Novel Two-dimensional Materials Utilized As Li-or Non-li-ion Batteries Anode From First-principles Study

Nowadays,in the field of energy storage devices,Li-or non-Li-ion batteries have obtained the same attention.Based on the density functional theory?DFT?,first-principle calculation not only explains the mechanisms behind the phenomenon,but also can predict whether the materials on the atomic scale can be used as Li-or non-Li-ion batteries electrode materials.Using first-principles,three kinds of two-dimensional materials utilized as Li-or non-Li-ion batteries anode were studied in the thesis on geometric structure,electronic structure and ion diffusion and theoretical capacity,etc.,the details are as follows:we studied the electronic properties and Li storage capability of V₂C and its F/OH functionalized derivatives.The bare V₂C monolayer is metallic with antiferromagnetic configuration,while its derived V₂CF₂ and V₂C?OH?₂ in their stable configurations are antiferromagnetic semiconductors.Li adsorption could result in the total net magnetic moments of the system,and could promote the electric conductivity of V₂C fluoride and hydroxide.The bare V₂C monolayer exhibits low diffusion barrier for Li atom and high Li storage capacity,whereas the passivated F or OH atoms on the surface tend to block Li diffusion and reduce the Li storage capacity.The average intercalation potentials for V₂C-based materials are calculated to be relatively low.Our results show that the V₂C monolayer could be a promising anode material for Li-ion batteries.To enhance its performance,methods need to be devised to reduce the surface functionalization of F and OH functional groups as much as possible.Using first-principles method,we then predict that 2D nitrogen electride materials can be served as anode materials for NIBs.Particularly,we show that Ca₂N meets almost all the requirements of a good NIBs anode.Each formula unit of a monolayer Ca₂N sheet can absorb up to four Na atoms,corresponding to a theoretical specific capacity of 1138mAh·g^-1.The metallic character for both pristine Ca₂N and its Na intercalated state Na_xCa₂N ensures good electronic conduction.Na diffusion along the 2D monolayer plane can be very fast even under room temperature,with a Na migration energy barrier as small as 0.084 e V.These properties are key to excellent rate performance of an anode material.The average open circuit voltage is relatively low,which is beneficial to the overall voltage of the cell.In addition,the 2D monolayers are almost zero-strain electrodes with very small lattice change upon Na intercalation,which ensures a very good cycling stability.All these results demonstrate that the Ca₂N monolayer is an excellent anode material for NIBs.Finally,first-principles calculations are performed to study the electronic properties and metal ion storage capabilities of the two-dimensional?2D?Nb₂C monolayer and its corresponding fluoride and hydroxide materials.We show that the Nb₂C monolayer and the derived Nb₂CF₂ and Nb₂C?OH?₂ are all metallic in their most stable configurations.We systematically investigate the adsorption and surface diffusion of different metal atom species A=?Li,Na,K,Be,Mg,Ca,Al?.We find that the bare Nb₂C monolayer has excellent performance in the case of Li or Mg:the material remains metallic after adsorption;the ion diffusion is fast with extremely low diffusion barrier;the storage capacity is high??542mAh/g for Li and?1084 mAh/g for Mg?;and the average intercalation potential is relatively low.Particularly,the diffusion barrier heights for the elements Li,Na,K,Mg,and Ca are all lower than 0.1 eV.In addition,the functional groups tend to strongly degrade the performance,which should be avoided in experiment as much as possible.Our results suggest that the Nb₂C monolayer is a promising anode material for Li-or non-Li-ion batteries.