To power a significant transition to electric vehicles, we need batteries that can store more energy, charge much faster, while being smaller and lighter. One way of doing that is to replace the graphite anode typically found in commercial lithium-ion cells with a material with much higher energy storage capacity, such as a lithium anode. However, simply replacing the anode, in a conventional battery results in dendritic growth of lithium which rapidly causes short circuits and catastrophic failure. While dendritic growth in a liquid electrolyte has been widely studied, it was thought that replacing the liquid electrolyte with a solid (and creating a solid-state battery) would alleviate this issue by acting as a physical barrier to dendrite growth. However, solid-state batteries also suffer from dendritic growth. Researchers at the University of Oxford and The Faraday Institution investigated the development of dendrites in solid-state batteries with lithium anodes and a solid ceramic electrolyte. In work recently published in Nature Materials, they used synchrotron X-ray techniques to uncover crucial information about how lithium grows into the ceramic electrolyte and sets off a process of crack formation and subsequent lithium ingress that can lead to battery failure. Their results provide critical information for the development of new materials for next-generation batteries.
Dendrites, tiny, rigid tree-like structures, of lithium metal that can grow inside a battery. Their presence reduces battery life and performance and can lead to short circuits and catastrophic battery failures. The formation of dendrites inside a standard lithium-ion cell with a liquid electrolyte is widely studied and linked to the interactions between the solid anode and electrolyte. Solid-state batteries do not contain liquid electrolyte, so it was believed that they would not be affected by dendrite growth. However, that proved not to be the case. Indeed, the formation of dendrites is one of the factors holding up the development of next-generation, solid-state batteries.
Researchers at the University of Oxford and The Faraday Institution wanted to understand the formation of dendrites in solid-state batteries. However, getting a good look at what's going on is challenging, particularly as operando studies are needed to watch what happens through charging and discharging cycles.
DPhil candidate Dominic Spencer Jolly from the Department of Materials at the University of Oxford was part of the team that used X-ray computed tomography (XCT) and X-ray diffraction (XRD) on beamline I12 to investigate.
Solid-state batteries have a critical charge density, below which we don't see dendrites forming. However, that's far too low for automotive applications, so we need to solve the dendrite problem. Our project is investigating new science, and we didn't know whether we would be able to get the data we needed, but XCT offered us the best chance of seeing what was happening inside the cell. It was a very challenging experiment, and we worked very closely with the beamline staff at Diamond - particularly Oxana Magdysyuk. That close collaboration ultimately led to good data and fascinating insights into lithium infiltration into our ceramic electrolyte.
The results showed that, above the critical charge density, pothole-like cracks called spallations formed on the electrolyte's surface. The team could see lithium causing these potholes, and on subsequent charging cycles lithium grows into these spallations. This is the critical initiation phase of dendrite formation.
Once spallations form, subsequent charging initiates the type of cracks that propagate through the electrolyte, setting up the conditions for short circuits. Lithium then spreads through the cracks, ultimately causing catastrophic failure.
One critical question was whether the presence of lithium at the crack tip drives their growth or whether lithium spreads into dry cracks. The results show that the latter is happening. This is important because avoiding the development of dry cracks is a well-studied aspect of ceramic science. Tweaking the ceramic electrolyte material to prevent the formation of dry cracks could be the key to making solid-state batteries commercially viable.
This study has added considerably to our understanding of lithium behaviour in solid-state batteries and offers considerable promise to next-generation battery technologies. The team have been back to Diamond to carry out follow-on studies, and the authors also used the Tomcat Beamline SLS for part of this published study They are now working towards real-world solutions and the ultimate goal of the higher current density needed for automotive applications.
Ning Z et al. Visualizing plating-induced cracking in lithium-anode solid-electrolyte cells. Nature Materials 1-9 (2021). DOI:10.1038/s41563-021-00967-8.
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