Lara Rasha

Monitoring the safety state of lithium-ion batteries (LIBs) in real time is critical for improving cell reliability and protecting end users. This study explores the capability of ultrasonic testing (UT) as a diagnostic tool during overcharging of NMC pouch cells. UT measurements are correlated with in-situ high speed synchrotron based X-ray radiography providing complementary insight into the evolution of internal degradation processes. Small 220 mAh cells were tested inside a custom-built chamber, allowing simultaneous ultrasonic and X-ray radiograph measurements. We observed clear links between structural changes captured by radiography and changes in the ultrasonic signals during overcharging. Spectral analysis showed an increase in low-frequency fast Fourier transform (FFT) magnitude, indicating internal cell degradation, while X-ray radiography revealed internal structure changes as cells transitioned from compact interlayer configurations to gas void accumulation. We then extended the ultrasonic monitoring to larger 56.3 Ah electric vehicle pouch cells, where radiographic imaging was not feasible. Similar trends in the ultrasonic data were detected, marking the first time such ultrasonic monitoring during an overcharging test has been demonstrated on EV-scale cells. This shows a potential for non-invasive, real-time monitoring of battery health during abuse.

Techniques using acoustic waves to interrogate batteries are increasingly investigated in the literature due to the appeal of three main properties: they are non-destructive, relatively low cost and have acquisition rates enabling operando testing. Popular demonstrations attempt to extract degradation markers from acoustic data, by continuous monitoring, and to attribute them to degradation modes. This is founded on the premise that the speed of sound depends on mechanical properties, such as the density and stiffness. Nevertheless, additional sensitivities of an acoustic time-of-flight analysis are often neglected, leading to incomplete experiments that can overstate the capabilities of the technique. In this work, such sensitivities are quantified and the use of pulse tests instead of CCCV protocols is recommended to elucidate the concurrent dynamic evolution of temperature, voltage and acoustic signals. A degradation experiment is performed, with pulse sequences incorporated in periodic reference performance tests. Dynamic parameters are extracted from each pulse; specifically, the dynamic rise of the time-of-flight (ΔToF_rise) and temperature (ΔTemp_rise) signals. Their evolution with degradation is traced and a statistical comparison of the main effects is performed. It is concluded that markers of degradation in the dynamic acoustic response are very subtle, masked by the effects of temperature.

Polymer electrolyte fuel cells (PEFCs) play a crucial role in approaching net-zero commitments due to their high-power density, rapid refuelling, and environmentally friendly operation. However, ensuring stable performance and durability relies on subtle water balance. Existing water management control strategies, including humidification, drainage, and cold starts, primarily depend on indirect feedback or calibration through the output voltage. The direct, real-time measurement of the water content inside a PEFC remains challenging, hindering the implementation of an efficient feedback control strategy for water management. To address this issue, synchronous measurement of neutron imaging and electrochemical impedance spectroscopy (EIS) is carried out at various water contents for the first time to the best of our knowledge. Machine learning is used to establish a nonlinear correlation between these two characterisation techniques. This enables the development of a more cost-effective and easily accessible real-time water content estimation technique suitable for high-power applications inferred from a universal EIS characterisation tool rather than relying solely on the limited availability of neutron imaging. This approach facilitates the optimisation and advancement of PEFCs.

A nature-inspired solution for water management in flow fields for electrochemical devices.

