Frictional properties of polymer binders for Li-ion batteries

Abstract

In this work, the frictional properties of various polymer binders were studied using atomic force microscopy to enhance the mechanical integrity of the silicon (Si) anode for lithium-ion batteries (LIBs). The binders used include polyacrylic acid (PAA), polyacrylonitrile (PAN), polyvinyl alcohol (PVA), carboxymethyl cellulose (CMC), and polyvinylidene fluoride. The interfacial shear strength of the polymer binder was determined against Si to understand the contact shearing and sliding behaviors. The results showed that due to electrolyte immersion, the intrinsic interfacial shear strengths and pressure coefficients decreased by factors of 1.3–7.5 and 1.6–23.7, respectively. Decreases in mechanical and adhesion properties in the electrolyte may be responsible for the decrease in the frictional properties. The results also showed that topography-induced friction fluctuation became significant in the electrolyte due to an increase in the slope of the surface asperities, which is associated with polymer swelling. The Si anodes of LIBs containing polymer binders with relatively large intrinsic interfacial shear strengths and pressure coefficients in the electrolyte (PAA, PAN, PVA, and CMC) had high capacity retention. Given that a polymer binder with high interfacial shear strength helps maintain the mechanical integrity of the Si anode, PAA with high intrinsic interfacial shear strength may be preferred when the stress level is low during lithiation, while PVA with a large pressure coefficient may be advantageous when the stress level is high during lithiation. The outcome of this work suggests that the interfacial shear strength is a better polymer binder performance indicator than just considering mechanical and adhesion properties.