Influence of Convective Drying Parameters on Electrode Performance and Physical Electrode Properties

As the need for nonpolluting individual mobility is rising, the production processes of lithium-ion batteries are gaining more and more attention. Although a lot of research for new battery materials or even for alternative battery technologies is done, the state of the art production process for large scale automotive lithium-ion batteries is still not completely understood. To get as close as possible to the theoretical performances of the used materials, an optimized process which includes a deep understanding of its influences on electrode structure and in the end on cell performance is essential. For this reason the drying process as one of the key-process-steps in the electrode production process is examined.

Since drying fixes the coating structure by evaporation of the solvent, the sedimentation of active material particles and the formation of the binder and carbon black matrix is influenced in a crucial way. In the present work some of the various drying parameters such as air temperature, outlet speed, air flow rate and residence time are varied and their influence on electrode structure is examined. A pilot-plant continuous convective dryer with three individual adjustable segments and a total length of six meters is basis of these investigations.

In order to analyze the resulting electrode structure measurements like mercury porosimetry (to determine the porediameter distribution of the coating), electrode conductivity (to describe the network of the conductive additives) and adhesion force measurements (to characterize the binder distribution) were carried out. As an example, a strong impact on the coating’s adhesion force to the current collector with increasing drying intensity (or faster solvent evaporation, respectively) and coating thickness could be found and explained by changes in electrode’s structure. Deeper insights in the processes affecting electrode structure can be obtained by determination of the electrochemical cell performance, thus allowing the establishment of process-structure-property relations.

Besides offline measurements to determine the electrode structure the drying process itself is monitored online by detecting the shift from constant rate drying period to falling rate period via infrared temperature sensors which measure the surface temperature of the coating. If the coating speed is kept constant the point of drying period change can be detected within a shorter distance from the dryer inlet when the air temperature is increased (see Fig. 1). The period change is identified for different drying parameters and electrode thicknesses as thicker electrodes tend to show a more distinct influence by the drying process.

In summary recent investigations show that electrode properties (e.g. adhesion force) resulting from solvent evaporation and structure fixation are clearly dependent on drying parameters and thus enabling a way for enhancing the battery’s performance.