Elevated Mixing and Coating Temperatures Yield Beneficial Processing Properties of Organic Solvent-Based Lithium-Ion Battery Cathode Slurries

Abstract

In traditional lithium-ion battery cathode manufacturing, the slurry mixing and coating stages bear tremendous influence on downstream processing steps and, consequently, the viability of the end product. Two avenues for improving these two stages include increasing the coating throughput to meet rising consumer demands and to make coatings with more consistent, homogeneous distributions of slurry components in order to create higher-performing devices. In state-of-the-art slot die coating operations, coating speed is maximized and imperfections (i.e. air bubbles and thickness variations) are minimized by reducing the viscosity of the material being coated. The active materials, which routinely form agglomerates with diameters on the order of 10 μm, must be prevented from settling under gravity to form a homogeneous active layer. In this work, heating the slurry during the mixing and coating stages is proposed as a scalable and effective solution to achieve faster and more reliable coating operations. This study demonstrates that increasing the mixing and coating temperature from 25°C to 60°C can reduce the slurry viscosity by 21 percent at practical industrial coating speeds. At low shear rates, slurries mixed at 60°C are also 10 percent more viscous, providing the additional benefit of sharper edge contours and, thus, less cut-off waste during processing. Oscillatory shear probing revealed that the gel strength of heated slurries is also greater, meaning there is greater sedimentation resistance due to increased polymer bridging. The discharge capacity of the cathode mixed and coated at 60°C was about 4% greater than that of the cathode mixed and coated at 25°C for all C-rates between C/10 and 5C, which could be indicative of an improved distribution of components. The results indicate that mild heating of the slurry (1) does not irreversibly gel the slurry such that it is unable to be processed, (2) provides rheological properties that correlate to quicker and more consistent coating operations, and (3) yields slightly higher-performing cathodes at C-rates of practical interest. Heating the slurry also provides the manufacturer with the option to increase the solid loading to reduce drying times and solvent recovery.