Investigation of the Effects of Formulation, Slurry Preparation and Coating Conditions on the Mechanical Properties and Electrochemistry of Carbon-Based Lithium-Ion Battery Electrodes

Abstract

Typically, new electrode materials for lithium-ion batteries are first tested on the laboratory-scale. Although this generally gives a good indication of the electrochemical performance of the active materials (e.g. specific charge and potential profile), it does not paint the whole picture: Electrode materials that look promising on the laboratory-scale might not necessarily turn out to be suitable for industrial applications, and vice versa. In particular, the upscaling to industrial quantities might pose significant challenges related to various processing issues (e.g. mixing, rheology and coatability). Therefore, it is essential to follow up on laboratory-scale experiments with larger-scale coating tests. We investigated various formulations containing synthetic graphite negative electrode material (IMERYS Graphite & Carbon) and tested the electrochemical performance of the resulting electrodes. Instead of applying the common laboratory-scale doctor blading process, the slurry was cast onto copper foil using the technically relevant comma-bar method on an industrial coating line. We investigated the effects of various parameters, such as formulation and preparation of the aqueous slurry (e.g. mixing conditions), binder and electrode additives, as well as coating conditions (e.g. continuous/intermittent and unpressed/pressed) on the homogeneity (e.g. thickness, mass and density distribution), adhesion (peeling strength) and electrochemistry (cycling behavior) of the resulting electrodes. The experiments showed that the solid content and the mixing conditions as well as the coating mode (continuous/intermittent) and calendering process (unpressed/pressed) have a significant influence on the properties of the final electrodes. A higher solid content generally resulted in more homogeneous electrodes due to less sagging (gravity induced flow). Increased shear rates and mixing times, on the other hand, led to better electrochemical properties due to a more effective dispersion process. Furthermore, continuous coating allowed the preparation of more homogeneous electrodes than intermittent coating. Finally, calendering (which, of course, reduces the electrode thickness and increases the electrode density) resulted in improved adhesion and a more homogeneous thickness distribution, while negatively affecting the density distribution. The methodology offers a useful way of further evaluating active material candidates that have been shown to be promising in laboratory-scale electrochemical experiments with regard to the electrode engineering and manufacturing on the pilot-scale and thus provides important information for the development of electrode materials.