How to Evaluate Lithium-ion Battery Electrode Slurry?

In lithium-ion battery production, slurry preparation is the first process. The suitability of the slurry after mixing directly affects subsequent coating and final battery performance, and is a crucial indicator of battery cost. Therefore, slurry preparation plays a central role in lithium-ion battery manufacturing. Research shows that the ideal microscopic particle distribution of a lithium-ion battery electrode is characterized by uniform dispersion of active materials without agglomeration, and thin-layer dispersion of conductive agent particles forming a conductive network. Lithium-ion batteries made from electrodes with uniform particle distribution exhibit excellent electrochemical performance and long lifespan. Therefore, the flowability, stability, and uniformity of lithium-ion battery slurries are typically tested and characterized to evaluate their suitability for subsequent coating processes and their contribution to the performance of the battery's active materials.

In lithium-ion battery production, slurry preparation is the first process. The suitability of the slurry after mixing directly affects subsequent coating and final battery performance, and is a crucial indicator of battery cost. Therefore, slurry preparation plays a central role in lithium-ion battery manufacturing. Studies have shown that the ideal microparticle distribution of lithium-ion battery electrodes is characterized by uniform dispersion of active materials without agglomeration, and thin-layer dispersion of conductive agent particles forming a conductive network. Lithium-ion batteries made from electrodes with uniform particle distribution exhibit excellent electrochemical performance and long lifespan. Therefore, the flowability, stability, and uniformity of lithium-ion battery slurries are typically tested and characterized to evaluate their suitability for subsequent coating processes and their effectiveness in enhancing the performance of the battery's active materials.

Electrode slurries are non-Newtonian fluids, and there are two types: positive electrode slurries and negative electrode slurries, which are classified into oil-based and water-based systems. The stirred slurry needs to possess good flowability, stability, and uniformity. The states of slurries with different flowability are shown in the figure. Excessively low or high viscosity, sedimentation, agglomeration, and uneven dispersion can significantly impact subsequent coating processes, causing defects in the macroscopic appearance of the electrode and inconsistencies in its microstructure. Unlike the requirements for slurry in traditional papermaking and coating industries, the uniformity and stability of lithium battery slurry ultimately affect the battery's electrical performance, leading to a series of problems such as voltage degradation, reduced cycle life, and poor battery consistency. Therefore, proper characterization and testing of the slurry before coating to determine its suitability for processing is crucial.

When a liquid flows, internal friction occurs between molecules; this property is called viscosity, measured numerically.

Viscosity = Shear Stress / Shear Rate

The functional relationship between shear rate and shear stress is called a rheological curve, commonly used to describe the variation of slurry (fluid) viscosity with shear characteristics. Rheological curves can be used to distinguish fluid types. For Newtonian fluids, the relationship between shear stress and shear rate on the rheological curve is a straight line passing through the origin.

In production, digital rotational viscometer and rheometers are commonly used to measure viscosity and rheological curves. Viscometers are simple to operate, while rheometers can measure a wider range and more comprehensive viscosity-shear rate/stress curves, and provide more accurate results. Battery slurries, as non-Newtonian fluids, exhibit various rheological properties, including shear thinning, viscoelasticity, and thixotropy.

Most lithium-ion electrode slurries exhibit shear thinning characteristics. As shown in the figure, shear viscosity decreases with increasing shear rate. This characteristic is more pronounced in slurries with high solids content. At higher shear rates, strong shear forces disrupt the network structure formed between materials, causing particles to rearrange into a more ordered structure parallel to the shear field, resulting in a decrease in viscosity and reaching a Newtonian plateau at higher shear rates.

This non-Newtonian characteristic is advantageous in the coating process of electrode preparation. Before using the electrode coating machine, the slurry is transported from the transfer tank to the coating chamber by a screw pump. During this process, the slurry is subjected to low shear force and remains in a high viscosity state to maintain its stability. When the slurry is instantaneously ejected from the coating head lip, it is subjected to a high shear rate, at which point the slurry viscosity decreases, ensuring smooth flow. After the slurry is transferred to the current collector, the shear rate decreases, and it remains in a high viscosity state. This can prevent or reduce particle sedimentation when the slurry is stationary or during the drying process, ensuring a consistent coating thickness.

