Multi-Length Scale X-Ray CT and Thermal Analysis for Battery Safety Assessment

Abstract

Lithium-ion battery failure occurs across multiple length and time scales. This work presents the properties of thermal and mechanical failure in batteries of cylindrical, pouch and coin cell format by using a combination of Accelerating Rate Calorimetry (ARC) and X-ray Computed Tomography (CT) at various length scales; from whole cell (macro) to bulk electrode (micro) to particle (nano).Typical features of a thermal runaway (TR) sequence in a commercial 18650 cell, such as the onset temperature of TR, the order of and rate of heat generation from cell component breakdown, such as SEI layer deformation, are shown using a heat-wait-search protocol in an ARC. Alongside this, pre- and post-mortem X-ray CT images reveal the effects of thermal failure on the bulk electrode structure within the cell, such as deformations due to gas formation and release. Electrode material characteristics, such as porosity, particle size distribution, tortuosity, and particle cracking, are also quantified and compared before and after cell failure in the bulk electrode (micro–X-ray CT) and individual particles (nano-X-ray CT)[1].A novel X-ray CT scanning protocol is also demonstrated for characterising bespoke pouch cells (single and multi-layer) with sufficient resolution to reconcile electrode morphology, for example to evaluate the morphology changes during thermal abuse by nail penetration.Furthermore, this work investigates the sequence of reactions leading up to thermal runaway using a novel coin cell Differential Scanning Calorimeter (DSC). Heat flux signals are used to quantify the contribution of various exothermic and endothermic reactions to the overall heat generated by a cell as it undergoes failure. Some key features of early-stage TR reactions, such as the decomposition of the SEI layer are revealed for NMC111, NMC622 and NMC811. Some of the typical thermal characteristics of cells as they undergo ‘normal’ charge and discharge are also shown and compared to abnormal operating conditions such as, temperatures up to 60 °C, high C-rates, and cycle numbers.[1] D. Patel, J. B. Robinson, S. Ball, D. J. L. Brett, and P. R. Shearing, J. Electrochem. Soc., 167, 090511 (2020). Figure 1