O- and S‑Terminated M₂C MXenes as Anode Materials for Na/K-Ion Batteries

Searching for high-performance anode materials is a critical issue for Na/K-ion batteries (NIBs/KIBs). First-principles calculations were carried out to evaluate O- and S-terminated M₂C MXenes as anode materials of NIBs and KIBs by a comparative analysis. For the metal (M) of MXenes in the same period on the periodic table, the adsorption strength of the Na/K-ion on M₂CO₂ increases when the number of valence electrons (Ne) of M increases from four to six, whereas the trend on M₂CS₂ is opposite to that on M₂CO₂; the diffusion barriers of the Na/K-ion on M₂CO₂ and M₂CS₂ both decrease with increasing Ne. On the same M₂C MXene, the diffusion barrier of the Na/K-ion on M₂CS₂ is less than that on M₂CO₂. Many MXenes show low average open circuit voltage (OCV), low diffusion barrier, and high capacity as anodes of NIBs. However, all studied MXenes are not suitable as anodes of KIBs owing to their low capacities.