Achieving superior high-capacity K-ion batteries with the C57 carbon monolayer anode by first-principles calculations

K-ion batteries (KIBs) have attracted much attention due to their low cost, high operating voltage and fast ionic conductivity in the electrolyte. However, the development of KIBs lags because the ion size and mass of K+ are larger than Li+ and Na+. Benefitting from the small molar weight of carbon and its structure diversity, the twodimensional carbon allotropes can be considered as a potential electrode material for batteries with large-radius ions. In the paper, we report a metallic carbon allotrope (C57 monolayer) as an excellent candidate for the anode material of KIBs. The low diffusion barrier and small OCV values indicate that the C57 monolayer has excellent mobility properties and a large driving force for K-ion diffusion in the charge/discharge process. Most remarkably, the maximum capacity up to 1117 mA h g(-1) is three times as high as the capacity of graphite-based commercial electrode. Based on our results, the C57 monolayer is ideal for an excellent anode material for KIBs.