(439g) Role of Powder Rheology in Dry Battery Electrode Processing

The novel dry battery electrode (DBE) process has several advantages compared to conventional and established solvent-based coating procedures for battery production including energy savings, environmental benefits, reduced costs and enhanced battery performance. In the DBE process, the active material, conductive additive, and polytetrafluoroethylene (PTFE) binder transition from fine powder to the free-standing electrode under high shear and elevated temperature, enabled by PTFE fibrillation. This transformation is critical for processability and the final battery performance, requiring a homogeneous mixture with the appropriate fibrillation before calendering¹. The goal of the study was to apply well-established powder rheological methods for characterizing the powder behaviour during this innovative battery production route as well as to develop a new method to characterize the fibrillation process of the PTFE binder.

During the first part of the study, conventional methods such as deaeration time, segregation, shear and compressibility testing were applied. For this purpose, mixed anode materials (graphite, carbon black, and PTFE) were characterized both before fibrillation (as premixes) and after fibrillation (as grinded particles) with varying binder contents of 1 wt.-%, 3 wt.-%, and 5 wt.-%. This sophisticated approach enabled to successfully predict the powder behaviour of the tested premixes during the different stages of the DBE process. The binder content played a significant role on the segregation behaviour where a higher binder proved to prevent segregation and was therefore preferable. Interestingly, the powder’s flowability as deduced from shear measurements was increased for the premix with the highest binder content with increased applied normal stress.

The second part of the study focused on developing a method for characterizing PTFE fibrillation. Pure PTFE samples were investigated. Wall friction measurements with an underlying temperature ramp revealed a strong increase in shear stress only once PTFE’s crystalline transition temperature below 20 °C² was transgressed (which is the prerequisite for fibrillation). This increase in shear stress could be successfully correlated with the powder’s fibrillation by scanning electron microscope pictures taken during the different stages of the temperature-controlled wall friction test.

References:

¹Y. Lu, C. Z. Zhao, H. Yuan, J. K. Hu, J. Q. Huang, Q. Zhang, Matter, 2022, 5, 876-898

²Conte, M., Pinedo, B., & Igartua, A. (2013). Role of crystallinity on wear behavior of PTFE composites. Wear, 307(1-2), 81-86.