E-Mobility Engineering 017 l ECE Doosan electric excavators dossier l In Conversation: Matt Faulks l Battery testing focus l Battery Show North America 2022 report l Ariel Hipercar digest l Cathode materials insight l Thermal management focus

Nick Flaherty explains the various non-invasive methods of analysing the workings of battery cells and their materials The inside story T esting battery cells, modules and packs presents a series of challenges. First of these is that taking measurements of the current and voltage changes the state of charge (SoC) of the cell and has an impact on its state of health. Also, because the battery is sealed, when it goes wrong it can be hard to see what faults occur. Taking the battery apart introduces internal changes that make it hard to see what the original flaw was or where it occurred. Non-invasive techniques, from X-rays to electron microscopes and even MRI scanners are therefore being used to examine how all kinds of cells operate without having to pull them apart. The data obtained using these technologies allows researchers to look at the materials being used, and even to see inside the cell while it’s in operation. The data is vital for building more detailed, accurate models of how the cell operates over time to gauge the reliability and lifetime of the cells and packs. It is also extremely helpful for the emerging machine learning (ML) frameworks that are also being used to improve the accuracy of models. X ray analysis X-ray analysis has helped to identify how materials work in a cell. One new cell compound, niobium pentoxide, has a novel crystalline structure and shows promise for speeding up charging while providing excellent storage capacity. During charging, lithium ions move from the cathode to the anode, which is commonly made of graphite. At higher charging speeds, lithium metal tends to accumulate on the graphite’s surface. This effect, known as plating, tends to degrade performance and can cause batteries to short-circuit, overheat and catch on fire. Niobium pentoxide is much less susceptible to plating, potentially making it safer and more durable than graphite. In addition, its atoms can arrange themselves in many different stable configurations that don’t require much energy to reconfigure them. A coin cell with niobium pentoxide as the electrode material has been built and tested using a range of X-ray technologies. The compound started out with an amorphous structure but as the cell was charged and discharged numerous times, the structure transformed into an ordered, crystalline one, which had never been seen previously. Compared with the disordered arrangement, the crystalline structure enabled easier and faster transport of lithium ions into the anode during charging. This points to the material’s promise for fast charging, and other measurements suggest it can store a large amount of charge. Because of the complex changes during the charge-discharge cycle, several complementary measurement tools were needed to gain a comprehensive understanding of the nanostructure. For example, transmission electron microscopy was used to verify the structural transformation from amorphous to crystalline, with high- energy electron beams creating digital Part of a synchrotron X-ray source for examining battery materials (Courtesy of Argonne National Laboratory) 34 January/February 2023 | E-Mobility Engineering