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Abstract: Solid-state lithium batteries are developing rapidly as a promising next-generation battery, while challenges still persist in understanding their degradation processes during cycling due to the difficulties in characterization. In this study, the 3D morphological evolution of the Li3PS4 solid electrolyte was tracked during electrochemical cycles (plating and stripping) until short circuit by utilizing in situ synchrotron X-ray computed tomography with sufficient spatial and temporal resolution. During the degradation process, cracks in the electrolyte alternately generated from the two electrode/electrolyte interfaces and propagated until shorting. The lithium dendrites filled in the electrolyte cracks but had a greatly reduced filling ratio after the first plating stage; therefore, the cell could continue working for some time after the solid electrolyte was fully fractured by cracks. The compression of the two lithium electrodes mainly occurred in initial cycles where a ca. 4–7 μm reduction in thickness was observed. The mechanical force and electric potential fields were modeled to visualize their redistributions in different stages of cycling. The release of strain energy after the first penetration and thereafter the subsequent driving forces are discussed. These results reveal a fast degradation of solid electrolyte in the initial cycles, providing insights for further modifications and improvements in solid-state batteries.

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Abstract: In this paper, we used synchrotron X-ray radiography and tomography to study in operando conditions the growth dynamics of the primary Al3Ni intermetallic phases in an Al-15wt%Ni alloy in the solidification process with magnetic pulses of up to 1.5 T. The real-time observations clearly revealed the growth dynamics of the intermetallics in time scale from millisecond to minutes, including phase growth instability, side branching, fragmentation and orientation alignment under different magnetic fluxes. A multiphysics numerical model was also developed to calculate time-evolved Lorentz forces and stresses acting on the Al3Ni phases and the nearby melt. The differential forces between the growing Al3Ni phases and the nearby melt can create slip dislocations at the growing crystal front which can be further developed into nm and μm crystal steps for initiating phase branching. Furthermore, the magnitudes of the shear stresses are strongly related to the size, morphological and geometric features of the growing Al3Ni phases. Dependent on the magnitude of the shear stresses, phase fragmentation could occur in a single pulse period or in multiple pulse periods via fatigue mechanism. The combined real-time experimental observation and modelling work allowed us to elucidate some of the long-time debated hypotheses concerning intermetallic phases growth instability and phase fragmentation in pulse magnetic fields.

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Abstract: Thermal runaway of lithium-ion batteries can involve various types of failure mechanisms each with their own unique characteristics. Using fractional thermal runaway calorimetry and high-speed radiography, the response of three different geometries of cylindrical cell (18650, 21700, and D-cell) to different abuse mechanisms (thermal, internal short circuiting, and nail penetration) are quantified and statistically examined. Correlations between the geometry of cells and their thermal behavior are identified, such as increasing heat output per amp-hour (kJ Ah-1) of cells with increasing cell diameter during nail penetration. High-speed radiography reveals that the rate of thermal runaway propagation within cells is generally highest for nail penetration where there is a relative increase in rate of propagation with increasing diameter, compared to thermal or internal short-circuiting abuse. For a given cell model tested under the same conditions, a distribution of heat output is observed with a trend of increasing heat output with increased mass ejection. Finally, internal temperature measurements using thermocouples embedded in the penetrating nail are shown to be unreliable thus demonstrating the need for care when using thermocouples where the temperature is rapidly changing. All data used in this manuscript are open access through the NREL and NASA Battery Failure Databank.

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Abstract: The magnesium–sulfur (Mg-S) battery may be a safer alternative for the lithium-sulfur battery because Mg plating usually proceeds without dendrite formation. Here, we correlate the thermal runaway of Mg-S battery with the associated change of electrolyte vapour pressure via battery testing calorimetery. Over-pressure builds up along with the programmed heating of the cell, and as a result, the thermal runaway is triggered at 20 to 45 K over the electrolyte boiling point, corresponding to 70 to 150 kPa pressure difference between the cell and the environment. The distinct performance-safety-cost behaviours of three ether type of electrolytes stems from the different CH2CH2O chain lengths. Such molecular insight will serve as a fundamental guideline in choosing and designing the desired electrolyte that simultaneously achieves a high explosion limit and good electrochemical performance.

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Abstract: This research studies the dynamic evolution of dendritic structures and intermetallic phases of four Al based alloys during the solidification under pulse electromagnetic fields (PMFs). An advanced PMF solidification device was upgraded, built, commissioned for the research. The alloys used were Al-15Cu, Al-35Cu, Al-15Ni and Al-5Cu-1.5Fe-1Si. Systematic in-situ and real-time observation and studies were carried out at the TOMCAT beamline of Swiss Light Source, I13-2 beamline of Diamond Light Source and ID19 beamline of European Synchrotron Radiation Facility in the duration of this project. Synchrotron X-ray radiography and tomography were used primarily to observe and study the influence of PMFs on the nucleation and growth of primary dendritic structures and intermetallic phases under different magnetic flux and solidification conditions for the four alloys. More than 20 TB images and tomography datasets have been obtained throughout this research. Much effort and time was spent on segmenting, visualising and analysing these huge datasets using the Hull University supercomputer cluster, Viper, and the software, Avizo, ImageJ (Fiji), etc to explore and extract new insights and new science from those datasets. In particular, the skeletonisation function available from Avizo was customised and used to quantify the complex 3D microstructures and interconnected networks of different phases for the alloys. The important new findings of the research are: (1) Fragmentation of primary Al dendrites in the Al-15%Cu alloy was found when the magnetic flux of PMF applied is above 0.75 T; similarly, the fragmentation of Al3Ni intermetallic phases in the Al-15%Ni alloy was also observed when the magnetic flux of PMF applied is above 0.8 T. The clear and real-time observation of the fragmentation events in both dendritic and intermetallic phases provide unambiguous evidence to demonstrate that PMFs play a dominant role in structure fragmentation and multiplication, which is one important mechanism for structure (grain) refinement. (2) PMFs also produces pinch pressure gradient inside the semi-solid melt. Due to the different magnetic anisotropic properties between the liquid and solid phases, shear stresses due to the pinch pressure gradient may be produced. In the case of Al-15%Ni alloy, shear stresses of up to 30 MPa is created, which is sufficient to fracture Al3Ni phases. For the first time, such fragmentation mechanism for the Al3Ni phases in the Al-15%Ni alloy was revealed in this research. (3) The transition (or change of growth modes) of Al columnar dendrites to seaweed type dendrites in Al-15Cu alloy; and the facet growth to dendritic growth of the Al3Ni phases in the Al-15%Ni alloy were also observed in real-time when the magnetic flux is in the range of 0.75~0.8 T. Again, such dynamic changes in structure growth under PMFs are due to the enhanced melt flow caused by the applied fields. (4) In-situ tomography observation of PMF processing of the Al-5Cu-1.5Fe-1Si alloy also shows the effect of PMF on the refinement of the Chinese script type Fe intermetallic phases. In addition, the true 3D morphologies of three different types of Fe intermetallic phases in this alloy were clarified, again for the first time, in this research.