What is the purpose of this article?
Show the importance of monitoring viscosity, density, and concentration of cathode and anode slurry during battery manufacturing. This article shows how the SRV and SRD can help control these parameters, improving the quality of the final product, the battery’s lifespan, and recyclability.
What products are involved?
SRV - Inline Viscosity Meter and SRD - Inline Density and Viscosity Meter

TABLE OF CONTENTS

Introduction

Battery slurry and electrode manufacturing:

Production parameter control and its importance:

Rheonics Inline Density and Viscosity sensors

Recommended Rheonics sensor locations

Comparing inline and lab readings:

Interpreting mixer data for quality control

Introduction

In our modern technology-driven world, batteries have become indispensable in daily life, powering everything from smartphones and electric vehicles to renewable energy storage systems.  As reliance on these energy sources grows, battery production has gained significant attention. Ensuring the quality of every component is key, as even minor deficiencies can lead to reduced battery performance, safety hazards, and shortened lifespans. In particular, the increased production of batteries introduces multiple environmental hazards through manufacturing and disposal. Process monitoring during fabrication can help ensure high-quality batteries which translates to smaller environmental footprints and a higher chance of producing long-living, recyclable batteries.

Figure 1: Typical cell production process includes electrode manufacturing (battery slurry mixing and manufacturing of battery electrode sheets), cell assembly (steps vary based on final cell type), and cell finishing (involves degassing, aging, and testing)


Battery production is a three-stage process with each stage containing several unit operations: 

  1. Electrode manufacturing (mixing, coating, drying calendaring, slitting, and vacuum drying), 

  2. Cell assembly (separating, stacking, packing, and electrolyte filling), and 

  3. Cell finishing (forming, degassing, aging, and testing). 

To ensure the production of cells of the highest quality, adequate controls must be applied throughout fabrication with close attention to the electrode manufacturing stage. Battery cells are comprised of multiple individual batteries, which are themselves made up of layered electrode sheets. A battery cell is, therefore, only as strong and durable as its weakest electrode sheet. 

Battery slurry and electrode manufacturing:

Electrode manufacturing begins with the formulation of battery slurry. This involves the dispersion of dry ingredients in solvents until a homogeneous slurry is obtained. The compositions vary for anode and cathode slurries but are comprised of active materials, conductive additives, solvents, and binders.

Figure 2: Simplified P&ID of Electrode manufacturing stage showing suitable installation points for the SRV and SRD.


In this first step of electrode manufacturing, raw materials are mixed and dispersed based on the electrode design desired. This process happens in a mixing vessel where different temperatures, mixing tools, mixing angles, atmospheres, and mixing times are employed. After this process, the battery slurry is then transported to the next step: coating. The transport can be achieved through pipework or storage in sealed tanks. Once at the coating step, slurry is applied onto a thin foil which undergoes drying, calendaring, slitting, and vacuum drying before being assembled into a cell and filled with electrolytes in the second stage.

In the final stage, cell finishing, the battery cell is encased and tested.

Production parameter control and its importance:

Electrode production is independent of the type of cell assembled. This means that closely monitoring the quality of electrodes is key to quality across all cell types and careful control of battery slurry formulation is the way to obtain the highest quality electrodes.


Figure 3: Inspection of high-capacity lithium-ion batteries for electric vehicles


According to the report on  LITHIUM-ION BATTERY CELL PRODUCTION PROCESS by RWTH Aachen [1], key features that determine the slurry quality are:

  • Homogeneity

  • Particle size

  • Purity 

  • Viscosity

Rheonics offers solutions that help monitor these variables.

Rheonics Inline Density and Viscosity sensors

Rheonics provides inline sensors for measuring viscosity and density, enabling comprehensive automation of slurry mixing and electrode coating processes. 

The Rheonics SRV sensor measures viscosity and temperature, whereas the SRD sensor measures density, viscosity, and temperature simultaneously. Both sensors are designed to operate accurately in various environments, delivering consistent and reliable results.

These Type-SR sensors offer users the following benefits:

  • Continuous inline monitoring of viscosity and density during slurry mixing and coating processes.

  • Zero recalibration is needed throughout the sensor's lifespan.

  • Reduce waste by removing sampling and measurement delays

  • High precision and repeatability

  • Enhanced potential for complete process automation.


