Ultrasonic detection of pre-existing thermal abuse in lithium-ion pouch cells

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The lithium-ion batteries used in high-power applications require thousands of cells arranged in arrays where failure of an individual cell may lead to total system failure via thermal runaway. One cause of thermal runaway is high-temperature abuse. This work explores the viability of ultrasonic inspection to detect whether lithium-ion cells have been previously subjected to localized thermal abuse by comparing ultrasonic features recorded during the charge–discharge cycling before and after thermal abuse. We employ 1 MHz Gaussian pulses propagating through mechanically-confined NMC lithium-ion cells as they undergo charge–discharge cycling, localized heating between 50°C and 150°C , and post-abuse charge–discharge cycling. Metrics of the transmitted ultrasonic signal, the time of flight shift (TOFS) and signal amplitude (SA), are analyzed to assess their utility in detecting pre-existing damage due to localized thermal abuse. We also present a transfer matrix method assuming periodic boundary conditions to derive a model for TOFS and SA during both normal cycling and thermal abuse. Results indicate that the SA and TOFS trends are strongly affected by previous, localized thermal abuse in the following charge–discharge cycles, while the model provides insight about which cell components are contributing to the observed changes in ultrasonic signals.