Rational design of surface termination of Ti₃C₂T₂ MXenes for lithium-ion battery anodes

Two-dimensional transition metal carbides, carbonitrides and nitrides (MXenes) have garnered increasing interest in the energy storage field due to their unique structural and electronic properties. However, the application performance is highly reliant on the surface termination, which is poorly understood from a chemical standpoint. In this work, the structural stability, chemical origin, electronic structure and lithium-ion (Li-ion) storage properties of 15 nonmetal terminated MXenes in the form of Ti₃C₂T₂ (T = B, C, Si, N, P, As, O, S, Se, Te, F, Cl, Br, I and OH) were investigated using first-principles calculations. The results indicate that the partially occupied d-orbital and zero pseudogap lead to the high chemical activity of surface Ti, and that surface terminations can diminish its chemical activity. Furthermore, a large pseudogap of the d-orbital promotes the structural stability of Ti₃C₂T₂. A useful descriptor, the antibonding state (E_σ*), was proposed to predict Li-ion adsorption energy. Combining the good electronic conductivity, high lithophilicity, low Li-ion diffusion barrier and high specific capacity, Ti₃C₂As₂, Ti₃C₂S₂ and Ti₃C₂Se₂ are considered as promising anode candidates for Li-ion batteries. Additionally, S, Se and As doping can improve the Li-ion storage performance of oxygen terminated Ti₃C₂O₂. This work offers insights into the chemical origin of the surface termination and paves the way for designing excellent Li-ion anode candidates based on MXenes.