Role of Binder Molecular Weight on the Rheology and Microstructure of Cathode Slurries for Lithium-Ion Batteries

Scaling up the processing of electrodes for lithium-ion batteries remains a challenge for novel cathode materials because the optimum formulation de- pends on the chemistry and morphology of the cathode material. The molecu- lar weight of the binder can be used to adjust the rheological properties of the resulting slurry, affecting the microstructure of the formed electrode. Here, the molecular weight of a polyvinylidene fluoride (PVDF) binder is system- atically varied from 180–1,300 kg mol−1 for a series of cathode slurries com- prising LiNi0.8Mn0.1Co0.1O2 (NMC) secondary particles and nano-particulate carbon black (CB) dispersed in N-methyl-2-pyrrolidone (NMP). The zero- shear viscosity increases with the molecular weight of the binder, but other bulk properties of the slurry, including conductivity and yield stress, exhibit maxima. These maxima coincide with a minimum in the measured diameter of CB aggregates, suggesting that smaller CB aggregates percolate more effec- tively. Scanning electron micrographs of the free surface of the cathode and the buried cathode–current collector interface show that higher molecular- weight PVDF prevents vertical stratification in the electrode. This finding validates the expected inverse relationship between the settling velocity of a particle and the zero-shear viscosity of the medium.