Figure 4: Rheonics SRV (left) and SRD (right) 3/4” NPT sensor probes

Rheonics density and viscosity sensors are recommended at various points in battery slurry preparation and coating. At each of these points, the sensor provides validation, and quality control, and can be used for process automation.


Figure 5: Electrode manufacturing process (left to right): Incoming raw materials, mixing, storing, coating, and slitting. [2]


Incoming raw materials - SRV or SRD is used to ensure incoming material is within specifications before entering the process. When the process involves a viscous material or a mixture, viscosity gives a great deal of information, and ensuring the material meets viscosity specifications results in high product consistency. However, SRD is used when density is known to be the key determinant of the quality of the raw material. 

Mixing - The SRV can be directly installed in the mixer. This allows real-time monitoring of viscosity as it develops through mixing. It is extremely useful as it allows immediate interventions if the viscosity deviates from the setpoint. It can also be used to indicate homogeneity, slurry aging, or the presence of impurities. Measurements here can work hand-in-hand with current sampling and slurry characterization.

Storage tank - For storage tanks, the SRD is often installed on recirculation lines. Keeping both the cathode and anode slurries in check with real-time monitoring of density and viscosity ensures consistent slurry that will then be transferred to the supply tank.

Supply tank - Installation of an SRD at the tank bottom can be used to monitor if sedimentation is occurring during transport.

Coating - Ensuring a uniformly thick layer of electrode slurry is easily done using an SRV to get consistent viscosity readings. Ensuring the same thickness through tight viscosity control is perhaps the most crucial factor for achieving the same cell build across batches of electrodes, translating to optimum performance and lifetime of the fully assembled battery pack. The SRV is an award-winning solution that is revolutionizing the process of coatings such as this. 

Cell filling and packaging - Rheonics SRV and SRD can be used to monitor electrolyte filling and dosing. Monitoring the viscosity and density of the electrolyte helps to obtain a homogeneous wetting process to properly activate the cell. Here the SRV and SRD are both possible solutions however, the SRV will be easier to integrate in the small lines of electrolyte filling systems.


Figure 6: Cell filling/wetting with an electrolyte solution. [3]


Follow these articles for installation guidelines:

Comparing inline and lab readings:

A Type-SR Sensor operating in a non-Newtonian fluid-like battery slurry will not give the same viscosity reading as one from a lab viscometer. The two operate at very different shear rates. The SRV and SRD are predominantly process control instruments, they can give you the smallest deviations from the baseline in production, which is ideal for process monitoring and control.

Figure 7: Process for achieving direct comparison between lab instruments and Rheonics inline viscometer and density meters. 


In other words, the SRV or SRD’s job is not to replace the laboratory rotational viscometers or rheometers. Rheonics' sensors are predominantly for process control. They are real-time indicators of changes in the fluid and allow the tracking of any change in the formulation of the fluid that is going through the line.

However, if the primary use is to meet lab measurements, then a correlation between the SRV or SRD viscosity measurements with lab values collected after sampling would be required. It is possible to upload this correlation model to the sensor so it gives you an equivalent reading.

Interpreting mixer data for quality control

When viscosity is measured continuously in a mixing tank, the measurements can allow operators to derive a great deal of additional information about the system. The reduction in measurement noise as the process reaches the setpoint viscosity is an indicator of the homogeneity of the system. Later deviation from the setpoint tolerance envelope can indicate artifacts, bubbles, or aging in a slurry. Ensuring viscosity and density remain in the setpoint envelope is an excellent way to confirm the end product is meeting the composition and consistency targets for quality control.


Figure 8: Sample viscosity data from an SRV in a mixing tank. Dark blue is the SRV measurements, black is the time-averaged data, and light blue is the setpoint tolerance envelope.


The correct distribution of solids in the slurry is ensured by the homogeneity of the density measurements with an SRD in the mixer. This allows for correct solids loading on the coated foils and as solids loading is a key performance indicator for battery cells, SRD density monitoring can also be used for this aspect of quality control. 

The tight control of viscosity similarly allows consistency in coating layer thickness. This aids in quality control for battery power density and recharge cycling.

References

[1]. LITHIUM-ION BATTERY CELL PRODUCTION PROCESS

[2]. Battery Materials 

[3]. Electrolyte Filling of a Lithium-Ion Cell