Lithium-ion batteries are everywhere today. You can find them in your computers, smart phones, power tools – and electric vehicles. Growing electric vehicle demand is helping to drive the development of more advanced batteries with higher energy density and lower cost. Collectively, different lithium-ion batteries are known as “lithium batteries” or “LBs.”
LB components and materials have been thoroughly researched in recent years, but further physical testing is needed to fully evaluate their performance. Testing requires manufacturing physical battery cells for evaluation. The most common cell formats used in testing are coins and pouches. Most labs manually produce these cells due to cost and resource restraints. This manual production can lead to significant variations in test results.
Our team recognized the need to further study coin and pouch cell fabrication, as it greatly impacts overall cell quality and performance. We identified key factors, parameters and protocols for coin and pouch cell fabrication processes. It is our hope that this research can provide general guidelines on reliable and reproducible cell fabrication and testing for the rest of the battery research community.
Electrodes are the most important component in LB cells and have an outsize impact on cell performance. Previous research has identified several best practices for electrode preparation, including mixing slurry with the appropriate equipment and timing. Some of the steps we take in creating this electrode slurry includes pre-grinding and sieving solid powders before wet mixing with a binder solution and ensuring that the solid content is consistent among batches.
Coating parameter variation is another factor that impacts performance, especially the thickness of the coating. A steady speed of coating application can help address variations in the slurry feeding method to the coating blade and evaporation rate of the solvent. That said, it is still important to carefully calibrate the gap of the coating blade as that can impact thickness variation along the coating width.
Component dryness also plays a critical role in cell performance. Uncontrolled moisture content in batteries can result in unstable active material structure, gas evolution and other potential safety issues. It’s therefore necessary to periodically check moisture content to ensure organic solvents and lithium salts are dry. We also recommend drying the separator before use, given its porous nature.
Coin cell parts, including cathode and anode cases, spacers and springs, also need to be carefully cleaned before drying. These metal parts can have metal and organic residues that can also lead to battery instability. Acetone/alcohol and a DI water rinse with an ultrasonic bath can help remove this residue before drying. Other cell component parts, including pouch materials and tabs/tapes, should be also pre-dried before each batch of cell fabrication to prevent moisture accumulation.
Coin Format Cell Preparation
Coin cells are the primary cell type used in battery research due to their simple configuration, easy preparation and relatively low material cost. How the cathodes and anodes are aligned within these cells can impact the overall cell quality. We’ve found that the anode should be slightly larger than the cathode to ensure good alignment of all cell components.
Another key factor is the amount of electrolyte material used in coin cell assembly. Generally, the electrolyte should fill all the pores in the electrode and separator membranes. However, some excess of electrolyte material can give better capacity, especially in certain novel chemistries. To ensure consistent testing results, an identical amount of electrolyte material should be used in all cell preparations.
The amount of pressure applied on internal parts like the electrodes and separator also impacts overall cell performance. In a coin cell, the applied pressure on electrodes comes from spring compression. A cell’s spring compression is determined by the total thickness of all its components and the thickness of the separator. When using similarly thick electrodes in different batches, the same separator thickness should be used to maintain consistent spring compression, which relates directly to the overall internal applied pressure inside the cell. A corresponding adjustment of the separator thickness is necessary to provide sufficient internal pressure when the thickness of the electrode coating, mass loading or lithium counter electrode thickness changes.
External pressure is applied by the crimping process. Unfortunately, it is hard to have a standard protocol for setting the crimping pressure when sealing cells, since the mechanics and designs of crimping tools vary. However, a consistent setup with an appropriate pressure setting and holding time can reduce the failure rate during cell making and can improve the data reproducibility of coin cells used for testing.
Pouch Format Cell Preparation
Pouch-style cells have become more common in testing as they more closely match their commercially produced counterparts compared to coin cells. Like coin cells, manually-made pouch cells can be significantly impacted by various factors in the cell fabrication process. The pouch cell fabrication process contains more steps than coin cell fabrication, increasing the risk of system and human errors. That said, many of the factors that impact coin cells also impact pouch cells.
Electrode alignment is still critical. We use a customized jig to apply pressure and geometrically confine what we call the “jelly roll” (electrode/separator stack) for uniform cell alignment.
Electrolyte wetting time is important, especially for cells with thickly coated electrodes. The wetting time should be controlled carefully to allow complete diffusion of the electrolyte. A vacuum sealed pouch can prevent electrolyte evaporation and penetration of external moisture or impurities during the wetting process.
Controlling the amount of electrolyte is another factor that impacts overall cell performance. The electrolyte amount should be accurately controlled and measured to create reproducible results in different pouch cells. To obtain the appropriate amount of electrolyte within the cell, several separate measurements are needed before and after the formation/resealing process.
Dryness remains an important factor in the performance of pouch cells. Considering the much larger surface area of the electrodes and separator compared to a coin cell, as well as the longer fabrication time, pouch cell components may absorb more moisture and impurities during fabrication. Immediate use of pre-dry components and shorter operation times are highly recommended to limit the impact from moisture.
Unlike coin cells, pressure applied on internal components in pouch cells comes from the vacuum effect inside the sealed pouch and external stacking pressure. Stacking pressure, which is usually provided by the cell fixture, is another critical factor for consistent cell performance and data reproducibility. Most prototype cell-making fixtures were created with different, customized designs to control pressure, and universal design standards are unfortunately not in place.
Plate texture and spring compression calibration are two things you should pay attention to when making and using jigs. When used as jig materials, metals like aluminum alloy or stainless-steel can provide better pressure distribution. In contrast, plastic or fiberglass plates are more flexible and bend at their corners when high pressure is applied.
Springs should be also carefully selected and calibrated. If necessary, multi-point pressure calibration on different locations is recommended to achieve better pressure distribution and spring compression control. A multi-point pressure check with a flat-shaped pressure sensor will further help control accuracy with external pressure, regardless of the spring type or type of cells being loaded into the jig.
When testing LB cells, a dedicated thermal chamber with accurate temperature control is necessary to provide a stable testing environment. In preliminary cell studies, many researchers use “room temperature,” which can fluctuate and affect overall cell performance. This is especially true with temperature-sensitive systems used in evaluating coin cells. For pouch cells, the heat exchange is slower within the cell-holding test jigs, making it necessary to build rest time into the testing process within the thermal chamber to stabilize the cell temperature.
The battery research community has recognized the critical role that reliable cell fabrication plays in creating reproducible battery studies. There are many factors that impact reproducible results in cell fabrication and more research is needed. We hope our work can help draw attention to these areas and provide some initial guidelines for the community. These efforts can help bridge the gap between study and practical application of LB technology.