The viscosity and microstructure of Li-ion battery slurries and the performance of the resulting electrodes have been shown to depend on the mixing protocol. This work applies rheology to understand ...the impact of shear during mixing and polymer molecular weight on slurry microstructure and electrode performance. Mixing protocols of different shear intensity are applied to slurries of LiNi0.33Mn0.33Co0.33O2 (NMC), carbon black (CB), and polyvinyldiene difluoride (PVDF) in N-methyl-2-pyrrolidinone (NMP), using both high-molecular-weight (HMW) and low-molecular-weight (LMW) PVDF. Slurries of both polymers are observed to form colloidal gels under high-shear mixing, even though unfavorable interactions between high molecular weight PVDF and CB should prevent this microstructure from forming. Theoretical analysis and experimental results show that increasing shear rate during the polymer and particle mixing steps causes polymer scission to decrease the polymer molecular weight and allow colloidal gelation. In general, electrodes made from high molecular weight PVDF generally show increased rate capability. However, high shear rates lead to increased cell variability, possibly due to the heterogeneities introduced by polymer scission.
Processing conditions of battery slurries into electrodes are known to affect final battery performance. However, there is a lack of fundamental understanding of how the relationships between ...processing conditions, the slurry microstructure, and the film microstructure affect electrode performance. This study determines the effects of the coating shear rate and drying temperature on battery electrode performance via discharge capacity. We use rheological measurements and energy dispersive X-ray spectroscopy (EDS) to correlate slurry and electrode microstructures to trends in discharge capacity. The radial distribution function is used to quantify differences in the electrode microstructure. More specifically, we show that the correlation between carbon and active material EDS detections to be the most relevant in understanding battery performance. Electrodes with both short- and long-range carbon/active material orders have the highest discharge capacities. This microstructure can be obtained through high shear rates, which induce better carbon dispersion via strong hydrodynamic forces, or through high temperature drying by preventing unwanted time-dependent structural changes after flow cessation. This analysis provides concrete evidence for the importance of both short-range and long-range contacts between the conductive additive and active material on battery performance.
The fabrication process of battery slurries into electrodes has been shown to impact final battery performance. While some fabrication steps have been studied, there is still a gap in understanding ...the influence of these steps on the slurry microstructure, the dried film microstructure and final electrode performance. In this study, two fabrication steps, coating and drying, are investigated for their influence on battery discharge capacity. Slurry and electrode microstructures are characterized by rheological measurements and energy-dispersive x-ray spectroscopy (EDS), respectively, and related to trends in discharge capacity. EDS elemental maps are processed through a radial distribution function to quantify the dried electrode microstructure. Through this analysis, the desired correlation between active material and conductive carbon particles for high discharge capacity was determined. Electrodes that produced the highest discharge capacities showed both short- and long-range order between active material and carbon particles. High shear rates allow for this microstructure to form by inducing strong hydrodynamic forces that lead to carbon dispersion. High temperature drying can also produce this structure by preventing time-dependent structural changes that occur for extended periods of time after shear. The results suggest the importance of both short- and long-range contacts between active material pieces and conductive carbon additive for optimal battery performance.