Viscoelasticity is also an important reference standard for evaluating slurry properties. It demonstrates the relationship between the viscosity (liquid) and elasticity (solid) of the slurry; simply put, it characterizes whether the slurry is more like a liquid or a solid. If the lithium battery slurry is only viscous and lacks elasticity, severe stringing will occur during coating, resulting in poor coating performance. Therefore, the lithium battery slurry must possess a certain degree of elasticity in addition to viscosity, allowing the broken filaments to quickly rebound during coating and ensuring uniform coating. If the slurry has very high elasticity or no viscosity at all, it indicates severe agglomeration or a solid-like structure, making coating impossible.

Viscoelasticity is generally measured using a rheometer. Small-amplitude vibrational shear forces, such as amplitude scanning and frequency scanning, are applied to measure the viscoelasticity of the slurry.

The shear rate is continuously increased from zero to a constant value, and the corresponding shear stress is recorded during this process. Then, the shear rate is gradually decreased to zero, and the corresponding shear stress value is recorded. A closed curve of shear rate versus shear stress can be plotted; this curve is called the thixotropic normal ring. The larger the area of ​​the normal ring, the greater the thixotropic performance, and vice versa.

Viscosity is a crucial factor in evaluating the stability of lithium-ion battery slurries. Both excessively high and low viscosities are detrimental to coating. High-viscosity slurries are less prone to settling and have better dispersibility, but excessively high viscosity results in poor leveling, making coating difficult and reducing coating speed. Appropriately reducing viscosity helps improve coating efficiency. Lower viscosity provides good slurry flowability and helps remove air bubbles, but excessively low viscosity leads to drying difficulties, reduces coating speed, exacerbates uneven coating, and may cause coating cracking and slurry agglomeration.

During production, especially after the slurry has been standing for a period of time, the viscosity often changes, mainly in three ways: increased viscosity, decreased viscosity, and special cases.

(1) Increased Viscosity: After prolonged standing, the colloid changes from a sol state to a gel state, causing the slurry viscosity to increase. Slowly stirring the slurry will restore the viscosity.

(2) Viscosity Reduction: Viscosity decreases when the binder absorbs moisture and undergoes qualitative changes, structural alterations during stirring, or degradation. Prolonged stirring, excessively fast stirring speed, and uneven dispersion of the slurry during homogenization can all lead to viscosity reduction. Adding slurry stabilizers, dispersants, or surfactants can improve slurry stability through the principles of charge repulsion and steric hindrance. Additionally, all components must be thoroughly dried before homogenization.

(3) Special Cases: Active materials may absorb moisture during storage or be stirred in high-humidity environments, causing PVDF to absorb moisture and produce a "jelly-like" slurry. When the slurry exhibits a particularly pronounced jelly-like consistency, it is unusable. In this case, the problematic slurry can be dried, the binder and NCM vaporized and separated, then sieved, ground, and proportionally mixed into normal slurry. It can then be used without affecting battery performance. Therefore, the most effective prevention method is to thoroughly dry the material before homogenization, measure the moisture content, and strictly control the humidity during homogenization.

The percentage of solid substances such as active materials, conductive agents, and binders in the total mass of the slurry is called the solid content of the slurry. Generally, the solid content is in the range of 40% to 70%. The main method for testing solid content is the drying method. Weigh a slurry with a mass of M, place it at a certain temperature to dry the solvent, and then weigh it again with a mass of m. The solid content is then calculated as m = M/M. A moisture meter can be used to quickly and relatively accurately measure the solid content of the slurry. The moisture meter uses the principle of thermal weight loss; it uniformly heats the sample with a halogen lamp to evaporate the solvent, and calculates the solid content by the change in mass before and after heating.

Under the same homogenization process and formulation, the higher the solid content of the slurry, the higher its viscosity. Similarly, the lower the solid content, the lower the viscosity. Within a defined range for solid content, a higher value indicates higher slurry stability. Increasing the solid content can reduce solvent usage, shorten slurry mixing time, and improve coating and drying efficiency. However, as the solid content increases, the viscosity increases, the fluidity decreases, increasing coating difficulty and causing more severe wear on equipment.

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