A study on the microstructure formation and drying behavior of Li-ion battery anode slurry

Abstract

The effect of binders used in Li-ion battery anode slurry on the microstructure formation and drying behavior was investigated. The microstructure formation of the anode slurry was studied by analyzing the rheological properties and cryo-SEM images. The graphite particles aggregated with each other due to their hydrophobicity and formed a gel structure in an aqueous slurry. In the case of graphite/SBR slurry, the microstructure was changed from the gel structure formed by graphite particles to dispersion due to the adsorption of SBR on the graphite surface. In the graphite/CMC slurry, the structural change was observed which was from gel to dispersion by adsorbing CMC on the graphite surface as in the graphite/SBR slurry. On the other hand, the dispersion was changed to gel structure again at high CMC concentration because the CMC molecules formed a network structure at high concentration and the polymeric network structure played a dominant role in the gel behavior of the battery slurry. In the graphite/CMC/SBR slurry, the SBR could affect the dispersion of the graphite particles at low CMC concentration in the slurry, while the microstructure was not changed regardless of adding SBR at high CMC concentration because the SBR could not adsorb on the graphite surface due to the CMC which was preferably adsorbed on the graphite. The drying behavior of the anode slurry was investigated by measuring the drying stress and by observing the microstructure in the dried film. The drying stress and the microstructure of the film were evaluated by a cantilever deflection method and a mercury intrusion porosimetry, respectively. We discovered that the SBR in the graphite/SBR slurry fills the voids among the graphite particles as the SBR concentration increases. In addition, the CMC in graphite/CMC slurry surrounded the particles by adsorbing on the surface of the graphite particles, thus helping the graphite particles to get together. In the case of graphite/CMC/SBR slurry, the effect of SBR on the drying stress depended on the CMC concentration. The residual stress increased as the SBR concentration was increased at low CMC concentration, while it maintained constant although the SBR concentration was increased at high CMC concentration. The residual stress in the film could be regarded as the mechanical strength of the film. In order to increase the mechanical strength of the film in low concentration region of CMC, a large amount of SBR was needed. On the other hand, in high concentration region of CMC, the SBR did not affect the mechanical strength of the film any longer. In other words, the CMC played a leading role in enhancing the mechanical strength of the film. Based on the drying stress of the slurries, we could draw a processing window map that clearly showed the effect of CMC and SBR on the mechanical strength of the film, which will be useful in the design of anode slurries. In addition, the latex migration in anode slurries during drying process was investigated by replacing SBR with fluorescent PS particles. The time evolution of the fluorescence signals revealed the particle migration in the film during drying. In CMC solution without graphite, the migration of the latex particles was suppressed above the entanglement concentration of CMC because the CMC molecules interrupted the latex migration. Thus, the uniform distribution of the particles in the film was produced at high CMC concentration, while the latex was distributed non-uniformly due to the migration at low concentration of CMC. In anode slurry, the latex migration was affected by the microstructure of the slurry. When the graphite particles were aggregated by depletion attraction, the latex migration was observed. On the other hand, when the graphite particles were well dispersed by adsorbed CMC on the graphite surface, the latex particles neither migrated nor segregated. This work provides the information about the effect of the binders, CMC and SBR used in anode slurry, on the microstructure formation and the drying behavior of the slurry. It is expected that the information about the microstructure and the drying behavior will be useful in designing the slurries for optimum performance of the Li-ion battery.