Lithium-ion (Li-ion) battery manufacturers must detect various physical flaws during production to ensure safe, reliable performance. Failure to detect 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, specifically, 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, the use of advanced Scanning Acoustic Microscopy (SAM) is an increasingly important technique to detect potential flaws in the manufacture of 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.
Early detection of flaws not only enables manufacturers to prevent defective products from entering the marketplace – reducing potential recalls, liability and reputational damage – but also facilitates design and production changes to eliminate future problems.
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.
“Scanning Acoustic Microscopy is detecting any degradation or change in the mechanical properties of the Li-ion battery cell. For example, is it swelling or disbonding? The technology effectively 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,” said Hari Polu, President of OKOS, a Virginia-based manufacturer of Scanning Acoustic Microscopy (SAM) and industrial ultrasonic non-destructive (NDT) systems.
OKOS, a wholly owned subsidiary of PVA TePla AG Germany, offers manual and automated inspection systems for batteries, flat panels, thin plates, circular discs, sputtering targets and special alloys.
With SAM, the sound hitting the object is either scattered, absorbed, reflected or transmitted. By detecting the direction of scattered pulses as well as the ‘time of flight,’ the presence of a boundary or object can be determined as well as 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 the need for time-consuming tomographic scan procedures and more costly X-rays. The images are then analyzed to detect and characterize flaws.
Scanning Acoustic Microscopy, acknowledged for its capability to identify defects as minuscule as 50-microns, is extensively embraced in the semiconductor industry as a metrology technology for failure analysis and reliability detection. Now the same high-speed technology is being applied to testing and failure analysis of Li-ion battery cells.
Manufacturers are increasingly integrating SAM inspection tools into their processes to catch defects at an early stage. For high volume operations, automated systems are also available that enable 100% inspection of battery cells, ensuring safety and performance.
According to Polu, effective testing for Li-ion battery flaws across the diverse form factors utilized requires both expertise in SAM technology as well as 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 how energy is stored and delivered. The different packaging requires adjustments in the manufacturing process along with reliable quality assurance to detect defects.
For some types of inspection, such as electric vehicle (EV) Li-ion batteries, an immersive type of SAM is proving effective. With this method, battery components are submerged in a fluid (typically water) to facilitate the transmission of ultrasonic waves during scanning.
For this application, custom, low-frequency transducers are utilized, serving as both 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 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, explained Polu.
“For thick battery packages like EV vehicles, we 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,” said Polu.
OKOS designs and manufactures a large variety of transducers up to 300 MHz for different applications. Some transducers require direct contact with a material in order to operate, others use an air gap or are immersed in a fluid, usually water, in order to better transmit the sound 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.
According to Polu, the utilization of special transducers and software enables efficient detection of Li-ion battery defects across a wide range of 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 need to detect one small feature [the potential defect] amidst a hell of a lot of noise,” explained Polu.
He anticipates that SAM technology will become increasingly automated to ensure Li-ion battery safety and performance as production volumes continue to rise. As an example, he comments on the trajectory of EV battery inspection.
“Initially, they’re trying to ascertain the failure modes [of Li-ion batteries]. Once that’s 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,” said Polu.
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.