DEM simulations and PEPT measurements of the dynamics of dry mixing NMC 622 cathode material in an Eirich mixer

Understanding of dry mixing of battery electrode materials has the potential to improve process sustainability by elimination of organic solvents, as well as the energy costs of slurry drying. The dry mixing dynamics of NMC 622, a cathode electrode material used in lithium-ion batteries, were obtained experimentally in an Eirich mixer using Positron Emission Particle Tracking (PEPT); the results were compared with simulations using the Discrete Element Method (DEM). The internal rotor tip speed was altered between 10, 20 and 30 m/s, with the rotational speed of the co-rotating outer mixing pan kept constant at 0.7 m/s. Depth-averaged heat maps of velocity obtained from PEPT were compared with DEM, focusing on key features of the mixer, namely the internal rotor, rotating pan wall and wall-scraper. Both PEPT and DEM data showed that the velocities were highest close to the rotor, and then dropped towards the wall-scraper, which disrupts material flow and improves dispersion. At the wall edge the velocity was constant due to the unchanged rotating motion of the pan. Both methods also capture a previously unobserved disengagement of the free surface of the electrode material from the rotor at higher tip-speeds, diminishing mixing performance, confirmed by estimating the transient mixing time using the Lacey mixing index. The gains in mixing performance are thus shown to have diminishing returns with increasing tip-speed, suggesting that mixing at lower speeds may, for the materials investigated here, prove more economical and thus more environmentally sustainable.