Local Ultrasonic Resonance Spectroscopy of Lithium Metal Batteries for Aerospace Applications

As next-generation aircraft and vehicles continue to develop, so do their associated energy
demands. Lithium metal batteries are a leading candidate to fulfill this energy requirement, but
these batteries are prone to internal dendrite defects that can lead to catastrophic thermal runaway
events. Current battery management systems are capable of mitigating such risks, but are unable
to detect such defects until thermal runaway has already begun. Various nondestructive evaluation
(NDE) techniques, particularly ultrasonic NDE, can directly monitor internal battery parameters
giving them the potential to detect critical defects prior to catastrophic failure. However, most of
the current battery NDE research has focused on improved battery state-of-charge (SOC) and stateof-
health (SOH) monitoring with little emphasis on critical defect detection. Thus, a measurement
technique sensitive to subtle battery defects is needed. In addition, the complex mechanics of
ultrasound in porous, thin, multilayered batteries prompt the use of physics-based simulation to
guide inspections.

In this work, an ultrasonic NDE technique has been developed utilizing frequency domain
analysis of local battery resonances to detect the presence of battery defects. This technique is a
practical extension of local ultrasonic resonance spectroscopy (LURS) – which previously required
non-contact laser ultrasonics – to measurements with piezoelectric contact and immersion scan
transducers. To extend the technique to work with piezoelectric transducers, ultrasonic battery
measurements were compared to a sans-battery calibration measurement. Then, a linear systems
deconvolution was used to eliminate the transfer functions of extraneous factors such as the
transducer and electronics, leaving only the frequency-dependent battery reflection coefficient.

The LURS technique was first validated on stainless steel and aluminum plates, producing
reflection coefficients in line with analytical and numerical finite element modeling (FEM) results.
Functioning Li-metal pouch cells were then seeded with lithium chip defects prior to LURS
measurements. The presence of these defects is shown to cause a measurable shift in the battery’s
through-thickness local resonances. 2D, frequency-domain poroelastic models of ultrasonic
propagation in a single-cell lithium metal pouch battery were created and corroborated these
findings. Thus, this work has both extended and proven the feasibility of the LURS technique in
the detection of local battery defects.