Mixing of lithium-ion battery slurry

The high-speed mixing and dispersion process accounts for more than 30 per cent of the impact on the quality of ternary material products throughout the entire lithium-ion battery production process, making it a critical stage in the production cycle. In the manufacture of lithium-ion battery electrodes, the cathode slurry consists of a binder, a conductive agent and cathode material (ternary material), whilst the anode slurry consists of a binder and graphite carbon powder. The preparation of both cathode and anode slurries involves a series of processes, including the mixing, dissolution and dispersion of liquid-liquid and liquid-solid materials, all of which are subject to variations in temperature, viscosity and environmental conditions. In both the cathode and anode slurries, the dispersion and uniformity of the particulate active material directly affect the movement of lithium ions between the battery’s electrodes. Consequently, the mixing and dispersion of the slurries for the electrode materials are of paramount importance in lithium-ion battery production; the quality of slurry dispersion directly impacts the quality of subsequent lithium-ion battery production and the performance of the final product.

MK-LB Series: A New Generation of Mixing Technology

The mixing and dispersion process of lithium-ion battery slurries can be divided into macroscopic mixing and microscopic dispersion; both processes are integral to the entire preparation of lithium-ion battery slurries. Based on the impeller shear and circulation characteristics of traditional processes, the function of the impeller in a high-speed lithium-ion battery mixer can be divided into two main categories: firstly, the shear effect generated in the vicinity of the impeller; and secondly, the circulation effect produced by the flow pumped out by the impeller. Further dispersion of the slurry relies primarily on the shear effect of the impeller, whilst the flow rate of the impeller determines its dispersion capacity. In areas further away from the impeller’s periphery, there is invariably a layer of slurry that remains stationary; this area is commonly referred to as the ‘dead zone’. The larger the working area of the dispersion equipment and the higher the viscosity of the slurry, the more pronounced the ‘dead zone’ problem becomes. Therefore, selecting a suitable high-speed mixer is crucial during the preparation of lithium-ion battery slurry. High-speed mixers feature a highly curved vessel bottom, with high-speed shear blades mounted at the base; the curvature is smooth and free of dead corners. The resulting slurry is uniformly mixed and dispersed, with powder particles in even contact with the binder, minimising issues such as stratification and the formation of hard precipitates. For a given slurry, at relatively low shear rates, no bulging flow occurs; however, at high shear rates, it may transform into a bulging-plastic fluid. Some non-Newtonian fluids may exhibit Newtonian behaviour at both low and high shear rates. This may be because, at low shear rates, the random thermal motion of molecules predominates, and the effect of shear rate on the rearrangement of material—which causes changes in apparent viscosity—is not evident. When the shear rate increases beyond a certain threshold, shear orientation reaches a sufficient degree, thereby stabilising the apparent viscosity so that it no longer varies with shear rate.

In high-speed mixing equipment for lithium-ion batteries, the energy applied to the liquid is generally highly concentrated, enabling the liquid to be subjected to high energy density. The type and intensity of the energy introduced must be sufficient to ensure that the dispersed phase particles are effectively and uniformly dispersed. The essence of uniform dispersion lies in the fragmentation and dispersion of the dispersed phase (solid particles, droplets, etc.) within the material through the combined action of hydrodynamic shear and pressure.

The direct cause of the fragmentation and dispersion of solid dispersed phase particles or droplets in a liquid material dispersion system is the combined action of shear forces and pressure. There are three main hydrodynamic effects that give rise to these shear forces and pressure: laminar flow effects, turbulent flow effects and cavitation effects. The laminar flow effect causes the solid dispersed phase particles or droplets to undergo shearing and elongation; the turbulent flow effect causes the solid dispersed phase particles or droplets to deform randomly under the influence of pressure fluctuations; whilst the cavitation effect causes the small air bubbles formed to collapse instantaneously, generating shock waves that induce vigorous agitation.

In summary, the high-speed mixer for lithium-ion batteries subjects the material to repeated compression effects under the action of high-frequency pressure waves, whilst simultaneously subjecting it to intense shear forces and rotational shear forces within the narrow gaps of the ultra-shear dispersion equipment. Through this combined and repeated action, the slurry being processed undergoes intense dispersion and comminution, thereby achieving the objective of rapid and fine dispersion.