Recycling spent Li-ion batteries (LiBs) guarantees the conservation of important metal resources by reducing demands on raw supply and offsetting the energy and environmental costs associated with its manufacture. However, selecting practical end of life solutions for battery waste management remains a challenge. Molten salts, on the other hand, are a well-established group of inorganic compounds and are understood to be one of the leading candidates in the recovery of valuable metals. They offer a much greener and simpler route to metal recovery by electrochemical means compared with the use of reducing agents for hydrometallurgical recovery [1]. Yet, the current mechanistic understanding of the electrochemical recovery of metals in molten salts needs to be improved. SEM, EDS and XRD are some of the most employed characterisation techniques used to reveal material composition and surface topography; however, they are known to expose the final product to air and moisture obscuring the results. X-ray computed tomography offers a non-destructive approach for 3D microstructure visualisation and subsequent quantification [2]. This technique can be used to study the morphological evolution of metal deposits from their oxide particles in a representative high-temperature medium to provide a wealth of information on the fundamentals of the electrochemical reduction process. Here, we demonstrate a study into the electrochemical deposition of recovered Co metal from lithium cobalt oxide, LiCoO₂ in LiCl-KCl eutectic (LKE) which has been previously studied by the authors’ using a fluidised cathode arrangement [3]. This novel diagnostic approach has been applied to LiCoO₂-LKE samples before and after electrolysis at 450 °C, yielding key insights into the morphological evolution of product formation. Different morphologies of recovered Co metal such as dendrite features and agglomerates have been captured and quantified. It has also been observed that CO/CO₂ gas formation occurs in copious quantities at the anode surface leading to operational consequences such as the required overpotential needed to drive the reaction. For the first time, the application of X-ray imaging offers the ability to understand the role of process conditions and cell configurations to optimise a pyro-electrochemical reprocessing technology that can play a key role in sustainable battery recycling as well as refractory metal recovery. [1] B. Zhang, H. Xie, B. Lu, X. Chen, P. Xing, J. Qu, Q. Song, H. Yin, A Green Electrochemical Process to Recover Co and Li from Spent LiCoO₂-Based Batteries in Molten Salts, ACS Sustain Chem Eng. 7 (2019) 13391–13399. [2] P.J. Withers, C. Bouman, S. Carmignato, V. Cnudde, D. Grimaldi, C.K. Hagen, E. Maire, M. Manley, A. du Plessis, S.R. Stock, X-ray computed tomography, Nature Reviews Methods Primers. 1 (2021) 1–21. [3] M. Mirza, R. Abdulaziz, W.C. Maskell, C. Tan, P.R. Shearing, D.J.L. Brett, Recovery of cobalt from lithium-ion batteries using fluidised cathode molten salt electrolysis, Electrochim Acta. 391 (2021) 138846.

