Correlating the Resulting Internal Structures of Lithium-Ion Battery Electrodes to the Prior-to-Coating Suspension Properties

Wednesday, 8 October 2014
Expo Center, 1st Floor, Center and Right Foyers (Moon Palace Resort)
H. Bockholt, B. Westphal, W. Haselrieder, and A. Kwade (Institute for Particle Technology, TU Braunschweig)
On the way to environmentally friendly and sustainable mobility lithium-ion batteries are in focus of various research works. Fundamental studies regarding the synthesis of new active materials or improvements of other important components, e.g. electrolytes or binders, are providing essential progress towards efficient e-mobility. In addition to all the material based developments, it is indispensable to gain a more detailed understanding of the main leverages which improve the electric and ionic transport processes inside the electrode and cell. One crucial property is the electrode’s inner structure. Since not only the materials but also the production processes have a considerable impact on the electrode structure, knowledge of the relationships between processes and structures is essential for a targeted electrode manufacturing.

Electrode’s inner structure is basically predetermined by the chosen components, particularly by their particle size distributions and aggregation status (active material, additives) or molecular weight (binder). In addition, some process steps in the manufacturing chain for lithium-ion battery electrodes, such as dispersing and calendering, show crucial impact on the resulting electrode structure. The process modalities while dispersing the powdery materials in a solvent control the desagglomeration degree of the carbon blacks and thus significantly affect the structure in the subsequently produced electrode. Changes in pore diameter distribution (see figure 1), porosity and tortuosity of the pores can be achieved by a more/less intensive or longer/shorter dispersing of the components in the solvent. To predict these structural alterations it is necessary to learn more about the suspension properties, especially about its structure.

Colloidal suspensions are commonly analyzed regarding their rheological characteristics in terms of flow behavior to estimate the properties for further processing steps like coating. In addition, rheological measurements can provide indirect structural information of battery suspensions, for instance concerning the fragmentation degree of the carbon black and the structural changes in the suspension while dispersing. Figure 2 exemplarily depicts the observed differences between suspensions with an aggregated particle-network (solid/gel like: phase angle 0°) to a more liquid-like (phase angle increasing up to 90°) suspension structure. A notable structural decrease can be identified after 45 minutes of dispersing compared to a shorter dispersing time of 10 minutes.

The correlation between suspension structure, analyzed via rheological measurements, and the resulting electrode structure, characterized via mercury porosimetry, is in the focus of the presented work. Additional investigations on the interrelations between structures and electric or ionic transport processes as well as electrochemical cell performances provide a basis for targeted electrode manufacturing.