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.