Editor's Note
During the preparation of the slurry, two main aspects need special attention. First, the primary materials must be uniformly dispersed to ensure consistency. Second, secondary agglomeration caused by interactions between raw materials should be prevented. This is crucial because the quality of the lithium battery electrode largely depends on the slurry preparation process.
The thickness of a lithium battery electrode typically ranges from 40 to 200 micrometers, and it varies depending on the type of battery—such as high-energy or high-power designs. For optimal electrochemical performance, the electrode must maintain uniform thickness, have a smooth surface, and be free of defects. In the coating process, slurry preparation plays a key role in determining the final quality. A good lithium battery slurry should have stable viscosity, fine particle size, no visible graininess after drying, and be free of bubbles or foreign particles. Therefore, during slurry preparation, ensuring uniform dispersion and preventing re-agglomeration are essential objectives.
Lithium battery electrodes are divided into positive and negative electrodes. The active materials, conductive agents, binders, and solvents used in these electrodes differ based on the battery system. Mixing such a complex multiphase system into a homogeneous slurry requires both practical experience and theoretical knowledge. Smaller particle sizes can make dispersion more challenging. For example, carbon black, a common conductive agent, tends to form large agglomerates during mixing, which not only reduces its conductivity but also negatively affects the battery’s energy density.
Carbon black is widely used in various applications, including plastics and polymers, so there are many studies on its dispersion. According to current research, the dispersion of agglomerates can be achieved through several methods, each with different mechanical forces and outcomes.
The first type of dispersion shown on the left occurs under weaker mechanical forces, where material flows and forms agglomerates of varying sizes. As mechanical force increases, smaller particles break off from larger agglomerates, forming secondary particles. When the external force exceeds a critical level, sudden particle rupture occurs, as seen in the middle of the image. However, if the mechanical energy is too high for an extended period, further fragmentation is less likely.
During the slurry preparation process, the interaction between materials may go through three distinct states. Initially, carbon black exists in a structured form (Structure 1) with relatively uniform distribution. As mechanical agitation continues, the conductive agent begins to disperse and coat the active material surfaces. This stage marks the transition from isolated particles to a homogeneously mixed substance.
This transformation can be divided into three stages: solid powder state (I), mixed wetting state (II), and finally, the formation of a suspension state (III). Each stage has unique characteristics, advantages, and challenges, as illustrated below.
The table below summarizes the features of each stage:
In the dry mixing stage, factors such as the input energy of the equipment, the mechanical force applied, and the particle size of the active material all influence the dispersion of the conductive agent. Additionally, humidity in the air significantly impacts material agglomeration.
The wetting stage begins when the solvent is first added to the solid powder. The degree of wetting depends on how well the powder absorbs the liquid and the adhesion and stress within the mixture. During this phase, both dispersion and agglomeration occur simultaneously as the mixture transitions between states. Once the gel point or full saturation is reached, further large-scale force input is unnecessary, and the particle interactions decrease, allowing the slurry to be diluted to the desired solids content.
In the suspension dilution stage, hydrodynamic forces and shear forces come into play. Under fluid flow conditions, the mixer applies shear forces that help stabilize the particles. Different processing control methods exert varying levels of force on the raw materials, influencing the uniformity of particle dispersion. This is especially important for carbon black, as the stress intensity directly affects the electrode’s electron conductivity, battery capacity, and current density. Therefore, understanding the mixing state during slurry preparation is essential for achieving high-quality lithium batteries.
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