Investigating the effect of thermal gradients on stress in solid oxide fuel cell anodes using combined synchrotron radiation and thermal imaging

Thermal gradients can arise within solid oxide fuel cells (SOFCs) due to start-up and shut-down, non-uniform gas distribution, fast cycling and operation under internal reforming conditions. Here, the effects of operationally relevant thermal gradients on Ni/YSZ SOFC anode half cells are investigated using combined synchrotron X-ray diffraction and thermal imaging. The combination of these techniques has identified significant deviation from linear thermal expansion behaviour in a sample exposed to a one dimensional thermal gradient. Stress gradients are identified along isothermal regions due to the presence of a proximate thermal gradient, with tensile stress deviations of up to 75Â MPa being observed across the sample at a constant temperature. Significant strain is also observed due to the presence of thermal gradients when compared to work carried out at isothermal conditions

Representative resolution analysis for X-ray CT: A Solid oxide fuel cell case study

A requirement to reduce dependency on high-carbon fuels has resulted in the rapid advancement of electrochemical devices. Considerable research has been applied to improve device performance and lifetime in order to compete with incumbent technologies. Of the portfolio of electrochemical conversion technologies, solid oxide fuel cells (SOFC) offer high fuel versatility and fast reaction kinetics without the requirement of expensive catalysts. However, degradation due to high temperature operation limits cell performance and lifetime, impeding widespread commercialisation. Due to the inherent link between microstructure and electrochemical performance, many three-dimensional (3D) characterisation techniques have been employed in the pursuit of the mitigation of degradation through rational electrode design. Instruments such as lab-based X-ray microscopes are now capable of imaging across multiple length scales, where the highest resolutions (i.e. smallest voxel lengths) are comparable to specialist synchrotron facilities. A widely used metric to describe electrode microstructure is the triple-phase boundary (TPB); the location where reactions occur within the SOFC electrode. The total TPB length is a vital metric in assessing the quality of an SOFC material, and thus many efforts have been made to determine accurate values. In order to map the TPB locations in 3D, the three constituent phases: metal, ceramic, and pore, need to be distinguished and segmented, requiring high resolutions. Although TPB values have been reported and compared extensively in the literature, the influence of the microscopic roughness is yet to be investigated. Using X-ray computed tomography (CT), here, for the first time, the effect of resolution is inspected for several key microstructural parameters. Moreover, the study is extended through the use of multiple instruments for a variety of sample structures. This work introduces the importance of the fractal properties of structures characterised using X-ray CT, which we expect to be influential across a broad range of materials. The choice of resolution when characterising a structure is important and determined by a variety of factors: instrument, feature size, image quality, etc., and should ultimately be chosen in order to efficaciously expose the features under investigation, in addition to this, metrics extracted should only be directly compared at the same resolution and, if possible, should be inspected for fractal properties via a representative resolution analysis. These conclusions are not restricted to SOFCs but should be applied to all fields of microstructural analysis

Data on the theoretical X-Ray attenuation and transmissions for lithium-ion battery cathodes

This article reports the data required for planning attenuation-based X-ray characterisation e.g. X-ray computed tomography (CT), of lithium-ion (Li-ion) battery cathodes. The data reported here is to accompany a co-submitted manuscript (10.1016/j.matdes.2020.108585 [1]) which compares two well-known X-ray attenuation data sources: Henke et al. and Hubbell et al., and applies methodology reported by Reiter et al. to extend this data towards the practical characterisation of prominent cathode materials. This data may be used to extend beyond the analysis reported in the accompanying manuscript, and may aid in the applications for other materials, not limited to Li-ion batteries

Theoretical transmissions for X-ray computed tomography studies of lithium-ion battery cathodes

X-ray computed tomography (CT) has emerged as a powerful tool for the 3D characterisation of materials. However, in order to obtain a useful tomogram, sufficient image quality should be achieved in the radiographs before reconstruction into a 3D dataset. The ratio of signal- and contrast-to-noise (SNR and CNR, respectively) quantify the image quality and are largely determined by the transmission and detection of photons that have undergone useful interactions with the sample. Theoretical transmission can be predicted if only a few variables are known: the material chemistry and penetrating thickness e.g. the particle diameter. This work discusses the calculations required to obtain transmission values for various Li(NiXMnYCoZ)O2 (NMC) lithium-ion battery cathodes. These calculations produce reference plots for quick assessment of beam parameters when designing an experiment. This is then extended to the theoretical material thicknesses for optimum image contrast. Finally, the theoretically predicted transmission is validated through comparison to experimentally determined values. These calculations are not exclusive to NMC, nor battery materials, but may be applied as a framework to calculate various sample transmissions and therefore may aid in the design and characterisation of numerous materials

