First-Principles Study Of Li-Ion Storage Of Ti₂CT₂?T=F,OH? Monolayer With Vacancies

In recent years,two-dimensional?2D?materials,such as graphene and silicene,have attracted great interest in the energy storage field.A new 2D carbide or nitride materials MXenes has been prepared in 2011 and considered as electrode materials with a good application prospect.During the preparation process,defects will inevitably be introduced into the system and will affect the properties of the materials.The effects of these defects on energy storage performance of MXene have not been systematically studied.In this paper,we use the first-principles calculation method to study the effect of vacancies(including carbon vacancies V_C and titanium vacancies V_Ti)on the adsorption and diffusion behavior of Li atom on Ti₂C monolayer.Since Ti₂C monolayer is usually etched in HF solution from its precursor Ti₂AlC,it has a mixture of-O,-F,and-OH terminations.After the introduction of functional groups,the monolayer Ti₂CT₂?T=F,OH?becomes non-magnetic.Firstly,the corresponding vacancy formation energy of defective monolayer Ti₂C,Ti₂CF₂,and Ti₂C?OH?₂ has been calculated theoretically.The results show that in the above three configurations,carbon vacancies have the lowest formation energy?2.42 eV?in Ti₂C?OH?₂ monolayer and titanium vacancies have the lowest formation energy?2.92eV?in Ti₂C monolayer.Secondly,the adsorption behaviors of Li atom at different sites on monolayer Ti₂C,Ti₂CF₂ and Ti₂C?OH?₂ are studied,and the optimal diffusion paths and their corresponding diffusion barriers are determined.Interestingly,we find that the carbon vacancies?V_C?tend to enhance the adsorption of Li atom in Ti₂C monolayer.While the titanium vacancies(V_Ti)play a similar role in Ti₂CT₂ when functional groups present.The existence of vacancies has a great influence on the diffusion behavior of Li atom.In this context,we propose an idea to mitigate the adverse effects on Li diffusion performance by regulating the functional groups.In the presence of V_C,the surface of Ti₂C monolayer is suggested to be modified with-OH groups due to its relatively low diffusion barrier in the range of 0.025⁰.037 eV when Li diffuses around V_C.While in the presence of V_Ti,the surface is suggested to remove the functional groups resulting in a decrease of energy barrier about 1 eV when Li atom diffuses around V_Ti.The paper points out that the diffusion energy barrier of Li atom on the surface of monolayer Ti₂C is lower than that of the other common 2D electrode materials.And different surfaces of monolayer Ti₂C can be regulated to overcome the adverse effect of the presence of vacancies,thereby obtaining better lithium storage properties.