The ABCs of Lithium Ion Cell Manufacturing
2021-05-14 | 17 min read
At a high level, manufacturing for mainstream lithium ion cells is generally a well-established process. But, like many things, it is increasingly “all in the details” in the manufacturing process where one can differentiate the performance, cost, and quality of cells, and provide a competitive advantage in the marketplace. Keysight recognizes this and continues to expand its differentiated portfolio of solutions for lithium ion cell manufacturing, to be a key supplier of choice for cell manufacturers.
So, what are the details and how can one differentiate themselves? To demonstrate this, we will first walk through the manufacturing process for lithium ion cells. We will then show where Keysight provides a wide range of solutions for the manufacturing process, and ways that these solutions provide differentiation for creating a competitive factory of the future.
The Lithium Ion Cell Manufacturing Process
The mainstream lithium ion cell manufacturing process is illustrated in Figure 1. Let’s look at each of these steps shown in greater detail.
Figure 1: The lithium ion cell manufacturing process
Here the electrodes’ active materials, binding agents, solvents, and other additives are mixed in specific ratios in preparation for being applied to the electrode. For the cathode the main active material is a lithium oxide compound. For the anode the main active material is carbon, typically in the form of graphite. Material particle size, mixing sequence, and additives all play a critical role in the performance of the active materials.
Once the active material slurries are prepared, they are applied to both sides of the electrodes’ conductive current-conducting base foils at a well-controlled thickness. Copper foil is used for the anode and aluminum foil is used for the cathode. This is done on rolls of foil in a continuous basis, with driers being utilized after application of the slurry to drive off the solvents and any moisture. It is imperative the electrode material is free of any moisture. An uncoated margin is left on the foils to serve as tabs to be interconnected later during cell assembly.
The dried electrodes are then run through rollers to compress the active material to a desired density and porosity for optimum cell performance. This also flattens the electrode’s surface to for more intimate contact with the cell separator while eliminating peaks that might break through the separator. The separator is a porous plastic membrane placed between the anode and cathode that allows ions in the electrolyte to flow between the electrodes while at the same time insulating against any electron conduction (i.e. current flow) within the cell.
For pouch cells the rolls of electrode material are cut into sheets of desired size and shape. The separator material is similarly cut into sheets or strips, depending on the construction method being used. Alternately, for cylindrical cells, the electrode and separator materials are cut into long strips of desired size and length to be rolled together.
Stacking or winding
For pouch cells the positive and negative electrodes are alternately stacked up with a separator sheet between them, for stacked construction. For Z-fold construction the positive and negative electrodes are alternately stacked up with a long strip of separator material continuously folded back over each electrode as they are added to the stack. The electrodes are stacked up such that all the positive tabs protrude from the stack together at one location and negative tabs at together at a second location.
For the cylindrical cells a strip of each electrode material together with two strips of separators are wound together in a “jelly roll” construction fashion. The electrode tabs are the full length of the strip with the positive and negative tabs protruding at opposite ends of the roll.
Welding and packaging
Once stacked all the tabs of a given polarity are welded or fused together along with heavier metal tabs to serve as the terminals that extend from the pouch of cell. The stack is then placed in a plastic pouch and the pouch is partially sealed except for a place where electrolyte can be added and for any out-gassing to escape during formation.
For cylindrical cells the jelly roll is inserted to a metal can with the electrode tabs getting connected to the case and endcap during assembly.
The cells are evacuated, and the electrolyte is then injected. The cells then rest for a specified period to allow the electrolyte to soak in to adequately wet the active material on the electrodes. Cells cannot sit too long at this stage however, as they are not chemically stable at this point.
The assembled cells then move onto the formation process, where a number electrical charging and discharging cycles are applied in a very specific regimen, up to days of time for some cells. This transforms the cells from an assembly of components into a working cell, capable of storing and delivering an electrical charge.
During formation, some of the electrolyte and lithium is consumed in the process of forming the solid electrolyte interface layer, or SEI layer, on the anode surface. This is vital to the performance and life of the cell. It serves as a barrier to prevents further consumption of electrolyte and lithium from the anode, which would quickly diminish the cell’s performance over time. The quality of the SEI layer is governed by the formation regimen. Out-gassing also takes place during formation, which is vented, followed by fully sealing the cells after formation. Cell charge capacity and open circuit voltage (OCV) electrical measurements are taken during formation to assure quality.
The cells are moved into a storage area where they rest for typically up to a couple of weeks. Electrical tests for cell OCV and self-discharge are made during this period to sort out potentially defective cells.
Testing and grading
Completed and aged cells are then electrically characterized for several parameters, including:
- Open circuit voltage (OCV)
- AC internal resistance (AC-IR)
- Pulsed current/power and DC internal resistance (DC-IR)
These electrical tests, along with self-discharge results from aging, determine the grades of the cells.
How Keysight contributes to the cell manufacturing formation and test
Keysight takes a very flexible and modular approach to lithium ion cell manufacturing, to provide a customizable offering tailored to the specific needs of the customer. This ranges from individual components, to integrated workstations for formation and test, all the up to complete formation and test line installations, including factory automation software.
Individual Keysight components for cell manufacturing formation and test
It is easy to see how cell formation is a key step in the manufacturing process. The charging and discharging regimen are vital to the cell’s quality and performance. Also, between the time spent in formation, the amount of equipment involved, and watt-hours of electricity consumed, formation also adds considerable cost to lithium ion cell manufacturing. Keysight provides the BT2200 Charge-Discharge Platform, pictured in Figure 2, for flexible and cost-effective cell formation.
