Comparative study on the rheological properties of natural and synthetic graphite-based anode slurries for lithium-ion batteries

Abstract

The rheological behavior of anode slurries for lithium-ion batteries, containing both natural and synthetic graphite as active material, was investigated with a focus on the different graphite morphologies. When the solid content is low, slurries containing synthetic graphite with a discotic shape display greater viscoelasticity than slurries containing natural graphite with a relatively more spherical shape. This result is attributed to the anisotropic geometry and interparticle force of the synthetic graphite. When the solid content is high, slurries comprising synthetic graphite exhibit lower viscoelasticity than slurries containing natural graphite. Tap density and sedimentation experiments reveal that, due to discotic shape and surface-to-surface attraction, synthetic graphite aggregates to a more densely packed aggregate than natural graphite. Consequently, in conditions of high solid contents where graphite has a greater chance of formation of densely packed aggregates, it is expected that synthetic graphite will have a more compact aggregate structure and a smaller effective volume. The smaller viscoelasticity of synthetic graphite slurries at more concentrated regions, where the effective volume of clusters plays more important role than in dilute regions, is attributed to the surface-to-surface aggregated structure of the synthetic graphite and the resulting small effective volume. Although the effective volume fraction of the graphite aggregates is reduced, slurries made of synthetic graphite demonstrate significant strain stiffening. Our findings suggest that the strain stiffening observed may originate from the anisotropic morphology, which possesses a significant surface area and is accompanied by jamming and high friction.