How to characterize the stability and dispersion of battery slurry?

The stability and dispersibility of battery slurry have an important impact on the properties of electrodes and finished battery products. So how to characterize the stability and dispersibility of battery slurry?

Characterization method of battery slurry stability

1. Solid content method

The solid content test method is a low-cost and easy-to-test method. Its principle is to place the slurry in a container and take samples at the same location at regular intervals to test and analyze the solid content. By judging the difference in solid content, the stability of the lithium battery slurry can be judged to see whether there are sedimentation, stratification and other phenomena.

2. Viscosity method

The viscosity test method can also basically reflect the stability of the slurry. Its principle is to place the slurry in a container and test the viscosity at regular intervals. The stability of the slurry can be judged by the change in viscosity.

3. Stability Analyzer

The use of stability analyzer can speak with data. For example, Sung et al. used a stability analyzer to monitor the light transmittance changes of different pH slurries using PAA as a binder within 12 hours. The initial light transmittance and 12-hour change valuesof the neutral slurry were smaller. Because carbon black materials have light absorption, lower light transmittance indicates better dispersion of carbon black particles, and smaller micro-agglomerates have larger specific surface areas, thereby improving light absorption efficiency. At the same time, the small change in the light transmittance of the slurry within 12 hours indicates that the slurry has good dispersion stability during the static process, as shown in the figure below.

4. Characterization of Zeta potential

Zeta potential refers to the potential of the Shear Plane, also known as electrokinetic potential or electromotive force, and is an important indicator for characterizing the stability of colloidal dispersions. The smaller the molecules or dispersed particles, the higher the absolute value of the Zeta potential (positive or negative), and the more stable the system, that is, the dissolution or dispersion can resist aggregation. Conversely, the lower the Zeta potential (positive or negative), the more it tends to coagulate or aggregate, that is, the attraction exceeds the repulsion, the dispersion is destroyed and coagulation or aggregation occurs.

Characterization method of battery slurry dispersion

1. Fineness

Fineness is an important performance indicator of battery slurry, which can reflect information such as slurry particle size and dispersion. The fineness value can be used to understand whether the particles in the slurry are dispersed and whether the agglomerates are deagglomerated.

2. Membrane impedance

Lithium battery slurry is a solid-liquid mixed system formed by dispersing electrode active materials and conductive agents in a binder solution. According to the principle of four-probe membrane impedance test, the slurry membrane impedance is tested. The distribution state of the conductive agent in the slurry can be quantitatively analyzed through resistivity to judge the dispersion effect of the slurry. The specific test process is: use a film applicator to evenly coat the slurry on the insulating film, then heat and dry it, measure the thickness of the coating after drying, cut the sample, and the size meets the infinite requirement. Finally, use four probes to measure the electrode membrane impedance and calculate the resistivity based on the thickness.

3. Scanning electron microscopy/energy spectrum analysis/cryo-electron microscopy

Scanning electron microscopy (SEM) can be used to directly observe the morphology of battery slurry, and cooperate with energy spectrum analysis (EDS) to analyze the dispersion of each component. However, when preparing samples, the drying of the slurry during this process may cause the redistribution of its own components. Cryo-electron microscopy (Cryo-SEM) can maintain the original distribution state of the slurry components, so it has recently begun to be used in slurry property analysis.

4. Electrode CT imaging

Electrode CT imaging can directly observe the dispersion state of particles in the electrode. As shown in the following figure, there are more large particles agglomerated in the electrode in Figure a, the agglomerated particles in the electrode in Figure b are significantly reduced, and there are almost no agglomerated large particles in the electrode in Figure c.

5. Laser diffraction measurement technology

Laser diffraction measurement technology uses Fresnel scattering theory and Fraunhofer theory to obtain particle size and distribution. The laser particle size analyzer based on this technology has high measurement accuracy, good repeatability, and short measurement time. It has been widely used in battery factories to test the particle size of slurry in batteries.

6. Electrochemical impedance spectroscopy analysis method

For example, Wang et al. used the electrochemical impedance spectroscopy analysis method (EIS) to directly analyze the impedance spectrum of the liquid slurry and obtained the electrochemical characteristics of the slurry at different particle concentrations. And through the impedance spectrum fitting results, an evaluation method for the internal particle distribution structure of the electrode slurry based on the parameter equivalent circuit model was established, which provided a new idea for the online measurement and online evaluation of the internal non-uniform structure of the lithium-ion battery slurry. The EIS test principle is shown in the figure.

