Combining ReaxFF Simulations and Experiments to Evaluate the Structure–Property Characteristics of Polymeric Binders in Si-Based Li-Ion Batteries

Abstract

High energy capacity silicon (Si) anodes in Li-ion batteries incorporate polymeric binders to improve cycle life, which is otherwise limited by large volume and stress fluctuations during charging/discharging cycles. Several properties of the polymeric binder play a role in achieving optimal battery performance, including interfacial adhesion strength, mechanical elasticity, and lithium-ion conduction rate. In this work, we utilize atomistic simulations with the ReaxFF force field and complementary experiments to investigate how these properties dictate the performance of Si/binder anodes. We study three C/N/H-based polymer binders with varying structures (pyrolyzed polyacrylonitrile (PPAN), polyacrylonitrile (PAN), and polyaniline (PANI)) to determine how the structure–property characteristics of the binder affect performance. The Si/binder adhesion analysis reveals some counter-intuitive results: although an individual PANI chain has a stronger affinity to Si compared to PPAN, the PANI bulk binds weaker to the Si surface. Interfacial structural analyses from simulations of the bulk phase show that PANI chains have poor stacking at the interface, while PPAN chains exhibit dense and highly ordered stacking behavior, leading to stronger adhesion. PPAN also has a lower Young’s modulus compared to PANI and PAN owing to its ordered and less entangled bulk structure. This added elasticity better accommodates volume changes associated with cycling, making it a more suitable candidate for Si anodes. Finally, both simulations and experimental measurements of Li-ion diffusion rates show higher Li mobility through PPAN than PAN and PANI because the ordered stacking of PPAN chains creates channels that are favorable for Li diffusion to the Si surface. Galvanostatic charge–discharge cycling experiments show that PPAN is indeed a highly promising binder for Si anodes in Li-ion batteries, retaining a capacity of ∼1400 mAh g–1 for 150 cycles. This work demonstrates that the orientation and structure of the polymer at and near the interface are essential for optimizing binder performance as well as showcases the initial steps for binder evaluation, selection, and application for electrodes in Li-ion batteries.

Cross-Linked γ-Polyglutamic Acid as an Aqueous SiOx Anode Binder for Long-Term Lithium-Ion Batteries

“…It has been demonstrated that weak interfacial adhesion between the active materials and the binders give rise to particle agglomeration. 52 Thus, the degree of the particle 3f). In contrast, the obvious agglomeration of active materials was observed in the SiO x / PGA (Figure 3g) and SiO x /CMC (Figure 3h) electrodes.…”

“…It has been demonstrated that weak interfacial adhesion between the active materials and the binders give rise to particle agglomeration . Thus, the degree of the particle agglomeration can predict the interfacial adhesion between SiO x particles and the binder.…”

Prelithiation Effects in Enhancing Silicon-Based Anodes for Full-Cell Lithium-Ion Batteries Using Stabilized Lithium Metal Particles

“…The fabrication of Si/PPAN anodes employed a similar procedure that had been reported previously. − Si microparticles (∼2 to 3 μm, Silgrain e-Si, Elkem) were mixed with polyacrylonitrile (PAN, M w ≈ 150,000, Sigma-Aldrich) at a 70:30 (% w/w) Si:PAN ratio, using N , N -dimethylformamide as the solvent (dimethylformamide (DMF), ≥99.9%, Sigma-Aldrich). The slurry was then spray-coated onto a copper current collector (MTI Corp.) utilizing an airbrush (Model G22, Master Airbrush).…”

Establishing a Resilient Conductive Binding Network for Si-Based Anodes via Molecular Engineering

“…85 Besides, Senftle et al found that pyrolyzed polyacrylonitrile (PPAN) chains exhibit dense and highly ordered stacking behavior at the anode interface due to repeated pyridine structures, leading to stronger adhesion and higher Li + conductivity. 86 In a subsequent study, Huo et al reported a stretchable conductive glue (CG) binder formed by cross-linking of D-sorbitol and vinyl acetate−acrylic (VAA) onto PEDOT:PSS chains. This binder can be stretched up to 400% in volume without losing conductivity (Figure 11C).…”