Rotational Rheometers Can Ensure High-Quality Battery Slurry

Rheology: Know Your Flow

Rheology, the study of the flow and deformation of matter, can provide valuable insights for products ranging from artificial cheeses to battery electrode slurries. Information about a substance’s rheological behavior can help to assure that a raw material can be processed in a certain way, or that a new formulation is within desired specifications, or that a product will perform well in a given application. While there are different techniques and instruments available to study the flow properties of various materials under different conditions, one device that stands out for its versatility is the rotational rheometer.  

A rotational rheometer works in the following way: the sample to be tested is loaded into the measuring geometry of the device, and the rheometer applies defined shear stresses or sheer deformation. The response of the sample to such mechanical forces is detected and used to calculate rheological parameters. A rotational rheometer can apply shear stresses or shear strains in the form of a continuous rotational motion via oscillation. The measurements obtained provide detailed information about the viscoelastic properties of the sample. These measurements are performed under well-defined conditions that include precise temperature control and consistent flow conditions throughout the entire test sample volume. 

A rotational rheometer usually consists of four main components: a measuring head that applies the shear stress or shear deformation and measures the response of the sample; a measuring geometry that holds the sample and transfers the torque or motion generated by the motor to the sample; a lift mechanism that moves the measuring head and the upper part of the measuring geometry into the correct measuring position; and a temperature control module that provides defined temperature conditions and supports the lower part of the geometry. 

Rotational rheometers outperform viscometers

Another type of instrument that is frequently used for measuring the flow properties of samples is a spindle viscometer. With this device, the flow of a sample is initiated by a rotating fixture–the spindle–and the viscosity value is calculated from the ratio of torque and rotation speed. However, compared to a rotational rheometer, a simple spindle viscometer is somewhat limited and shows several shortcomings: 

• A spindle viscometer is usually used to measure viscosities at a single rotation speed–a single point measurement. In those cases, it will not generate a complete flow and viscosity curve and won’t reveal any complex non-Newtonian flow behavior. 
• A spindle viscometer does not generate a well-defined and uniform flow field throughout the entire sample volume. The obtained results must be considered as relative or apparent viscosity data instead of absolute values. 
• In its standard setup a spindle viscometer does not come with any temperature control. Measurements are usually performed in a lab beaker or in the original sample container at room temperature. 
• A spindle viscometer cannot be used to investigate and quantify elastic properties of a material. 

Only a rotational rheometer is capable of all the above. 

Applications of rheometers in lithium-ion battery manufacturing

The manufacturing process of lithium-ion batteries involves many steps, but one key step is the production of a high-performing anode or cathode slurry. The slurries are what allows the battery to charge and discharge, and so for the battery to achieve peak performance, the slurries need to possess the optimal distribution of particles (no agglomeration or sedimentation) and exhibit the proper behavior when stressed. 

In the field of lithium-ion battery manufacturing, rotational rheometers can be used to measure the flow properties of those electrode slurries. The following aspects of the slurry production and processing are key considerations: 

• Quality control: Manufacturers need to make sure that the produced slurries always show the same flow properties. 
• Developing new formulations: In order to perform well in the electrode production process, the rheological behavior of a slurry needs to fall within a certain range of parameters. Using rheological measurements in the development process of new formulations avoids expensive and time-consuming trial series in the actual production of a pilot scale setup. 
• Predicting slurry stability: After mixing and during storage, electrode slurries can demonstrate undesired behavior such as sedimentation or the agglomeration of particles into clusters. These behaviors could cause battery slurries to need further agitation during storage to avoid any quality issues with the final product. Rheological data can help to predict the stability of electrode slurries, helping to know if continuous agitation during storage might be required. 
• Optimizing the coating process: To deliver the desired final battery properties, a slurry needs to perform well when it is coated onto a current collector foil. If the rheological properties of the electrode slurries are known, adjustments to the settings of the coating process can be made to ensure the best possible coating quality. 

Rheometers and twin-screw extruders: a small but mighty combination

The compact size of a rotational rheometer allows it to be deployed in small areas. For battery research, this makes it an ideal companion to a twin-screw extruder. The small dimensions of these instruments provide researchers the ability to perform R&D on a more cost-effective scale or in a more easily controlled environment. In fact, a glovebox can accommodate both a rotational rheometer and a twin-screw extruder simultaneously, allowing for the production and investigation of air-sensitive electrode slurries under an inert atmosphere. The use of a rheometer in line with the extruded product enables the level of control over the electrode slurry to be that much better. 

To summarize, a rotational rheometer is the optimal device to measure viscosity as a function of shear rate and temperature as well as for oscillatory characterization of viscoelastic material behavior. In contrast, a spindle viscometer can provide only an apparent viscosity value at a single shear rate. The rheometer’s ability to analyze characteristics of battery paste makes it ideal for quality control and for saving time and reducing costs when developing electrode slurry formulations. 

Additional resources

• Webinar: Rotational Rheometry in Battery Manufacturing and Research
• Poster: Continuous twin-screw extrusion and rheological analysis of electrode slurries for optimizing lithium-ion battery manufacturing 
• Application Note: Rheometer in a glovebox