Developments in X-ray tomography characterization for electrochemical devices

Over the last century, X-ray imaging instruments and their accompanying tomographic reconstruction algorithms have developed considerably. With improved tomogram quality and resolution, voxel sizes down to tens of nanometers can now be achieved. Moreover, recent advancements in readily accessible lab-based X-ray computed tomography (X-ray CT) instruments have produced spatial resolutions comparable to specialist synchrotron facilities. Electrochemical energy conversion devices, such as fuel cells and batteries, have inherently complex electrode microstructures to achieve competitive power delivery for consideration as replacements for conventional sources. With resolution capabilities spanning tens of microns to tens of nanometers, X-ray CT has become widely employed in the three-dimensional (3D) characterization of electrochemical materials. The ability to perform multiscale imaging has enabled characterization from system-down to particle-level, with the ability to resolve critical features within device microstructures. X-ray characterization presents a favorable alternative to other 3D methods, such as focused ion beam scanning electron microscopy, due to its non-destructive nature, which allows four-dimensional (4D) studies, three spatial dimensions plus time, linking structural dynamics to device performance and lifetime. X-ray CT has accelerated research from fundamental understanding of the links between cell structure and performance, to the improvement in manufacturing and scale-up of full electrochemical cells. Furthermore, this has aided in the mitigation of degradation and cell-level failures, such as thermal runaway. This review presents recent developments in the use of X-ray CT as a characterization method and its role in the advancement of electrochemical materials engineering

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

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

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

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

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

Correlative electrochemical acoustic time-of-flight spectroscopy and X-ray imaging to monitor the performance of single-crystal and polycrystalline NMC811/Gr lithium-ion batteries

LiNixMnyCozO2 (NMC) electrodes typically consist of anisotropic single-crystal primary particles aggregated to form polycrystalline secondary particles. Electrodes composed of polycrystalline NMC particles have a comparatively high gravimetric capacity and good rate capabilities but do not perform as well as single crystal equivalents in terms of volumetric energy density and cycling stability. This has prompted research into well-dispersed single-crystalline NMC products as an alternative solution for high-energy-density batteries. Here, for the first time known to the authors, electrochemical acoustic time-of-flight (EA-ToF) spectroscopy has been shown to be effective in distinguishing between Li-ion batteries composed of either single-crystal NMC811 (SC-NMC811) or polycrystalline NMC811 (PC-NMC811) electrodes. Cells composed of PC-NMC811 electrodes had a higher degree of gas evolution compared to cells containing SC-NMC811 electrodes. Cells composed of PC-NMC811 electrodes also underwent larger changes in the acoustic signal's time-of-flight (ToF) during constant current cycling at a range of C-rates indicating expansion, fracture or dislocation of the reflective interfaces inside the cell. In addition, X-ray computed tomography (X-ray CT) has been used to confirm significant morphological differences between SC-NMC811 electrodes and PC-NMC811 electrodes including the electrode's particle size distribution (PSD) that is suggested to have an effect on acoustic signal interaction with these electrode interfaces

Data for an Advanced Microstructural and Electrochemical Datasheet on 18650 Li-ion Batteries with Nickel-Rich NMC811 Cathodes and Graphite-Silicon Anodes

The data presented here were collected from a commercial LG Chem cylindrical INR18650 MJ1 lithium-ion (Li-ion) battery (approximate nominal specifications: 3.5 Ah, 3.6 V, 12.2 Wh). Electrochemical and microstructural information is presented, the latter collected across several length scales using X-ray computed tomography (CT): from cell to particle. One cell-level tomogram, four assembly-level and two electrode/particle-level 3D datasets are available; all data was collected in the pristine state. The electrochemical data consists of the full current and voltage charge-discharge curves for 400 operational cycles. All data has been made freely available via a repository [10.5522/04/c.4994651] in order to aid in the development of improved computational models for commercially-relevant Li-ion battery materials

A greyscale erosion algorithm for tomography (GREAT) to rapidly detect battery particle defects

Particle micro-cracking is a major source of performance loss within lithium-ion batteries, however early detection before full particle fracture is highly challenging, requiring time consuming high-resolution imaging with poor statistics. Here, various electrochemical cycling (e.g., voltage cut-off, cycle number, C-rate) has been conducted to study the degradation of Ni-rich NMC811 (LiNi0.8Mn0.1Co0.1O2) cathodes characterized using laboratory X-ray micro-computed tomography. An algorithm has been developed that calculates inter- and intra-particle density variations to produce integrity measurements for each secondary particle, individually. Hundreds of data points have been produced per electrochemical history from a relatively short period of characterization (ca. 1400 particles per day), an order of magnitude throughput improvement compared to conventional nano-scale analysis (ca. 130 particles per day). The particle integrity approximations correlated well with electrochemical capacity losses suggesting that the proposed algorithm permits the rapid detection of sub-particle defects with superior materials statistics not possible with conventional analysis

The Detection of Monoclinic Zirconia and Non-Uniform 3D Crystallographic Strain in a Re-Oxidized Ni-YSZ Solid Oxide Fuel Cell Anode

The solid oxide fuel cell (SOFC) anode is often composed of nickel (Ni) and yttria-stabilized zirconia (YSZ). The yttria is added in small quantities (e.g., 8 mol %) to maintain the crystallographic structure throughout the operating temperatures (e.g., room-temperature to >800 °C). The YSZ skeleton provides a constraining structural support that inhibits degradation mechanisms such as Ni agglomeration and thermal expansion miss-match between the anode and electrolyte layers. Within this structure, the Ni is deposited in the oxide form and then reduced during start-up; however, exposure to oxygen (e.g., during gasket failure) readily re-oxidizes the Ni back to NiO, impeding electrochemical performance and introducing complex structural stresses. In this work, we correlate lab-based X-ray computed tomography using zone plate focusing optics, with X-ray synchrotron diffraction computed tomography to explore the crystal structure of a partially re-oxidized Ni/NiO-YSZ electrode. These state-of-the-art techniques expose several novel findings: non-isotropic YSZ lattice distributions; the presence of monoclinic zirconia around the oxidation boundary; and metallic strain complications in the presence of variable yttria content. This work provides evidence that the reduction–oxidation processes may destabilize the YSZ structure, producing monoclinic zirconia and microscopic YSZ strain, which has implications upon the electrode’s mechanical integrity and thus lifetime of the SOFC