Tuning the electrochemical performance of TiC and HfC monolayer by functional groups for metal-ion battery applications.

It is extremely important to design and explore high-efficiency anode materials in metal-ion batteries with strong stability, good electronic conductivity, and high storage capacity. Mxenes are susceptible to functionalization due to the presence of dangling bonds on the surface; thus, their chemical properties can be tuned accordingly by functional groups, which provide an opportunity to design novel materials with good electrochemical performance. The geometry and stability of Ti3C2X2 and Hf3C2X2 (X = Si, P, S, and Cl) monolayers are explored with the aid of density functional theory and the ab initio molecular dynamics (AIMD) simulations. Ti3C2X2 and Hf3C2X2 (X = S, Cl) exhibit high thermodynamic stability than Ti3C2X2 and Hf3C2X2 (X = Si, P) as found from formation energy and AIMD simulations. Then, the electrochemical performance of S- and Cl-functionalized Ti3C2 and Hf3C2 monolayers was further explored for use as anode materials in metal-ion batteries (including Li, Na, K, Mg, Ca, and Al). The high structural stability, metallic nature, low diffusion energy barrier, and proper open circuit voltage make Ti3C2 and Hf3C2 monolayer-functionalized with S and Cl as rechargeable metal-ion anode materials. More importantly, the stable multilayer adsorption of Li and Na (Li and Na: up to two layers) ensures high capacities for the Ti3C2S2 monolayer in Li- and Na-ion batteries (462.86 and 462.86 mA h g-1, respectively). In particular, compared with other 2D materials, Ti3C2S2 monolayer exhibits a higher capacity when used as an anode electrode material for Mg-ion batteries, mainly due to the perfect matching of the diameter of Mg and the lattice constant of Ti3C2S2. The results show that S- and Cl-functionalized Mxenes are promising metal-ion anode materials and provide valuable insights into the next generation of energy storage and conversion devices. This discovery is of positive significance for the design of new MXenes.