Rheological properties and stability of NMP based cathode slurries for lithium ion batteries

Slurries for the manufacturing of cathodes for lithium ion batteries are compared regarding to their colloidal stability by means of rheology. Model formulations with nanoscaled LiFePO4 (LFP) and micron scaled Li(Ni,Mn,Co)O2 (NMC) were prepared by using N-methyl-2-pyrrolidone (NMP) as solvent. Polyvinylidene difluoride (PVDF) binder and carbon black (CB) conducting additive were added at typical amounts of a few weight percent. The influence of these inactive electrode components on the physical stability of the dispersions was investigated by steady state and oscillation experiments. It is demonstrated that the addition of a high molecular weight PVDF binder is sufficient to establish gel formation by bridging flocculation in case of the nanoscaled cathode material. For the larger micron scaled particles, the formation of a stable coagulated state is also feasible but it requires the combination of a particulate CB gel and a strengthening PVDF polymer network.