Department of Energy Science, BK21 Physics Division, Graphene Center, Sungkyunkwan Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon, Republic of South Korea.
J Am Chem Soc. 2012 May 23;134(20):8646-54. doi: 10.1021/ja301586m. Epub 2012 May 11.
Coexistence of both edge plane and basal plane in graphite often hinders the understanding of lithium ion diffusion mechanism. In this report, two types of graphene samples were prepared by chemical vapor deposition (CVD): (i) well-defined basal plane graphene grown on Cu foil and (ii) edge plane-enriched graphene layers grown on Ni film. Electrochemical performance of the graphene electrode can be split into two regimes depending on the number of graphene layers: (i) the corrosion-dominant regime and (ii) the lithiation-dominant regime. Li ion diffusion perpendicular to the basal plane of graphene is facilitated by defects, whereas diffusion parallel to the plane is limited by the steric hindrance that originates from aggregated Li ions adsorbed on the abundant defect sites. The critical layer thickness (l(c)) to effectively prohibit substrate reaction using CVD-grown graphene layers was predicted to be ∼6 layers, independent of defect population. Our density functional theory calculations demonstrate that divacancies and higher order defects have reasonable diffusion barrier heights allowing lithium diffusion through the basal plane but neither monovacancies nor Stone-Wales defect.
在石墨中，边缘平面和基面共存通常会阻碍对锂离子扩散机制的理解。在本报告中，通过化学气相沉积 (CVD) 制备了两种类型的石墨烯样品：(i) 在 Cu 箔上生长的具有明确基面的石墨烯和 (ii) 在 Ni 膜上生长的富含边缘平面的石墨烯层。根据石墨烯层数的不同，石墨烯电极的电化学性能可以分为两个区域：(i) 腐蚀主导区域和 (ii) 锂化主导区域。缺陷有利于锂离子在石墨烯基面垂直方向上的扩散，而在平行于平面的方向上扩散则受到源于吸附在丰富缺陷位上的聚集 Li 离子的空间位阻的限制。使用 CVD 生长的石墨烯层有效阻止基底反应的临界层厚度 (l(c)) 预测为约 6 层，与缺陷密度无关。我们的密度泛函理论计算表明，双空位和更高阶缺陷具有合理的扩散势垒高度，允许锂离子通过基面扩散，但单空位和 Stone-Wales 缺陷则不行。
