Loftager Simon, García-Lastra Juan María, Vegge Tejs
Department of Energy Conversion and Storage, Technical University of Denmark, Fysikvej, 2800 Kgs. Lyngby, Denmark.
Phys Chem Chem Phys. 2017 Jan 18;19(3):2087-2094. doi: 10.1039/c6cp06312h.
Lithium iron borate (LiFeBO) is a promising cathode material due to its high theoretical specific capacity, inexpensive components and small volume change during operation. Yet, challenges related to severe air- and moisture-induced degradation have prompted the utilization of a protective coating on the electrode which also improves the electronic conductivity. However, not much is known about the preferential geometries of the coating as well as how these coating-electrode interfaces influence the lithium diffusion between the coating and the electrode. Here, we therefore present a density functional theory (DFT) study of the anchoring configurations of carbon coating on the LiFeBO electrode and its implications on the interfacial lithium diffusion. Due to large barriers associated with Li-ion diffusion through a parallel-oriented pristine graphene coating on the FeBO and LiFeBO electrode surfaces, large structural defects in the graphene coating are required for fast Li-ion diffusion. However, such defects are expected to exist only in small concentrations due to their high formation energies. Alternative coating geometries were therefore investigated, and the configuration in which the coating layers were anchored normal to the electrode surface at B and O atoms was found to be most stable. Nudged elastic band (NEB) calculations of the lithium diffusion barriers across the interface between the optimally oriented coating layers and the electrode show no kinetic limitations for lithium extraction and insertion. Additionally, this graphite-coating configuration showed partial blocking of electrode-degrading species.
硼酸铁锂（LiFeBO）因其较高的理论比容量、廉价的成分以及运行过程中较小的体积变化，是一种很有前景的阴极材料。然而，与空气和水分引起的严重降解相关的挑战促使人们在电极上使用保护涂层，这也提高了电子导电性。然而，关于涂层的优先几何形状以及这些涂层 - 电极界面如何影响锂在涂层和电极之间的扩散，人们了解得并不多。因此，在这里我们提出了一项关于碳涂层在LiFeBO电极上的锚固构型及其对界面锂扩散影响的密度泛函理论（DFT）研究。由于锂离子通过FeBO和LiFeBO电极表面上平行取向的原始石墨烯涂层扩散存在较大能垒，石墨烯涂层中需要有大的结构缺陷才能实现快速锂离子扩散。然而，由于这些缺陷的形成能很高，预计它们仅以低浓度存在。因此研究了替代的涂层几何形状，发现涂层在B和O原子处垂直于电极表面锚固的构型最稳定。对最佳取向的涂层与电极之间界面上锂扩散势垒的推挤弹性带（NEB）计算表明，锂的脱出和嵌入没有动力学限制。此外，这种石墨涂层构型显示出对电极降解物种有部分阻挡作用。
