Prevention, Mitigation and Correlative Acoustic Spectroscopy of Lithium-Ion Battery Thermal Runaway
Authors: Pham Martin T.M. , Finegan Donal P. , Darst John J , Hinds Gareth , Darcy Eric , Shearing Paul R 
Published: July 2022

Scrutiny of lithium-ion battery failure has seen heavy regulations and restrictions with well-reported headlines of catastrophic thermal runaway of commercial electronic devices and electric vehicles. Thermal runaway occurs at a critical temperature when the rate of heat generation exceeds that of dissipation, highly exothermic decomposition reactions occur and temperatures in excess of 600 ᵒC can be measured 1. These concerns continue to grow with applications of greater energy density and capacity, such as the proliferation of electric vehicles.
Isolation of electronically conductive components within the electrode assembly to prevent short circuit has shown promise to prevent the onset of thermal runaway within lithium-ion cells. Here 2, the authors present a metal-coated polymer current collector (PCC) and thermally stable separator (TSS) to prevent and mitigate thermal runaway. 2.10 Ah capacity 18650-geometry cells were subject to thermal, internal short circuit and nail penetration abuse techniques with these materials and their commercial counterparts for individual comparison of the PCCs and TSS. Reductions in cell mass were observed on the order of 5%, due to the majority of the PCC consisting of the polymer substrate rather than pure aluminium and copper in commercial current collectors. High speed synchrotron facilities provided insights into the function of the polymer current collectors with post- mortem X-ray computed tomography. During mitigation testing, cells were taken to failure with thermal and with an internal short circuit device (ISCD) within a Fraction Thermal Runaway Calorimeter (FTRC) to measure the reduction in calorific output. This highlighted the difficulty that exists with detecting gas-induced and internal structural degradation within lithium-ion batteries, especially outside specialized diagnostic laboratories to characterize these rapid changes with elapse within seconds. Attenuation and wave velocity determined by the propagative properties of ultrasound through different phases, prominently highlights structural, temperature and formation of different phases which occur during thermal runaway. Such analysis provided by acoustic spectroscopy are characterised by variations in the attenuation and signal peak shifts. Further information which can be derived from spatial mapping using acoustic spectroscopy 3,4 include: state of charge, state of health, electrochemical performance.
Cells made with the aluminium-coated PCC demonstrated 100% success in prevention of thermal runaway during nail penetration, retaining a voltage above 4.00 V. Cells manufactured with the PCC, both PCC and TSS, exhibited a reduction in total energy output on average of 19.4% and 41.3% respectively during thermal abuse. ISCD triggered thermal runaways also observed reductions of 24.6% and 30.3% with these respective cell material configurations. Prevention of thermal runaway is a transformative advancement in improving lithium-ion battery safety, especially in the cylindrical 18650-format cell. Additionally, significant reductions in the calorific output during failure were measured. These safety innovations provides efficacy of these materials independent of cell chemistry (provided they are stable in the operating environment) while also increasing the energy density and be assimilated into existing manufacturing technology.
Furthermore, the authors demonstrate correlative high-speed acoustic spectroscopy during routine cycling of a commercial 210 mAh lithium-ion pouch cell and during thermal abuse 5. Mechanical deformation such as gas-induced delaminations, which have been linked to degradation in electrochemical performance, capacity retention, cell state of health and impending thermal runaway, were detected. This degradation was observed with characteristic acoustic spectroscopy signal peaks changes during cycling due to cell manufacturing defects and during induced thermal abuse, were corroborated with high speed synchrotron X-ray imaging. This analysis can be observed immediately after application of the acoustic probe which emphasises the rapid diagnostics insights provided by acoustic spectroscopy as a robust field deployable technique. Thus, integration into battery management systems, second-life evaluation/recycling or as in-line cell metrology during manufacturing would provide real time direct insight into these metrics.

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