Magnesium battery cathode could benefit from a bit of disorder

Tiny, disordered particles of magnesium chromium oxide may hold the key to a new magnesium battery energy storage technology, which could possess enhanced capacity compared with conventional lithium-ion batteries. This is according to a study by researchers from University College London (UCL) in the UK and the University of Illinois at Chicago. In a paper in Nanoscale, the researchers report a new, scalable method for making a material that can reversibly store magnesium ions at high-voltage, the defining feature of a cathode. While the work is at an early stage, the researchers say it represents a significant development in moving towards magnesium-based batteries. To date, very few inorganic materials have shown reversible magnesium removal and insertion, which is key for a magnesium battery to function. "Lithium-ion technology is reaching the boundary of its capability, so it's important to look for other chemistries that will allow us to build batteries with a bigger storage capacity and a slimmer design," said co-lead author Ian Johnson at UCL. "Magnesium battery technology has been championed as a possible solution to provide longer-lasting phone and electric car batteries, but getting a practical material to use as a cathode has been a challenge." One factor limiting lithium-ion batteries is the anode. Low-capacity carbon anodes have to be used in lithium-ion batteries for safety reasons, as the use of pure lithium metal anodes can cause dangerous short circuits and fires. In contrast, magnesium metal anodes are much safer, so partnering magnesium metal with a functioning cathode material would produce a smaller battery able to store more energy. Previous research using computational models predicted that magnesium chromium oxide (MgCr2O4) could be a promising candidate for Mg battery cathodes. Inspired by this work, UCL researchers produced disordered magnesium chromium oxide crystals just 5nm in size, using a very rapid and relatively low temperature reaction. Collaborators at the University of Illinois at Chicago then compared its magnesium activity with a conventional, ordered magnesium chromium oxide crystal that was 7nm in size. The researchers utilized a range of different techniques, including X-ray diffraction, X-ray absorption spectroscopy and cutting-edge electrochemical methods, to investigate structural and chemical changes when the two materials were tested for magnesium activity in a cell. They found that the two types of crystals behaved very differently: the disordered crystals displayed reversible magnesium extraction and insertion, whereas the larger, ordered crystals didn’t. "This suggests the future of batteries might lie in disordered and unconventional structures, which is an exciting prospect and one we've not explored before as usually disorder gives rise to issues in battery materials. It highlights the importance of seeing if other structurally defective materials might give further opportunities for reversible battery chemistry," explained co-author Jawwad Darr at UCL. "We see increasing the surface area and including disorder in the crystal structure offers novel avenues for important chemistry to take place compared to ordered crystals. Conventionally, order is desired to provide clear diffusion pathways, allowing cells to be charged and discharged easily – but what we've seen suggests that a disordered structure introduces new, accessible diffusion pathways that need to be further investigated," said co-author Jordi Cabana at the University of Illinois at Chicago. The researchers at UCL and the University of Illinois at Chicago now intend to expand their studies to other disordered, high surface area materials, with the aim of achieving further gains in magnesium storage capability and developing a practical magnesium battery. This story is adapted from material from UCL, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.