title: Self-generated concentration and modulus gradient coating design to protect Si nano-wire electrodes during lithiation.
abstract: 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.
authors.affiliation: Department of Chemical engineering & Material Science, Michigan State University, East Lansing, MI 48824, USA. yueqi@egr.msu.edu.
authors.forename: Sung-Yup
authors.initials: SY
authors.lastname: Kim
authors.forename: Alireza
authors.initials: A
authors.lastname: Ostadhossein
authors.forename: Adri C T
authors.initials: AC
authors.lastname: van Duin
authors.forename: Xingcheng
authors.initials: X
authors.lastname: Xiao
authors.forename: Huajian
authors.initials: H
authors.lastname: Gao
authors.forename: Yue
authors.initials: Y
authors.lastname: Qi