Beneficial rheological properties of lithium-ion battery cathode slurries from elevated mixing and coating temperatures

It is imperative that lithium-ion battery manufacturers implement strategies to expedite production without sacrificing quality due to rising consumer demand. Cathode coating is commonly performed at the industrial scale with a slot-die coater. In slot-die coating, substrate velocity is maximized and imperfections (such as air entrainment and thickness variations) are minimized by reducing the viscosity of the material being coated. A simple, scalable method of reducing the viscosity of the cathode slurry is to increase its temperature, though it is dire that this heat does not cause irreversible gelation or otherwise deteriorate the slurry constituents. Cathode slurries were prepared at different mixing temperatures between 25 °C and 75 °C and their flow behavior was studied at their mixing temperature. At practical shear rates, the slurry coated at 60 °C was 23% less viscous than that coated at 25 °C, meaning the critical coating speed could be increased by roughly 14% at 60 °C. Between 25 °C and 60 °C, the slurries’ yield stress and equilibrium storage modulus increased monotonically, providing the additional benefit of higher sedimentation resistance of the active materials. To examine the influence of temperature on coating morphology and electrochemical performance, slurries were prepared and coated at 25 °C and 60 °C. Micrographs revealed no superficial differences between coatings. The electrode coated at 60 °C demonstrated comparable capacity retention during long-term cycling and high-rate discharge testing when compared to the electrode coated at 25 °C. The results of this study indicate that warmer mixing and coating operations serve to maximize cathode productivity, particularly if advancements can be made in industrial-scale electrode drying.