Nonlinear Acoustic Shear Wave Detection of Zero Gap Kissing Bonds in Encapsulated Packs
Nonlinear acoustic shear wave harmonic detection resolves zero gap kissing bonds in encapsulated battery packs by measuring contact acoustic nonlinearity.
Acoustic Impedance Mismatch
Meaning
The difference in the resistance of two adjacent materials to the passage of high frequency sound waves determines the portion of acoustic energy reflected at their boundary. When ultrasonic testing is performed on lithium-ion cells during production, an acoustic impedance mismatch dictates how much of the ultrasonic signal passes through the layers of the anode, cathode, and separator. High mismatch values indicate poor physical contact, often resulting from gas pockets, delamination, or dry areas in the electrolyte.
Signal Reflection
Ultrasonic pulses encountering a boundary between materials with very different densities and sound velocities are mostly reflected back to the transducer. In battery cell inspection, this phenomenon occurs when an air gap exists between the electrode layers and the separator. Since air has a very low acoustic impedance compared to copper, aluminum, or liquid-electrolyte-soaked porous media, almost the entire signal is returned, leaving a negligible remainder to transmit through the cell.
This high reflectivity allows quality control systems to pinpoint dry spots and pockets of trapped gas.
Measurement Procedure
Acquiring these data requires an ultrasonic transmitter on one side of the battery cell and a receiver on the opposite side. The amplitude of the transmitted sound waves is recorded as a function of position across the surface of the cell. If the amplitude drops below a predefined calibration threshold, the region is flagged for inspection.
Specialized software maps these transmission losses to create a spatial image of the internal wetting and mechanical contact quality.
Commercial Influence
Sourcing teams use this ultrasonic mapping to reject cells that exhibit incomplete electrolyte wetting before they are assembled into packs. Cells with poor internal contact degrade rapidly under fast-charging conditions. Detecting these manufacturing flaws early prevents premature field failures and protects lifetime warranties.