石墨嵌锂的结构转变和弹性性质计算研究
Calculation of structural transformations and elastic properties of graphite in the insertion process of lithium

采用基于密度泛函理论的第一性原理平面波赝势方法，计算了锂离子电池石墨负极在嵌Li过程中形成石墨嵌层化合物LixC6(0≤x≤1)的形成能、嵌锂平台、晶体结构、电子结构和弹性性质的变化规律。结果表明，随着嵌Li量x增加，LixC6的体系总量能逐渐降低，形成能逐渐增大，嵌Li反应逐渐变得困难；计算得到石墨的嵌Li电位逐渐降低，这与实验测得的充放电曲线具有良好的一致性。石墨嵌Li导致碳层发生滑移，晶胞体积逐渐增大，当x为1时晶胞体积增大约12.75%。随着x增加，LixC的费米能态密度几乎呈现增大趋势，有利于增强电子导电性。随着x增加，平行于碳平面的杨氏模量Ea和Eb都呈现小幅下降趋势，而垂直于碳平面的杨氏模量Ec呈现大幅度增加趋势，导致石墨刚性逐渐增大，结构稳定性变差。
The formation energies, average intercalation voltages, crystal structures, electronic structures and elastic properties of graphite intercalate compounds LixC6 (0≤x≤1) formed in the process of lithium insert into graphite for lithium ion battery anode were investigated by means of the first-principles plane-wave pseudopotentials method based on the density functional theory(DFT). The results show that the total energies of LixC6 decrease gradually and the formation energies of LixC6 increase gradually with increasing Li concentration, indicating the reaction of lithium insert into graphite become more difficulty. The average lithium intercalation voltages of LixC6 decrease gradually, which are well consistent with the charge/discharge curves of graphite obtained from experiment. The carbon layers of graphite slip in the process of lithium insert into graphite and the cell volumes of LixC6 expand, the maximum of expansion rate could reach 12.75% as x is 1. The density of states of LixC at the Fermi level increase with increasing Li concentration, indicating the improvement of electrical conductivity. The Young's modulus Ea and Eb which are parallel to carbon surface drop slightly and Ec which are perpendicular to carbon surface increase obviously, which results the rigidity of graphite increase and structure stability become worse.