Monitoring and Controlling the Electrode Particle Characteristics and Viscosity of Battery Slurries

Batteries play a major part in modern life and reliance on them is increasing. Thus, guaranteeing optimal battery performance through formulation optimization and manufacturing control is of growing importance. In earlier articles, Malvern Panalytical has covered the significance of controlling the size of particles used in the manufacture of battery materials [1] and the influence of carbon microstructure in graphite electrodes on battery performance [2].

Particle shape is also a significant factor to think about and control, as irregular-shaped particles not only decrease packing density, but they can also result in the formation of a high viscosity electrode slurry. In this third article on batteries, Malvern Panalytical looks at the role of shape and size on the viscosity of the electrode slurry.

Electrode Composition

The usual structure of a battery electrode is shown in Figure 1. The electrode is typically fabricated by applying a slurry of particles in suspension onto a metal foil.

Typical structure of the Li-ion battery.

Figure 1. Typical structure of the Li-ion battery.

The slurry in question is made up of electrode particles (cathode or anode), small carbon particles to help conduction and binder material (made up of polymer and solvent) to hold the structure together. The concentration of particles in the slurry is high, representing between 20-40% of the total by weight. Therefore, the particle properties have a substantial influence on the physical properties of the resultant slurry.

Particle Properties and Slurry Viscosity

The viscosity, concentration, dispersibility and compactability of the slurry are crucial parameters in establishing how effective the slurry will be during application. A high viscosity slurry causes problems in the coating process and poor dispersibility results in low film uniformity; the compactability and concentration of the slurry controls the film density. Uniformity of the coating thickness and the layer density are crucial to guarantee control over the life time (recharge cycle time) and ion transfer rate of the battery, while regulating the layer thickness enables a smaller battery to be created.

As shown in Figure 2, the presence of a high proportion of irregularly-shaped particles will result in a higher viscosity slurry because of the greater impact of particle friction and interlocking, but also because of the additional flow energy necessary for the fluid to circumvent the particles.

Irregularly-shaped particles experience greater interlocking and friction, leading to a higher viscosity.

Figure 2. Irregularly-shaped particles experience greater interlocking and friction, leading to a higher viscosity.

Particle shape also influences packing density since uneven particles pack less efficiently than spheres. Hence, lesser particles can be incorporated to the liquid prior to the viscosity starting to increase, as shown in Figure 3. Particle size can also be essential regarding packing since a polydisperse sample will pack more efficiently compared to a monodisperse sample at the same concentration, and hence lower viscosity. Smaller uneven particles, however, may boost viscosity because of their higher surface area, which will emphasize particle-particle and particle-liquid interactions. Thus, it is crucial to be able to track and control the proportion of erratically shaped particles and fine material within an electrode material sample to lessen the viscosity.

The influence of particle shape on viscosity.

Figure 3. The influence of particle shape on viscosity.

Case Study

In this article, two types of carbon material were studied for use as a carbon electrode material: Carbon A, from natural sources and synthetically produced Carbon B. Both materials were integrated with the same binder (2.5% PVDF by weight in NMP) to form two slurries at a concentration of 22% by weight.

Viscosity Measurement

Viscosity measurements were done using a Kinexus rotatonal rheometer at shear rates ranging from 0.1 to 1000 s-1. Figure 4 illustrates that the incorporation of PVDF to NMP increases the viscosity by two orders of magnitude (approximately 200 times) relative to NMP alone and the viscosity remains largely independent of shear rate (Newtonian behavior).

Slurry with Carbon A (naturally occurring) has a much higher viscosity than Carbon B (synthetically produced)

Figure 4. Slurry with Carbon A (naturally occurring) has a much higher viscosity than Carbon B (synthetically produced).

Adding carbon black further increased the viscosity and the resulting slurries both revealed shear rate dependence (non-Newtonian behavior). The slurry made with Carbon A showed a much higher viscosity compared to Carbon B at high and low shear rates, which would probably increase resistance to sedimentation on standing (low shear process) and deliver a thicker electrode film on coating (high shear process). The higher viscosity may also render the coating process more hard to control, possibly leading to an irregular coating and variable layer density, which in turn brings about a variable ion transfer rate and hence battery life time (and recharge cycle time).

Size and Shape Measurement

To determine the reason behind the differences in viscosity, both carbon powder samples were tested using the Morphologi G3. The samples were spread using a low energy dispersion of 1 bar and over 70,000 particles were automatically measured using the 10x objective.

As Figure 5 shows, it was noticed that the carbon material attained from natural sources had more fine material compared to the carbon sample produced synthetically.

Size distributions for the naturally occurring carbon (red) and synthetically produced carbon (green).

Figure 5. Size distributions for the naturally occurring carbon (red) and synthetically produced carbon (green).

Additionally, it was discovered that although there was minimal difference in aspect ratio between the two carbon samples, comparing circularity revealed that Carbon B, the artificial carbon material, has a higher circularity than that from natural sources, Carbon A, as illustrated in Figure 6. This is confirmed from the particle illustrated in Figure 7.

The synthetically produced carbon (green) is more circular than the natural occurring carbon material (red) but there is little difference in aspect ratio.

Figure 6. The synthetically produced carbon (green) is more circular than the natural occurring carbon material (red) but there is little difference in aspect ratio.

Particle images illustrating the differences observed in particle shape - the naturally occurring Carbon A has a much lower circularity than the synthetically produced Carbon B.

Figure 7. Particle images illustrating the differences observed in particle shape - the naturally occurring Carbon A has a much lower circularity than the synthetically produced Carbon B.

Conclusion

Two carbon-based electrode materials have shown quite different viscosities when converted into a slurry, resulting in varied application behaviors during the production of batteries. Using the Morphologi G3, Malvern Panalytical has been able to reveal that the natural-source carbon has a higher proportion of fine material and irregular particles. Hence, when dispersed into a slurry the natural source carbon will yield lower packing fractions and higher viscosities.

A higher viscosity slurry decreases coating control during application to the electrode foil, which consecutively can result in an irregular coating of varying density. This influences battery performance, as the succeeding variability in ion-transfer rate causes an erratic battery lifetime. Thus, use of the Morphologi G3 to observe the slurry particle features can guarantee that these factors are controlled.

References

[1] Malvern Panalytical, “Characterization of Battery Materials using Laser Diffraction Particle Size Analysis”

[2] Malvern Panalytical, “Exploring the Effect of Carbon Microstructure on Lithium-ion Battery Performance”

[3] Malvern Panalytical, "Ten Ways to…..Control Rheology by Changing Particle Properties"

This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.

For more information on this source, please visit Malvern Panalytical.

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