(439d) The Optimization of Carbon Distribution in Dry Mixed Lithium-Ion Cathodes Utilizing Conical Tumble Milling

Lithium-ion batteries (LIB), due to their combination of high energy density and long cycle-life have become the dominant energy storage chemistry, powering almost all consumer electronics and helping drive the adoption of electric vehicles. However, one of the barriers to widespread adoption has been the high-costs and lower travel ranges compared to conventional internal combustion vehicles. One investigation route to address both issues has been in the dry processing of the electrodes. While a completely solvent free dry coating process would be ideal, it still faces technological hurdles preventing widescale implementation. Dry mixing, however, has become widely adopted for the cathodes as a method to reduce the total slurry mixing time and improve energy density. The non-conductive cathode active materials are typically dry mixed through ball-milling (BM) with a conductive carbon, making use of the high energy process to rapidly disperse the particles and affix a conductive layer over the cathode powder prior to introduction of solvent. However, research has shown that this process requires significant optimization as BM not only disperses agglomerates but also breaks apart the secondary particles of the active material. Because many cathode powders are optimized by the manufacturer, the subsequent dry milling process can disrupt functionalized surfaces and the sphericity of the powder. In addition, the excess conductive carbon coated onto the cathode surface during the dry process reduces the carbon available for the binder matrix of the final electrode, resulting in a net loss in performance. Hence, it is necessary to obtain an effectively homogeneous mixture without damage to the cathode active material.

This work investigates the conical tumble-milling (CTM) dry-mixing tool, which is more common in organic synthesis and pharmacology, and compares it to both previously reported BM procedures and conventional wet mixing for the NMC811 cathode slurry. As CTM is less energetic than BM, the mixing may take longer, but the equipment also has lower energy consumption requirements and has been validated at large-scale. This low-energy milling allows the soft carbon agglomerates to be dispersed and coated over the active materials without major disruption to the cathode morphology. In addition, for the two dry mixing processes (CTM and BM), conductive carbon is incorporated in both the dry and wet steps in several ratios, creating a targeted distribution of the additive between the final electrode’s coating’s polymer matrix and the surface of the cathode particles. The level of dispersion and changes in resultant morphology of the coated electrodes are characterized through SEM. BET analysis is used to identify changes in specific surface area of the powders and help quantify the particle size-reduction, and pycnometry is performed on all feed powders to allow for accurate calculations of the electrode porosity. Half-cells constructed using coated cathodes of uniform mass loading and density from each mixing approach demonstrate their resulting rate performance with electrochemical impedance spectroscopy analysis to identify the changes in charge transfer resistance and contact resistance. Finally, full pouch cells fabricated with paired graphite anodes provide cycle life comparison across slurry mix methods. This study aims to understand the effects of mixing procedure on electrode morphology and related cell performance and provides optimized parameters for CTM dry mixing in cathode powder processing.