Tuning the electrochemical performance of Ti3C2 and Hf3C2 monolayer by functional groups for metal-ion battery applications

Abstract

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.