The growing demand for electric vehicles necessitates significant improvements to present-day battery technologies - especially in terms of cost and energy density. Silicon (Si) materials for anodes in Li-ion batteries are cost-effective and have energy densities that are ten times that of conventional graphite anode materials. However, Si anodes undergo ~300% volume expansion during charging, which leads to rapid material degradation and catastrophic performance loss. Porous Si materials combined with conductive pyrolyzed polyacrylonitrile (PPAN) binders have shown promising results in improving the cycle life and coulombic efficiency of Si-based batteries. Although promising, there is little understanding of the atomic-level structure of Si/PPAN interface, making it difficult to optimize performance by rationally tuning porosity, ratio of Si to PPAN, and degree PAN pyrolysis. Reactive force fields, such as ReaxFF, are well-suited for investigating such composite battery materials because they allow chemical bonds to form and break during a simulation and can treat interfaces separating dissimilar phases, such as Si and PPAN. In our present work, we have developed a Si/C/N/H force field by merging Si/C/H and Si/N/H force fields optimized against training data generated from quantum chemical calculations. Initial molecular dynamics simulations using this force field show that the elasticity of the composite material is enhanced by the PPAN matrix, thus allowing the anode material to retain structural integrity over repeated volume deformations. This supports experimental observations suggesting that the material properties of the PPAN matrix are essential to improving the cycle life of the battery. The Si/C/N/H force field allows us to perform large-scale simulations of complex Si/PPAN geometries to further investigate electrochemical events occurring at the Si/PPAN interface, which in turn yield atomistic insight that can be used to optimize anode design to enhance battery stability.

Multi-scale modeling of Li-ion battery anode materials using reactive force fields

2019 Spring National Meeting

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