Prevention and mitigation of thermal runaway presents one of the greatest challenges for the safe operation of lithium-ion batteries. Here, we demonstrate for the first time the application of high-speed synchrotron X-ray computed tomography and radiography, in conjunction with thermal imaging, to track the evolution of internal structural damage and thermal behaviour during initiation and propagation of thermal runaway in lithium-ion batteries. This diagnostic approach is applied to commercial lithium-ion batteries (LG 18650 NMC cells), yielding insights into key degradation modes including gas-induced delamination, electrode layer collapse and propagation of structural degradation. It is envisaged that the use of these techniques will lead to major improvements in the design of Li-ion batteries and their safety features.

Operando microbeam Raman spectroscopy is used to map the changes in hard carbon during sodiation and desodiation in unprecedented detail, elucidating several important and unresolved aspects of the sodiation mechanism. On sodiation a substantial, reversible decrease in G-peak energy is observed, which corresponds directly to the sloping part of the voltage profile and we argue can only be due to steady intercalation of sodium between the turbostratic layers of the hard carbon. Corresponding reversibility of the D-peak energy change is consistent with intercalation rather than representing a permanent increase in disorder. No change in energy of the graphitic phonons occurs over the low voltage plateau, indicating that intercalation saturates before sodium clusters form in micropores in this region. At the start of the initial sodiation there is no change in G-and D-peak energy as the Solid Electrolyte Interphase (SEI) forms. After SEI formation, the background slope of the spectra increases irreversibly, due to fluorescence. The importance of in situ/operando experiments over ex situ studies is demonstrated; washing the samples or air exposure causes the G-and D-peaks to revert back to their original states owing to SEI removal and sodium de-intercalation and confirming no permanent damage to the carbon structure.

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A polymer electrolyte membrane water electrolyser (PEMWE) employing a segmented current collector made from a printed circuit board (PCB) with optical access to the channel has been demonstrated for the first time. The cell allows the local current density, flow regime and bubble formation dynamics to be studied in real time. Transition from bubbly to slug flow is observed towards the end of the channel under higher bubble formation conditions which is associated with a significant increase in local current density.

a b s t r a c tWater uptake, sorption mechanics and swelling characteristics of thin-film Nafion and a commercially available Tokuyama alkaline anion exchange membrane ionomer from the vapour phase is explored using a quartz crystal microbalance (QCM). The water uptake measures the number of water molecules adsorbed by the ionomer per functional group and is determined in-situ using the QCM frequency responses allowing for comparison with nanogram precision. Crystal admittance spectroscopy, along with equivalent circuit fitting, is applied to both thin films for the first time and is used to investigate the ionomer's viscoelastic changes during hydration; to elucidate the mechanisms at play during low, medium and high relative humidities.

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