Figure 2: The Keysight BT2200 Charge-Discharge Platform for cell formation
The BT2200 Charge-Discharge platform has industry-leading efficiency with regeneration to recycle energy back to the AC line. Coupled with its space-savings size and flexibility, it can bring significant advantages and savings to the cell formation process. To learn more about the formation process and differences provided by the BT2200, check out the posting: “Innovative Keysight Solution Platform Enhances Li-Ion Cell Formation in Manufacturing” (click on text to access).
All cells have some self-discharge. However, excessive self-discharge is a strong indicator of underlying defects within the cell. Hence all cells get screened for self-discharge in manufacturing. Traditionally this is accomplished by measuring the loss of OCV over weeks of time, referred to as the delta OCV method. An alternate, and faster way is using a potentiostatic test method to directly measure the cell’s internal self-discharge current. The Keysight BT2152B Self-Discharge Analyzer, pictured in Figure 3, is based on the potentiostatic method for making faster cell self-discharge measurements.
Figure 3: Keysight BT2152B Self-Discharge Analyzer
To learn more about lithium ion self-discharge, and details of the delta OCV and potentiostatic test methods, check out the postings “Keysight Solutions for Measuring Self-Discharge of Lithium Ion Cells Achieves Revolutionary Reduction in Test Time”, and more recently “Denoising algorithms enhance self-discharge current measurements on Li-Ion cells”. (Click on text to access.)
Testing a cell’s DC-IR, pulsed current, and pulsed power capabilities requires equipment having high power, high current, and high slew rate loading capabilities coupled with synchronized fast digitizing measurement. Keysight has fast two-quadrant DC sources ideally suited for DC-IR, pulsed current, and pulsed power testing on cells. An advantage of using a two-quadrant DC source is that it can test cells for either pulsed discharging or pulsed charging conditions. One example of a two-quadrant DC source is the N7950A, 20V, +/-100A Advanced Power System, pictured in Figure 4. With built in ARB and digitizing measurement capabilities, the N7950A is a complete solution onto itself, well suited for testing DC-IR, and pulsed discharging and pulsed charging on cells.
Figure 4: Keysight N7950A 20V, +/-100A Advanced Power System
In addition, Keysight has a complete portfolio of DMMs, SMUs, and data acquisition equipment for performing voltage, AC-IR, temperature, and most any other measurement on cells in manufacturing, as needed.
Integrated stations for formation and test
Lithium Ion manufacturing lines range from semi-automated for small pilot lines, to fully automated for high volume production. Stations are required along the manufacturing line for performing the formation and test steps on trays of cells. Stations typically incorporate, but are not limited to, the following:
- AC power distribution.
- Electronics for control and automation.
- A chamber/fixture for receiving a tray of cells from an automated or semi-automated loader.
- Electrical contacting to connect to the contacts of all the cells in the tray.
- Station controller.
- Equipment for formation charging and discharging.
- Equipment for making test measurements.
- Temperature monitoring.
- Safety features.
Stations developed and integrated by Keysight saves time and cost. As they are built and tested as an integrated system there are no surprises. They are designed for flexibility, being adaptable for a wide range of cell form factors, and for easy maintenance.
Software for the cell formation and test floor
Inevitably in-house software often grows from a basic initial effort and is hurriedly built on to support a growing business. This leads to making a lot of compromises. Keysight’s software for the cell formation and test manufacturing floor starts with a fresh, new approach to define what it truly takes to create a manufacturing process for a factory of the future. It is modular, making it extremely adaptable and flexible. Various software components can be provided as needed as well as integrate in and make use of the manufacturer’s existing software components. It is scalable. It can support from just one station to hundreds of stations. It works within the manufacturer’s existing environment. It supports manual, semi-automated, or fully automated production. It uniquely supports all different types of users, from operators to system administrators. It tracks and manages the status and workflow of the cells as they are manufactured. It uniquely supports the various station functions, such as formation stations and measurement stations. It supports data storage, whether on local servers, or in the cloud, or mixed. Advanced analytics help to identify obscure root causes of problems. The list goes on.
Complete turnkey solutions
When needed, Keysight also provides complete turnkey solutions for the factory formation and test process, as this may not be a core competency of the manufacturer. Between the conveyer systems, handlers, formation and testing stations, storage and aging, and software, Keysight’s turnkey solutions take care of all aspects of the factory manufacturing floor. Cells come in as an unfinished assembly and leave completed, tested, and sorted, ready for shipment.
While there continues to be significant improvements, breakthroughs, and altogether other innovative new design concepts coming out for lithium ion cells, the mainstream of cells being manufactured follow a well-established process that has been covered here. For these mainstream cells it’s all about making improvements in the details where one can differentiate the performance, cost, and quality of cells, to provide a competitive advantage. Correspondingly, Keysight provides a range of solutions for cell manufacturing, paying close attention to all the details that provide the differentiation needed for that competitive factory of the future. Examples of ways Keysight differentiates itself shown here and in other postings include:
- the BT2152B Self-Discharge Analyzer, which measures cell self-discharge in about an hour compared to the traditional approach that takes days to weeks.
- The BT2200 Charge-Discharge platform, which provides industry-leading energy efficiency, space savings size, and flexibility.
- Test and measurement equipment for accurate measurement of any cell parameter needed.
- Fully integrated formation and test stations for quickly getting up and running with no surprises.
- Complete formation and test line installations where this may not be an in-house expertise.
- System software, which starts with a fresh new highly flexible approach to address the needs for that factory of the future.