Written by Dan Caesar

 

Separator Film Testing for Electric Vehicle Batteries

  

Separator films are used in EV battery production to separate the anode and cathode in lithium-ion and other liquid electrolyte batteries. These films are required in high volumes and are extremely critical to product safety. Because testing standards specific to EV batteries have not yet been established, ISO 527-3 is commonly used as a proxy standard to characterize their frictional properties. The most common material used for separator film is polyolefin, a polymer film that is strong enough to withstand the winding operation during assembly as well as uneven plating of lithium on the anode due to extensive use. Safer and stronger separator material more effectively prevents contact between the anode and cathode, while thinner material helps reduce the weight of each battery and improve energy density.

Because thin film specimens break at relatively low forces, ISO 527-3 testing is often performed on a single column universal testing machine. However, due to the fact that many thin film specimens exhibit high elongation at break, an extra height single column frame or possibly a dual column frame may be required in order to test until break. Instron's 3400 and 6800 Series include a variety of test frames suitable for thin film testing, with the 6800 Series offering many advanced features for those wishing to maximize repeatability and throughput.

Due to the flexible, delicate nature of thin film specimens, specimen gripping can be a challenge. Pneumatic side action grips are an ideal solution because they provide an adjustable inlet air pressure to ensure that specimens can be clamped sufficiently throughout the test. When using pneumatic grips with the Smart-Close Air Kit, operators can define the inlet clamping pressure in the Bluehill Universal test method, replacing the need to manually change the pressure at the air supply’s regulator. The Smart-Close Air Kit not only promotes a safer use of pneumatic grips, but it can make testing more efficient and repeatable.

For those looking for a non-pneumatic solution for ISO 527-3 testing, Instron recommends self-tightening roller grips. Manually-tightened screw grips are not recommended for thin film testing, because the variability they add to the clamping pressure can negatively impact the repeatability of results and cause a higher frequency of improper gripping (jaw breaks or slippage). 

Specimen Alignment

Tensile testing of thin films and foils presents more challenges than the testing of rigid specimens. The majority of thin films and foils used in EV battery manufacturing have a thickness of no more than 15 microns, making it extremely easy to damage them during the specimen insertion process. Correctly aligning such delicate specimens is a major pain point for many testing labs, and the use of a precision specimen loader device is recommended. These devices use a detachable alignment clip and linear rail to guide the specimens into the grips, and are available in multiple sizes to accommodate a wide range of specimen dimensions.

Extensometers enable us to provide the most accurate strain measurement. Non-contacting video extensometry is the best solution for thin film testing, as typical clip-on extensometers weigh down the specimen causing premature failure. Instron's Advanced Video Extensometer (AVE2) can measure strain as accurately as a clip-on extensometer without physically contacting the specimen.

Instron's Bluehill® Universal testing software makes it easy to report calculations in compliance with the standard. Customers who choose the Plastics method suite will receive a pre-configured test method for ISO 527-3 testing, complete with specific test parameters and calculations to help ensure compliance.

For labs with high-volume testing needs, several modifications to the tensile machine setup can be made to speed up the testing process and increase throughput, up to and including fully automated test systems. Fully automated systems such as Instron's AT3 are designed to incorporate specimen measurement, specimen loading, testing, and removal, and are able to run for hours without operator interaction. These systems help to reduce variability due to human error and can be left running after a shift ends to continue getting results when operators go home.