attention as alternatives to Li-ion batteries. However, the lack of a high-performance anode limits the success of K-ion batteries. Here, we study the performance of monolayer C 5 N as a potential building block for the anode. Our theoretical data show that the metallic monolayer can accommodate 2.25 K atoms per formula unit, which corresponds to a specific capacity of 814 mA h g −1 ,

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... nd provides a low diffusion barrier of only 0.11 eV. The volume change of bulk C 5 N (47%) is found to be smaller than that of graphite (61%; commercial anode material). These predictions will serve the design of future anode materials for K-ion batteries.