For professional electronics design engineers and developers, assuring lithium-ion (Li-ion) battery safety, reliability, and performance is critical. This means battery manufacturers must detect various physical flaws during production. Failure to notice even minute defects could lead to internal short circuits, overheating, fire, explosion, diminished function, costly product recalls, and serious reputation damage.
Li-ion batteries can produce gas under stress, and a process called gas evolution can lead to swelling and structural changes over time that reduce safety, efficiency, and capacity. Batteries encased in pouch cells are more susceptible to swelling and structural issues than rigid cells. The stress from swelling and gas evolution can cause layers within the battery to separate or disbond.
Today, advanced scanning acoustic microscopy (SAM) is an increasingly important technique for detecting potential flaws in manufacturing Li-ion batteries. SAM can quickly and effectively image the material forms and internal structures of up to 100% of batteries to identify areas where layers are improperly bonded or otherwise physically defective (Figure 1).
Early detection of flaws enables manufacturers to prevent defective products from entering the marketplace, reducing potential recalls, liability, and reputation-related damage and facilitating design and production changes to eliminate future problems.
Figure 1. Automated scanning acoustic microscopy (SAM) systems allow for scalable inspection, ensuring 100% quality control in high-volume production.
Detecting Li-ion battery problems
The need to assure quality is driving the adoption of non-destructive battery inspection techniques such as scanning acoustic microscopy.
SAM detects any degradation or change in the mechanical properties of the Li-ion battery cell. For example, is it swelling or disbonding? The technology monitors what the chemistry is doing to the mechanical construction of the package. This becomes critical as Li-ion battery production ramps up and there’s increasing variation in physical form factors.
With SAM, the sound hitting the object is scattered, absorbed, reflected, or transmitted. By detecting the direction of scattered pulses and the “time of flight,” the presence of a boundary or object can be determined, including its distance.
Samples are scanned point by point and line by line to produce an image. Scanning modes range from single-layer views to tray scans and cross-sections. Multi-layer scans can include up to 50 independent layers. Depth-specific information can be extracted and applied to create two- and three-dimensional images without time-consuming tomographic scan procedures and more costly X-rays. The images are then analyzed to detect and characterize flaws.
Recognized for its ability to detect defects as small as 50 microns, SAM is widely used in the semiconductor industry for failure analysis and reliability assessment. Now, the same high-speed technology is being applied to test and analyze failures of Li-ion battery cells.
Manufacturers are increasingly integrating SAM inspection tools into their processes to catch defects at an early stage (Figure 2). For high-volume operations, automated systems are also available that enable 100% inspection of battery cells, ensuring safety and performance.
Effective testing for Li-ion battery flaws across diverse form factors requires expertise in SAM technology and customization to the specific application. Li-ion battery cells can be packaged in various sizes and shapes, such as square, round, and pouch, to optimize energy storage and delivery. The different packaging requires adjustments in the manufacturing process and reliable quality assurance to detect defects.
Immersive EV battery inspection
An immersive type of SAM is proving effective for some kinds of inspection, such as electric vehicle (EV) Li-ion batteries. With this method, battery components are submerged in a fluid (typically water) to facilitate the transmission of ultrasonic waves during scanning.
Figure 2. SAM uses ultrasonic waves to scan and create detailed images, detecting structural flaws in batteries, even at the micron level.
For this application, custom, low-frequency transducers are used to serve as a transmitter and receiver of ultrasonic sound waves. For thick materials, high-frequency ultrasound (which provides high resolution) cannot penetrate deeply enough. The lower-frequency ultrasound can penetrate deeper into thick packages but has lower spatial resolution.
A transducer is an energy conversion device that generates ultrasonic waves when a voltage is applied to them and can turn them back into voltage. Combined with the shape of the lens on the transducer, frequency and focal length can be controlled to provide the best results when inspecting the internal characteristics of samples.
For thick battery packages like EV vehicles, some companies use relatively low-frequency, highly customized transducers to penetrate through the parts. The special transducers need to have an extremely high surface penetration to approximately five millimeters while maintaining resolution.
A transducer is an energy conversion device that generates ultrasonic waves when a voltage is applied to them and can turn ultrasonic waves back into voltage. Combined with the shape of the lens on the transducer, frequency and focal length can be controlled to provide the best results when inspecting the internal characteristics of samples.
For thick battery packages like EV vehicles, some companies use relatively low-frequency, highly customized transducers to penetrate through the parts. The special transducers need to have very high surface penetration to a depth of approximately five millimeters while still maintaining resolution.
Some companies design and manufacture a large variety of transducers up to 300 MHz for different applications. Some transducers require direct contact with a material to operate, while others use an air gap or are immersed in a fluid, usually water, to transmit sound better through a material. Transducers come in a variety of sizes and shapes for different applications.
Among the specialized transducers for custom applications are phased array transducers, which contain multiple elements, unlike the single element in other transducers. The transducer can also be curved in shape, which allows for faster scans as the elements simultaneously brush over samples and faster scans of curved surfaces. Using constructive interference between the elements, focal lengths can be changed at any time to achieve the best results. Phased array transducers are typically 20 MHz or below.
Special transducers and software enable efficient detection of Li-ion battery defects across various form factors.
It’s a combination of using the transducers to measure signals along with software to extract features out of a very noisy environment. It’s like the Hubble Space Telescope: you must detect one small feature, the potential defect, amidst a ton of noise.
SAM technology will become increasingly automated to ensure Li-ion battery safety and performance as production volumes continue to rise. Take, for example, the trajectory of EV battery inspection. Initially, they’re trying to ascertain the failure modes of Li-ion batteries. Once that is achieved, implementing quality assurance is the focus, ideally at an automated level, because no one wants to drive an EV if battery safety is in question.
As technology progresses, new variations and form factors of Li-ion batteries are emerging. Manufacturers across a wide range of industries that work with an expert provider of SAM, which can customize the technology to their specific application, will have an advantage in ensuring the safety, quality, and reliability of their products.
Filed Under: Batteries, FAQs, Featured Contributions