Identifying Adhesion Properties at Si/Polymer Interfaces with ReaxFF
M Bhati and TP Senftle, JOURNAL OF PHYSICAL CHEMISTRY C, 123, 27036-27047 (2019).
High capacity lithium-ion battery anodes based on silicon (Si) undergo large volume fluctuations during operation that can compromise the structural integrity of the electrode. This issue can be mitigated by using flexible polymers to encapsulate the active Si material so that the electrode can accommodate significant volume expansion and contraction during battery cycling. Such designs require a stable interface between the polymer and Si that can undergo repeated deformations. To help design such interfaces, we have developed a ReaxFF force field to investigate the interfacial adhesion properties of polymers on Si surfaces at the atomistic scale. We consider three C/N/H-based polymers in this study that have been shown to improve battery performance when used as a binder for the active Si component in battery electrodes: polyacrylonitrile (PAN), pyrolyzed PAN (PPAN), and polypyrrole (PPy). Molecular dynamics simulations with the newly developed ReaxFF parameters show that single chains of PPy bind more strongly to Si compared to those of PAN or PPAN, which is validated by adsorption energies computed with density functional theory. This trend reverses when considering the interface between bulk polymers and Si, with bulk PPy binding least strongly to the surface. We show that this reversal is caused by the interaction of individual polymer chains at the interface, where the first layer of PPy binds so strongly that it prevents the next layer of chains from accessing the surface. This work offers insight into atomistic interfacial phenomena in composite electrode materials and provides simulation tools that can be readily extended to other Si/polymer systems.
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