The safety of concerns of lithium-ion batteries continues to be a prevalent obstacle toward their widespread application from vehicle electrification to space exploration. Aside from the highly oxidising and reducing electrode materials, their safety is compounded by an inherent drawback of poor heat dissipation [1]. High-speed imaging with in-situ/operando X-ray CT has been used extensively to study various lithium-ion battery safety features and failure mechanisms [2][3], including thermal failure [4]. However, these are exclusively using synchrotron X-ray sources which are limited in terms of both access and data recording capabilities: high frame rates require the data collection window to be restricted to a few seconds. During lithium-ion battery failure, there are several changes to a cell structure leading up to thermal runaway (TR) which can take minutes, and as a result are often missed. Here, we present an instrument that simulates thermal failure for lab-based radiography at slower imaging speeds and longer recording lengths, which has been validated by correlative synchrotron measurements. The failure mechanisms within a fully charged (100 % SOC, 4.2 V) commercially available LiCoO₂ cathode and graphite anode pouch cell (651628-2C, AA Portable Power Corp) rated at 210 mAh are investigated. Three samples are studied using lab-based radiography at a frame rate of 3.75 fps with a 16.1 µm pixel resolution and, for comparison, an additional three samples are studied using synchrotron X-ray sources at a higher speed of 20,000 fps with a 13.3 µm pixel resolution. For the six samples investigated, the total time taken from a start temperature of 80 °C to TR is approximately 20 minutes and the onset temperatures for TR are recorded within the range of 196 °C to 210 °C. The beginning of the TR event (defined as a sample temperature increase greater than 15 °C s^-1), where the effects to the electrode structure are the most catastrophic, lasts for approximately 1 s. Operando radiographic images during this event reveal that the structural displacement of electrode layers begins at the centre of the cell and propagates outwards in a wave-like motion. The electrode displacement, as a result, is quantified by cross-correlating Gabor signals and spatiotemporal mapping [5] in both types of datasets. For the lab-based radiography, data is recorded from the start temperature to TR (lasting approximately 20 minutes), and reactions such as the electrolyte decomposition, ca. 105 °C, and separator melting, ca.130 °C are characterised in the context of electrode deformation and gas evolution. Investigations of pre- and post-failure 3D X-ray CT images further verify the uniformity of the pristine (or pre-failure) cell assembly as well as the estimated post-failure behaviour between samples. Finally, by comparison with correlative synchrotron measurements, the instrument for inducing thermal failure for lab-based X-ray CT is proven to be a viable and more accessible method to investigate thermal failure within a 210 mAh pouch cell. While synchrotron data has a higher-speed imaging advantage, it is limited to only recording the short TR event at a high temporal resolution. Whereas continuous imaging in lab-based radiography has the benefit of measuring the slower architectural changes taking place up to TR, albeit at a marginally lower spatial resolution. References [1] D. H. Doughty and E. P. Roth, Interface Mag., 21, 37–44 (2012). [2] D. P. Finegan, M. Scheel, J. B. Robinson, B. Tjaden, M. Di Michiel, G. Hinds, D. J. L. Brett, and P. R. Shearing, Phys. Chem. Chem. Phys., 18, 30912–30919 (2016). [3] D. P. Finegan, M. Scheel, J. B. Robinson, B. Tjaden, I. Hunt, T. J. Mason, J. Millichamp, M. Di Michiel, G. J. Offer, G. Hinds, D. J. L. Brett, and P. R. Shearing, Nat. Commun., 6, 6924 (2015). [4] M. T. M. Pham, J. J. Darst, D. P. Finegan, J. B. Robinson, T. M. M. Heenan, M. D. R. Kok, F. Iacoviello, R. Owen, W. Q. Walker, O. V. Magdysyuk, T. Connolley, E. Darcy, G. Hinds, D. J. L. Brett, and P. R. Shearing, J. Power Sources, 470, 228039 (2020). [5] A. N. P. Radhakrishnan, M. Buckwell, M. Pham, D. P. Finegan, A. Rack, G. Hinds, D. J. L. Brett, and P. R. Shearing, ChemRxiv (2021).

In recent years, low-temperature polymer electrolyte fuel cells have become an increasingly important pillar in a zero-carbon strategy for curbing climate change, with their potential to power multiscale stationary and mobile applications. The performance improvement is a particular focus of research and engineering roadmaps, with water management being one of the major areas of interest for development. Appropriate characterisation tools for mapping the evolution, motion and removal of water are of high importance to tackle shortcomings. This article demonstrates the development of a 4D high-speed neutron imaging technique, which enables a quantitative analysis of the local water evolution. 4D visualisation allows the time-resolved studies of droplet formation in the flow fields and water quantification in various cell parts. Performance parameters for water management are identified that offer a method of cell classification, which will, in turn, support computer modelling and the engineering of next-generation flow field designs.

Polymer electrolyte fuel cells (PEFCs) are a promising replacement for the fossil fuel–dependent automotive and energy sectors. They have become increasingly commercialized in the last decade; however, significant limitations on durability and performance limit their commercial uptake. Catalyst layer (CL) design is commonly reported to impact device power density and durability; although, a consensus is rarely reached due to differences in testing conditions, experimental design, and types of data reported. This is further exacerbated by aspects of CL design such as catalyst support, proton conduction, catalyst, fabrication, and morphology, being significantly interdependent; hence, a wider appreciation is required in order to optimize performance, improve durability, and reduce costs. Here, the cutting‐edge research within the field of PEFCs is reviewed, investigating the effect of different manufacturing techniques, electrolyte distribution, support materials, surface chemistries, and total porosity on power density and durability. These are critically appraised from an applied perspective to inform the most relevant and promising pathways to make and test commercially viable cells. This holistic view of the competing aspects of CL design and preparation will facilitate the development of optimized CLs, especially the incorporation of novel catalyst support materials.