Methods for characterizing both slurry stability and dispersibility

1. Rheometer

(1) Viscoelasticity test

The viscoelastic characteristics of the slurry are characterized by the relative valuesof the storage modulus (G′) and the loss modulus (G″). The storage modulus G′, also known as the elastic modulus, represents the capacity stored when the slurry undergoes reversible elastic deformation and is a measure of the elastic deformation of the slurry. The loss modulus G″, also known as the viscous modulus, represents the energy consumed when the slurry undergoes irreversible deformation and is a measure of the viscous deformation of the slurry. In the frequency scan, based on the relative size of G′and G″and evaluating the sensitivity of G′to the angular frequency, it is possible to reflect whether the slurry is in a fluid state or a solid-like state. In the low-frequency range, G′>G″and the larger the difference, the better the stability of the slurry. As shown in the figure below, the stability of natural graphite slurry is better than that of synthetic graphite slurry.

(2) Changes in viscosity with shear rate

The viscosity of a slurry usually changes with shear rate. When shear thinning behavior exists, there are soft agglomerates in the slurry that are easily destroyed by shear stress. On the contrary, the presence of shear thickening usually indicates that there are hard aggregated particles in the slurry. Generally speaking, slurries with faster shear thinning rates tend to have better dispersibility, ignoring the destruction of the binder by shear force. As shown in the figure below, the slurry represented by the hollow circle has better dispersibility than the other two slurries.

(3) Yield stress test

The yield stress in rheology is defined as the applied stress at which irreversible plastic deformation is first observed on the sample. Theoretically, the yield stress is the minimum stress required to initiate flow. Yield analysis is important for all complex structured fluids. It helps to better understand product performance, such as shelf life and stability against sedimentation or phase separation. There are a variety of rheological methods that can be used to determine the yield stress. The figure below shows the yield stress analysis using the shear flow ramp-down method. From the test results, it can be seen that at moderate shear rates, the shear stress decreases as the shear rate decreases. However, when the shear rate is further reduced, the stress curve reaches a stable level and is independent of the rate. This stable stress value is called the yield point. At the same time, the measured "apparent viscosity" curve becomes infinite and has a linear relationship with the shear rate when the slope is -1.

Since synthetic graphite has a larger particle size and more irregular particle shape, the slurry exhibits a lower yield stress and a weaker network structure. Therefore, this synthetic graphite slurry sample will be more susceptible to sedimentation and phase separation. Slurry sedimentation can lead to uneven distribution of active materials on the electrode, thereby reducing battery performance.

(4) Thixotropy

After coating, the battery slurry will level under the action of gravity and surface tension on the current collector. In the low shear rate range, it is hoped that the viscosity will gradually return to the high viscosity before coating. Before it returns to high viscosity, the viscosity of the slurry is still relatively low, easy to level, and the coating surface is smooth and uniform in thickness. The recovery time should not be too long or too short. If the recovery time is too long, the viscosity of the slurry will be too low during the leveling process, and it is easy to have tailing or the thickness of the lower edge is higher than the thickness of the upper coating. If the time is too short, the slurry will not have time to level.

2. Slurry resistance meter

The slurry resistivity parameter has a significant correlation with the slurry formula, the type and content of the conductive agent, the type and content of the binder, etc. After the slurry is stirred and left to stand for a period of time, gel sedimentation may occur, and the resistivity value will also show different degrees of change. Therefore, the slurry resistivity can be used as a method to characterize the uniformity and stability of the slurry's electrical properties.

Test method: put a certain volume of slurry (about 80mL) into the measuring glass cup, insert a clean electrode pen, start the software, test the change of slurry resistivity at three pairs of electrodes over time and save it to the document.

Test parameters: resistivity, temperature, time

Calculation formula: Resistivity (Ω*cm):Ρe=U/I * S/L

Features:

1. Separate the voltage and current lines, eliminate the influence of inductance on voltage measurement, and improve the resistivity detection accuracy.

2. The 10mm diameter disk electrode ensures a relatively large contact area with the sample and reduces the test error.

3. The resistivity change at three positions in the vertical direction of the slurry over time can be monitored in real time.

Resistivity measurement range: 2.5Ω*cm~50MΩ*cm

Resistivity measurement accuracy:±0.5%