Leadership and approaches to the management of workplace bullying

Leadership behaviour has been identified as an important antecedent of workplace bullying, since managers may prevent, permit or engage in the mistreatment of others. However, the issue of how managers respond when bullying occurs has received limited attention. With this in mind, the aim of this study was to explore how managers behave when bullying occurs in their work group, and to elucidate the contextual issues that underlie this behaviour. This was achieved through analysis of in-depth interviews with individuals involved in cases of bullying. The findings revealed a typology of four types of management behaviour in cases of bullying, each underpinned by contextual factors at the individual, group and organizational level. The study shows that the role of leadership in workplace bullying is more complex than previously thought, and suggests several ways in which managers and organizations could deal with bullying behaviour

First Cycle Cracking Behaviour Within Ni-Rich Cathodes During High-Voltage Charging

Increasing the operating voltage of lithium-ion batteries unlocks access to a higher charge capacity and therefore increases the driving range in electric vehicles, but doing so results in accelerated degradation via various mechanisms. A mechanism of particular interest is particle cracking in the positive electrode, resulting in losses in capacity, disconnection of active material, electrolyte side reactions, and gas formation. In this study, NMC811 (LiNi0.8Mn0.1Co0.1O2) half-cells are charged to increasing cut-off voltages, and ex situ X-ray diffraction and X-ray computed tomography are used to conduct post-mortem analysis of electrodes after their first charge in the delithiated state. In doing so, the lattice changes and extent of cracking that occur in early operation are uncovered. The reversibility of these effects is assessed through comparison to discharged cathodes undergoing a full cycle and have been relithiated. Comparisons to pristine lithiated electrodes show an increase in cracking for all electrodes as the voltage increases during delithiation, with the majority of cracks then closing upon lithiation

Design of a miniature flow cell for in situ x-ray imaging of redox flow batteries

Flow batteries represent a possible grid-scale energy storage solution, having many advantages such as scalability, separation of power and energy capabilities, and simple operation. However, they can suffer from degradation during operation and the characteristics of the felt electrodes are little understood in terms of wetting, compression and pressure drops. Presented here is the design of a miniature flow cell that allows the use of x-ray computed tomography (CT) to study carbon felt materials in situ and operando, in both lab-based and synchrotron CT. Through application of the bespoke cell it is possible to observe felt fibres, electrolyte and pore phases and therefore enables non-destructive characterisation of an array of microstructural parameters during the operation of flow batteries. Furthermore, we expect this design can be readily adapted to the study of other electrochemical systems

A Dilatometric Study of Graphite Electrodes during Cycling with X-ray Computed Tomography

Graphite is the most commonly used anode material in commercial lithium-ion batteries (LiBs). Understanding the mechanisms driving the dimensional changes of graphite can pave the way to methods for inhibiting degradation pathways and possibly predict electrochemical performance loss. In this study, correlative microscopy tools were used alongside electrochemical dilatometry (ECD) to provide new insights into the dimensional changes during galvanostatic cycling. X-ray computed tomography (CT) provided a morphological perspective of the cycled electrode so that the effects of dilation and contraction on effective diffusivity and electrode pore phase volume fraction could be examined. During the first cycle, the graphite electrode underwent thickness changes close to 9% after lithiation and, moreover, it did not return to its initial thickness after subsequent delithiation. The irreversible dilation increased over subsequent cycles. It is suggested the primary reason for this dilation is electrode delamination. This is supported by the finding that the electrode porosity remained mostly unchanged during cycling, as revealed by X-ray CT

Nanoscale state-of-charge heterogeneities within polycrystalline nickel-rich layered oxide cathode materials

Nickel-rich cathodes (LiNixMnyCo1-x-yO2, x > 0.6) permit higher energy in lithium-ion rechargeable batteries but suffer from accelerated degradation at potentials above 4.1 V versus Li/Li+. Here, we present a proof-of-concept in situ pouch cell and methodology for correlative 2D synchrotron transmission X-ray microscopy with 3D lab-based micro-CT. XANES analysis of the TXM data enables tracking of Ni edge energy within and between the polycrystalline NMC811 particles embedded in the operating electrode through its initial delithiation. By using edge energy as a proxy, state-of-charge heterogeneities can be tracked at the nanoscale, revealing the role of cracked particles as potential nucleation points for failure and highlighting the challenges in achieving uniform (de-)lithiation. We propose, in future work, to leverage the pouch cell design presented here for longitudinal TXM-XANES studies of nickel-rich cathodes across multiple cycles and operating variables and investigate the effect of dopants and microstructural optimization in mitigating degradation