The health of a battery is a critical parameter that determines its remaining useful life and performance. Among various techniques for state-of-health (SOH) prediction, ultrasonic methods have emerged as a promising non-invasive approach. These techniques rely on the interaction of acoustic waves with internal battery components, providing insights into structural and chemical changes that correlate with degradation.

Acoustic wave propagation in batteries is influenced by several factors, including electrode porosity, particle cracking, and solid-electrolyte interphase (SEI) growth. As ultrasonic waves travel through battery materials, their speed and amplitude are altered by changes in mechanical properties. Electrode porosity, for instance, affects wave velocity due to variations in material density. Fresh electrodes with uniform porosity exhibit consistent wave propagation, whereas aged electrodes with increased porosity from particle cracking or binder degradation show slower wave speeds and higher attenuation.

Particle cracking in active materials, such as silicon or graphite anodes, introduces discontinuities that scatter ultrasonic waves. This scattering results in measurable changes in time-of-flight (TOF) and signal amplitude. Similarly, SEI growth on electrode surfaces alters acoustic impedance, further modifying wave reflection and transmission characteristics. The SEI layer, being mechanically distinct from the bulk electrode, creates an additional interface that affects ultrasonic signatures.

Time-of-flight measurements are a fundamental ultrasonic technique for SOH prediction. By transmitting an ultrasonic pulse through a battery and measuring the time taken for the wave to traverse the cell, researchers can detect changes in electrode thickness, stiffness, or delamination. Increased TOF often indicates electrode swelling or porosity changes, while reduced signal amplitude suggests internal damage or gas formation.

Attenuation analysis complements TOF by quantifying energy loss in the acoustic signal. High attenuation may indicate particle fracture, electrolyte drying, or gas accumulation, all of which contribute to capacity fade. By comparing attenuation patterns across different cycling conditions, correlations between ultrasonic metrics and SOH can be established. For example, batteries cycled at high rates often exhibit greater attenuation due to accelerated particle cracking and SEI growth.

Integrating ultrasonic transducers into battery systems presents several challenges. Transducers must operate reliably in the battery environment, which may involve exposure to temperature fluctuations, mechanical stress, and electrochemical activity. Piezoelectric transducers are commonly used due to their high sensitivity, but their coupling to flexible pouch cells can be problematic. Poor acoustic coupling leads to signal loss, reducing measurement accuracy. Additionally, transducer placement must account for cell geometry to ensure consistent wave propagation paths.

Case studies have demonstrated the effectiveness of ultrasonic techniques in SOH prediction. In one study involving lithium-ion pouch cells, researchers observed a linear relationship between TOF increase and capacity fade over 500 cycles. Cells subjected to fast charging showed more pronounced TOF shifts compared to those charged at standard rates, aligning with post-mortem analysis revealing severe particle cracking. Another study on nickel-rich cathodes correlated attenuation changes with SEI growth, enabling early detection of performance degradation before significant capacity loss occurred.

The advantages of ultrasonic SOH prediction are significant. Unlike electrochemical impedance spectroscopy or voltage-based methods, ultrasonic techniques provide direct mechanical insights without requiring electrical disconnection. This non-invasive nature allows for continuous monitoring in operational systems, such as electric vehicles or grid storage. Furthermore, ultrasonic data can be combined with machine learning models to improve prediction accuracy by identifying subtle degradation patterns.

However, limitations exist, particularly in pouch cell form factors. The flexible nature of pouch cells complicates transducer coupling, as inconsistent pressure or bending alters acoustic transmission. Multi-layer pouch cells also introduce additional interfaces that can obscure ultrasonic signals, making data interpretation more complex. Cylindrical and prismatic cells, with their rigid structures, generally yield more consistent results but may still require careful transducer placement.

In summary, ultrasonic techniques offer a powerful tool for battery SOH prediction by leveraging acoustic wave interactions with evolving electrode structures. Time-of-flight and attenuation measurements provide quantifiable indicators of porosity changes, particle cracking, and SEI growth, enabling early detection of degradation mechanisms. While challenges remain in transducer integration and signal interpretation, the non-invasive nature and potential for real-time monitoring make ultrasonics a valuable addition to battery diagnostics. Continued research into advanced signal processing and transducer materials will further enhance the reliability and applicability of this method across diverse battery systems.