Effect of Drying in Electrode Processing for Energy Storage

Despite the wide range of ideal applications for LIBs, there is still room for improvement in their performance. The effects of varying the composition and chemistry of LIBs are well documented [1-2], however a critical aspect that is often overlooked is the processing steps during fabrication. Typical processing parameters such as film thickness or evaporation have a large impact on critical parameters such as porosity of the electrodes, and thus can affect the electrochemical performance. During the solvent evaporation stage, a non-uniform distribution of electrode constituents can develop in cases where the solvent evaporation rate exceeds that of the diffusion rate of the mobile electrode constituents [3]. An even distribution of carbon black and binder throughout the electrode is crucial to ensuring the minimization of internal cell resistance, and therefore the maximum performance for a given electrode composition. 

In this work, we evaluate the impact of evaporation rate on the distribution of binder and carbon black in the electrode microstructure and the subsequent electrochemical performance. Both single-stage and two-stage evaporation was considered, where the two-stage process consisted of a slow rate followed by  oven-drying, and the single-stage consisted solely of immediate oven drying, with two different rates. Figure 1 shows the evaporation rate as a function of overall electrode sheet weight percent. Here the first stage for the two-stage sheet occurs over approximately 16 hours, with a loss of roughly 20% by weight. 

SEM images of the resulting microstructures indicate the vertical accumulation of carbon and binder on the surface of electrodes, with the increased carbon content in the case of the fast rate cases leading to the formation of a binder/carbon black composite that does not coat the active material particle surface, as shown in Figure 2. This through-plane (along the thickness) accumulation has been shown to decrease the rate performance, as shown in Figure 3.

The experimental results combined with theoretical analysis is expected to shed insight into the optimal drying conditions that may yield an optimum distribution of electrode constituents. 

References:

1.  M. Armand and J.M. Tarascon, Issues and Challenges  facing Rechargable Lithium Batteries. Nature, 414,      359 (2001).

2.  Handbook of Battery Materials, Second Edition. Edited by Claus Daniel and Jurgen O. Besenhard. © 2011 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2011 by Wiley-VCH Verlag GmbH & Co. KGaA.

3.  A.F. Routh, Drying of Thin Colloidal Films. Rep. Prog.Phys., 76 (2013)