In battery cell development and design, the attainment of fast-charging capability is regarded as a significant challenge, especially when it must be surmounted without compromising performance indicators such as cost and energy density. Metal plating (e.g. lithium-plating for lithium-ion cells) has proven to be the limiting factor when aiming to achieve fast charging goals. Extensive research has shown that metal plating is accompanied by a massive loss of capacity and can even lead to a safety-critical state. Consequently, it is imperative to steer clear of cell designs in the developmental stages that are susceptible to metal plating. Recent literature has explored electrical methods to discern whether plating occurred during the previous charging cycle. Regrettably, these methods have been limited in their ability to pinpoint the underlying cause of plating, thus offering only limited guidance in the development process.

In this work, we present a scalable method that non-destructively images the inside of a battery cell in real time. This is achieved through the utilization of ultrasonic waves, which are emitted and received at various points on the surface of the battery cell using ultrasound transducers, a technique commonly referred to as scanning acoustic microscopy. Some of the core properties of the ultrasonic waves, like velocity or amplitude, will change depending on the medium they are traveling through. This effect is used to detect the acoustic properties of the cell components at each position approached by the transducer(s). Combined with a robust signal processing toolchain, this method produces a real-time image with a resolution in the micrometer range. The significant advantage of this technology compared to other imaging techniques is that it is non-destructive, cost-effective, and fast. This makes it suitable to assist cell development processes.

Using the developed method, we demonstrate the real-time visualization of lithium plating of a high-nickel 1.4 Ah pouch-bag cell. To achieve this, we perform multiple charging procedures, effectively visualizing lithium plating and identifying the specific design element responsible for it. To validate our findings, we carry out numerous reference electrode measurements employing identical charging protocols. Additionally, we undertake a comprehensive post-mortem analysis, incorporating optical and scanning electron microscopy (SEM) images. The detection of metallic lithium plating is confirmed through the use of an Ultim Extreme lithium detector, manufactured by Oxford Instruments plc, employed for energy-dispersive X-ray (EDX) analysis.