Yielding behavior of concentrated lithium-ion battery anode slurry

Abstract

The nonlinear rheology of a concentrated lithium-ion battery anode slurry was examined under large amplitude oscillatory shear and interpreted with a sequence of physical process (SPP) analysis. A complex interplay of three anode slurry components—graphite (Gr) as an active material, carbon black (CB) as a conductive additive, and carboxymethyl cellulose (CMC) as a binder—leads to a two-step yielding behavior, represented as the secondary plateau in dynamic strain and stress sweep tests. We demonstrate that a two-step yielding behavior is manifested as double deltoids in SPP analysis through the study of intra-cycle rheological transition under oscillatory shear flow. Slurries of Gr-CMC exhibit the two-step yielding behavior; slurries of CB-CMC do not, suggesting that Gr and CMC are the primary causes of two-step yielding in an anode slurry. A sedimentation test on a dilute Gr-CMC solution yielded phase separation between graphite particles, with CMC adsorbed on their surface and graphite particles aggregated via hydrophobic attraction. This indicates two possible types of interactions in a concentrated slurry: a hydrophobic interaction between graphite particles and a physicochemical interaction caused by CMC adsorbed on graphite particles. The first yielding step relates to the hydrophobic attraction between graphite particles, resulting in a network structure that is expected to be brittle and rupture at a small strain. The second yielding step is attributed to the interaction between concentrated CMC, which is corroborated by the overlap of the secondary deltoid of the anode slurry and the single deltoid of the concentrated CMC solution in SPP analysis.