Optimization of Polyacrylic acid-based Binders for Silicon-Graphite Electrodes in Li-ion Batteries
Abstract:
Silicon-rich anodes offer a promising avenue for increasing the energy density of lithium-ion batteries, but their implementation remains challenging due to the significant volume changes in silicon during charging and discharging cycles, which promote mechanical degradation and the continuous formation of the solid electrolyte interphase (SEI). Polyacrylic acid (PAA) is a widely studied binder for silicon-based electrodes due to its strong interactions with the native silicon oxide surface, which enhance electrode cohesion and may contribute to the formation of an artificial passivation layer. This thesis investigates the relationships between the characteristics of the PAA binder, the electrode fabrication process, the electrodes’ structure, their interfacial properties, and the electrochemical performance of silicon-graphite composite anodes.
The main PAA formulation parameters were systematically studied, including binder content, molecular weight, degree of neutralization, and the nature of the counterion associated with the carboxylate group. Increasing the PAA content improved the stability of the suspension and the electrode fabrication process up to an optimal binder coverage ratio, beyond which polymer agglomeration increased the electrode’s resistivity and reduced its electrochemical performance. The molecular weight of the binder and the mass fraction of solids in the suspension exhibited coupled effects on the dispersion of active materials, water retention, electrode cohesion, and SEI formation. Although different molecular weights provided distinct advantages in terms of processability and mechanical properties, comparable electrochemical performance was achieved by appropriately optimizing the solid content of the ink.
The degree of PAA neutralization was also found to have a significant influence on electrode properties and cell performance. Partial neutralization at higher pH improved processing and electrochemical performance, whereas a lower degree of neutralization was associated with accelerated degradation under practical cell conditions. Finally, the identity of the counterion played a decisive role in the suspension’s performance. Although divalent cations promoted ionic interactions between PAA chains, the resulting polymer aggregation adversely affected the suspension’s rheology, electrode properties, and cycling performance. Conversely, monovalent cations improved the processing and performance of the studied formulation, with Na+ promoting the formation of a more stable SEI rich in LiF.
Overall, this work demonstrates the complex interaction between the various parameters studied and the benefits of optimizing them collectively to improve the electrochemical performance of silicon-rich anodes. The efficiency of the binder was improved by optimizing the mass fraction of solids to 36%, increasing the degree of neutralization to 66%, and using Na+ as the counterion. This combination maximized electrochemical performance while providing a practical path toward reducing the inactive mass in this high-energy-density electrode formulation. The systematic optimization of this formulation highlights several strategies for refining the binder formulation, thereby representing a step toward the commercialization of silicon-rich anodes.
Keywords: lithium-ion batteries; silicon anodes; silicon-graphite anodes; polyacrylic acid; PAA; aqueous synthesis; aqueous binder; binder neutralization; solid fraction of the ink; solid-electrolyte interface
