Effect of Binder Content on Silicon Microparticle Anodes for Lithium-Ion Batteries

Abstract

Electrode formulation directly influences the resulting composite electrode structure and properties, ultimately impacting the electrode performance and durability. Although there have been many significant advancements in the development and understanding of novel and effective binder materials for silicon anodes1–4, few studies report more than one formulation, or ratio of active material to conductive additive to polymeric binder5,6.In this study, we focus on investigating the impact of polyimide binder weight ratio in silicon microparticle electrodes. Electrode adhesion to the current collector, cohesion, porosity, electrical resistivity, and electrical tortuosity were characterized in pristine electrodes. These properties were then correlated to electrode cycle life and rate performance in Si-NMC622 full cells. We show that there is an optimal binder weight ratio for a given type of polymer binder. Increasing binder weight ratio improves electrode adhesion and cohesion, resulting in improved cycle life. However, increased binder weight ratio decreases electrical conductivity and porosity, leading to poorer silicon particle utilization, capacity, and rate performance.In the formulations we investigated, the silicon to carbon weight ratio was fixed, while binder content was varied. The optimal binder ratio maximizes silicon particle utilization, achieving 1084 ± 25 mAh/gSi gravimetric capacity after the formation cycles. When binder content was increased three-fold, the discharge capacity decreased by 27%, which was correlated to lower electrode porosity and lower initial coulombic efficiency. We hypothesize that insufficient porosity limits lithium-ion access to the silicon particles, thus reducing the capacity at the given C-rate. We also hypothesize that the decreased coulombic efficiency indicates that the polyimide binder contributes to the consumption of available lithium.In contrast, capacity retention improved with additional binder content. This is partly due to increased electrode cohesion with the added binder and partly due to lower overall capacity in the silicon particles, resulting in less volumetric strain. In electrodes with the binder content reduced by one-third, capacity retention dropped dramatically. We hypothesize that there is a lower limit of binder content, below which the electrode lacks sufficient structural integrity to withstand volume strains during cycling.These findings point towards important design principles for the optimization of binder content in silicon electrode formulations.