Interplay of Adsorbed and Free Polymer Governs Microstructure and Processability of Battery Slurries

Abstract

In Li-ion battery anodes, sodium carboxymethyl cellulose (CMC) serves as an indispensable binder despite its minimal electrochemical contribution. Even at low concentrations, CMC imparts colloidal stability, ensures adequate flow during coating, and promotes uniform distribution of active materials. Within the slurry, CMC simultaneously exists in adsorbed and nonadsorbed (free) states, each interacting with active components through distinct mechanisms. This coexistence gives rise to a complexity that belies the apparent simplicity of binder chemistry, raising fundamental questions about how binders govern slurry microstructure and rheological behavior. In this study, we systematically vary CMC molecular weight and concentration to control the degree of intermolecular interactions between CMC and active components, and explicitly decouple the roles of adsorbed and free polymer fractions in governing dispersion quality and processability. Through comprehensive characterization of graphite anode slurry microstructurescombining small-angle scattering, rheology, and dispersion stability analysiswe demonstrate that short CMC chains primarily stabilize graphite aggregates through electrostatic and steric repulsion, while long CMC chains bridge neighboring aggregates through continuous polymer–polymer association, yielding a well-defined network structure. Removing free CMC reveals that long-adsorbed chains alone can sustain the bridged aggregate structure, whereas short adsorbed chains fail to maintain dispersion stability without the support of free polymer in the continuous phase. Furthermore, in situ rheo-SAXS experiments show that the interplay between adsorbed and free CMC drives distinct flow-induced structural evolutions, ultimately governing macroscopic processability and electrochemical performance of the resulting electrodes.