Kim Sung-Yup, Ostadhossein Alireza, van Duin Adri C T, Xiao Xingcheng, Gao Huajian, Qi Yue
Department of Chemical engineering & Material Science, Michigan State University, East Lansing, MI 48824, USA.
Phys Chem Chem Phys. 2016 Feb 7;18(5):3706-15. doi: 10.1039/c5cp07219k. Epub 2016 Jan 13.
Surface coatings as artificial solid electrolyte interphases have been actively pursued as an effective way to improve the cycle efficiency of nanostructured Si electrodes for high energy density lithium ion batteries, where the mechanical stability of the surface coatings on Si is as critical as Si itself. However, the chemical composition and mechanical property change of coating materials during the lithiation and delithiation process imposed a grand challenge to design coating/Si nanostructure as an integrated electrode system. In our work, we first developed reactive force field (ReaxFF) parameters for Li-Si-Al-O materials to simulate the lithiation process of Si-core/Al2O3-shell and Si-core/SiO2-shell nanostructures. With reactive dynamics simulations, we were able to simultaneously track and correlate the lithiation rate, compositional change, mechanical property evolution, stress distributions, and fracture. A new mechanics model based on these varying properties was developed to determine how to stabilize the coating with a critical size ratio. Furthermore, we discovered that the self-accelerating Li diffusion in Al2O3 coating forms a well-defined Li concentration gradient, leading to an elastic modulus gradient, which effectively avoids local stress concentration and mitigates crack propagation. Based on these results, we propose a modulus gradient coating, softer outside, harder inside, as the most efficient coating to protect the Si electrode surface and improve its current efficiency.
作为人工固体电解质界面的表面涂层，已被积极探索为提高高能量密度锂离子电池纳米结构硅电极循环效率的有效方法，其中硅表面涂层的机械稳定性与硅本身同样关键。然而，在锂化和脱锂过程中涂层材料的化学成分和机械性能变化，对设计作为集成电极系统的涂层/硅纳米结构构成了巨大挑战。在我们的工作中，我们首先开发了用于Li-Si-Al-O材料的反应力场（ReaxFF）参数，以模拟硅核/Al2O3壳和硅核/SiO2壳纳米结构的锂化过程。通过反应动力学模拟，我们能够同时跟踪并关联锂化速率、成分变化、机械性能演变、应力分布和断裂情况。基于这些变化特性，开发了一种新的力学模型，以确定如何通过临界尺寸比来稳定涂层。此外，我们发现Al2O3涂层中锂的自加速扩散形成了明确的锂浓度梯度，进而导致弹性模量梯度，这有效地避免了局部应力集中并减轻了裂纹扩展。基于这些结果，我们提出一种外软内硬的模量梯度涂层，作为保护硅电极表面并提高其电流效率的最有效涂层。
