title: Surface Coating Constraint Induced Self-Discharging of Silicon Nanoparticles as Anodes for Lithium Ion Batteries.
abstract: One of the key challenges of Si-based anodes for lithium ion batteries is the large volume change upon lithiation and delithiation, which commonly leads to electrochemi-mechanical degradation and subsequent fast capacity fading. Recent studies have shown that applying nanometer-thick coating layers on Si nanoparticle (SiNPs) enhances cyclability and capacity retention. However, it is far from clear how the coating layer function from the point of view of both surface chemistry and electrochemi-mechanical effect. Herein, we use in situ transmission electron microscopy to investigate the lithiation/delithiation kinetics of SiNPs coated with a conductive polymer, polypyrrole (PPy). We discovered that this coating layer can lead to "self-delithiation" or "self-discharging" at different stages of lithiation. We rationalized that the self-discharging is driven by the internal compressive stress generated inside the lithiated SiNPs due to the constraint effect of the coating layer. We also noticed that the critical size of lithiation-induced fracture of SiNPs is increased from ∼150 nm for bare SiNPs to ∼380 nm for the PPy-coated SiNPs, showing a mechanically protective role of the coating layer. These observations demonstrate both beneficial and detrimental roles of the surface coatings, shedding light on rational design of surface coatings for silicon to retain high-power and high capacity as anode for lithium ion batteries.
authors.affiliation: Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory , 902 Battelle Boulevard, Richland, Washington 99352, United States.
authors.forename: Langli
authors.initials: L
authors.lastname: Luo
authors.affiliation: Engineering Science and Mechanics and Bioengineering, Pennsylvania State University , University Park, Pennsylvania 16801, United States.
authors.forename: Peng
authors.initials: P
authors.lastname: Zhao
authors.affiliation: Engineering Science and Mechanics and Bioengineering, Pennsylvania State University , University Park, Pennsylvania 16801, United States.
authors.forename: Hui
authors.initials: H
authors.lastname: Yang
authors.affiliation: Materials Science and Engineering Program and Department of Mechanical Engineering, The University of Texas at Austin , Austin, Texas 78712, United States.
authors.forename: Borui
authors.initials: B
authors.lastname: Liu
authors.affiliation: Energy and Environmental Directorate, Pacific Northwest National Laboratory , 902 Battelle Boulevard, Richland, Washington 99352, United States.
authors.forename: Ji-Guang
authors.initials: JG
authors.lastname: Zhang
authors.affiliation: Department of Materials Science and Engineering, Stanford University , Stanford, California 94305, United States.|Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory , 2575 Sand Hill Road, Menlo Park, California 94025, United States.
authors.forename: Yi
authors.initials: Y
authors.lastname: Cui
authors.affiliation: Materials Science and Engineering Program and Department of Mechanical Engineering, The University of Texas at Austin , Austin, Texas 78712, United States.
authors.forename: Guihua
authors.initials: G
authors.lastname: Yu
authors.affiliation: Engineering Science and Mechanics and Bioengineering, Pennsylvania State University , University Park, Pennsylvania 16801, United States.
authors.forename: Sulin
authors.initials: S
authors.lastname: Zhang
authors.affiliation: Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory , 902 Battelle Boulevard, Richland, Washington 99352, United States.
authors.forename: Chong-Min
authors.initials: CM
authors.lastname: Wang
keywords: Si anode; mechanical constraint; polymer coating; self-discharge