First-Principle Investigation Of Li Diffusion In Copper Thin Film

Copper thin film is the most commonly used material for current collector of anode for Li-ion batteries because it is stable during charge/discharge processes. Recently. Suzuki et al discovered that the mass transfer of lithium ion in the copper lattice is possible, which arouses people's intensive interesting. Based on the density functional theory, the first principles calculation was performed to investigate the bulk, surface copper and the diffusion in the copper thin film. The related results are obtained by the computer simulation and design and calculating, including the copper lattice constant, surface energy, vacancy formation energy, vacancy diffusion barriers and the lithium diffusion barriers that is the major purpose of the paper, respectively. General speaking, some up to date results are presented here:First principles calculations are carried out to study the basic properties of bulk copper, which is also the start of the work. The calculated results, which are in good agreement with experimental and theoretical results, showed that GGA can give more accurate calculations, and copper lattice constant and cohesive energy are 0.3635nm, 3.728eV respectively. As a result, the lattice constant is often used in the following work.Ab initio calculations are performed, using a slab model with 6 atom-layers, to investigate the character of clean surface, surface with vacancy. Calculated results reveal that the surface energy of Cu(111) is lowest and the relaxation is smallest, which also agree well with the experimental results. The valence band moves towards the Fermi energy. The viewpoints on the copper vacancies form more easily in the surface are presented due to the vacancy formation energy of surface, subsurface and middlesurface layer respectively.Ab initio molecular dynamics are used to study the diffusion mechanism of Li motion in copper thin film. Two methods are applied in the work: First, adiabatic trajectory method was used to determine the diffusion barriers corresponding to various diffusion mechanisms, and the lowest value is about 0.6eV. According to the calculated results, for the first time, we present that the nearest neighbor vacancy assisted jumping is d the most probable diffusion mechanism. It is found that freer diffusion may be observed by increasing the number of copper vacancies in the thin film, and it is also found there exits a relation between the thickness of copper thin film and lithium diffusion barrier. Second, full molecular dynamics is performed to discuss the effect of temperature. After local relaxations have been finished, simulation begin with T=273K and 1273K. respectively. The mean square displacements (MSD) of the Li and Cu atoms as a function of time are clearly shown. It indicatives when the temperature is increased Cu atoms are oscillating around their original sites, while Li shows a remarkable diffusion behavior.