From Lab Scale to Large Volume Production: Substitution of Toxic NMP/PVDF with H2o/CMC/SBR in Lithium-Ion Cells

Lithium-ion batteries (LIBs) are becoming more and more important for mobile energy storage devices, with respect to their energy density. For large size applications such as in electric vehicles (EVs) or hybrid electric vehicles (HEVs), both higher energy density and low-cost materials are required also taking environmental friendliness into account. In order to enhance the performance of lithium-ion batteries, researchers and battery-manufacturers are trying to create new electrode materials and new electrolyte compositions. Nevertheless, battery efficiency strongly depends on the electrode engineering and additionally on the production process with the optimization of each individual step.^[1,2]

Polyvinylidene fluoride (PVDF) based electrodes on the cathode as well as on the anode side are widely used. One major drawback of PVDF based electrodes is the usage of volatile organic solvents that are often toxic (like N‑methyl-2-pyrrolidone (NMP)) and are difficult to dispose at the end of the production process. Alternatively, water soluble natural and/or synthetical based binders have been proposed which also cope the increasing demand of a more environmentally-friendly production process and recycling of LIBs. Processing of active materials for negative as well as positive electrodes is possible using carboxy methyl cellulose (CMC) and/or styrene-butadiene (SBR).^[3]

In our study we present the results of our experiments to substitute NMP/PVDF with a water based CMC/SBR binder solution. Moreover, the work comprises the development and adaption of mixing processes, coating techniques, and appropriate drying procedures. To optimize the mixing process for water based slurries we had to figure out ideal substitution grades, mixing ratios between CMC/SBR, and the right pH-value. With the focus on viscosity, mixer types, and shear rates we figured out an optimized slurry configuration for the adaption to our battery pilot plants including the manufacturing of different cell types. Furthermore, the adjustment of coating techniques is one essential issue towards large scale production processes. Nevertheless, we also had to pay attention to the challenging production step, the drying procedure by the combination of different drying duration and temperatures as well as the adapted drying technique.

This work demonstrates the difficulties and production steps going from lab scale to large scale manufacturing.

We kindly thank the European Regional Development Fund for the funding this project “ProLiBat” (EM-1041H) and also the project partners for support and cooperation.

[1] M. Broussely, J. Power Sources, 81-82 (1999), 140.

[2] X. Zhang, et al.,  J. Electrochem. Soc., 148 (2001), A463.

[3] Lux, S. F., et al J. Electrochem. Soc., 157.3 (2010), A320-A325.