Solid-state batteries are poised to transform the future of energy storage offering a potentially safer, more energy dense alternative to traditional lithium-ion batteries.
At the forefront of next generation battery technologies, solid-state batteries using lithium metal anodes could provide a leap forward in the range capabilities of electric vehicles and could even help advance electrically powered aviation.
However, despite their immense potential, solid-state batteries using lithium metal anodes face some hurdles. A major challenge is their limited charging speed. The use of a lithium metal anode imposes a strict charging current threshold. If this limit is exceeded, the battery will fail.
The key reason for this problem is ‘dendritic cracking’, which occurs when tiny, needle-like Li filaments (dendrites) grow from the metal anode, through the solid electrolyte, eventually reaching the positive electrode causing an internal short circuit.
One solution is to change the materials inside the battery cell or modify the cell’s mechanical properties to inhibit dendrites from growing through completely.
Recently, researchers used cutting-edge synchrotron X-ray imaging at Diamond to obtain exciting new insights on how lithium dendrites form and how they propagate, increasing our fundamental understanding of how they work and making it possible to develop mitigation strategies.
Experiments to investigate what is going on inside the battery cell during operation employed a combination of in situ synchrotron X-ray computed tomography (XCT) and X-ray diffraction techniques using Diamond’s I12-JEEP (Joint Engineering, Environmental, and Processing) beamline [4] and the Diamond Manchester Imaging Branchline I13-2 [5].
The findings were published in the high-impact peer-reviewed scientific journal Joule on the 18th of September 2024 [1] and build on previous research at Diamond [2,3]. They reveal that dendritic cracking is slowing charging times, and that the mechanical properties of the materials play a pivotal role.
The research was a collaborative effort among scientists from the University of Oxford (UK), the University of Birmingham (UK), University of Manchester (UK), Diamond Light Source Ltd (UK), STFC-Rutherford Appleton Laboratory (ISIS Facility) (UK), the Fujian Science & Technology Innovation Laboratory for Energy Devices (PRC), and Shanghai Jiao Tong University (PRC).
Solid-state batteries hold the potential to address the growing need for high-energy batteries while mitigating the safety risks associated with traditional lithium-ion batteries.
This is because unlike the conventional electrolytes in lithium-ion batteries, which are liquids, solid-state batteries instead use a solid material, typically a ceramic.
The big advantage of using a ceramic electrolyte is that it potentially enables the use of a lithium metal anode (this is the negative electrode in the battery), which is theoretically the best type of anode for producing high energy.
In terms of safety, ceramics are considered much safer than liquids because the liquid electrolyte in conventional lithium-ion batteries has flammable components which can cause uncontrollable heating and explosion.
However, the charging current densities of solid-state batteries with lithium metal anodes and ceramic electrolytes are severely limited due to lithium dendrites that penetrate the solid electrolyte at practical charging currents, leading to battery failure.
How does the process of lithium dendritic cracking work? When charging, the dense lithium metal layer of the solid-state battery is being ‘plated’, which leads to pressure building within defects (pores and cracks) of the ceramic electrolyte close to the lithium metal anode. This causes fractures in the ceramic which enables the plated lithium metal (the dendrite) to grow inwards through the ceramic.
As cracks grow across the entire width of the ceramic electrolyte, they move towards the positive electrode in the battery – the cathode.
Once the lithium metal dendrite reaches the cathode, it causes an internal short circuit because lithium metal is highly electronically conductive.
One of the difficulties in understanding dendrites in solid-state batteries, however, is that they occur in between solid layers of material, in effect occurring in a buried interface. This means it is difficult to get a clear picture of what is happening during battery cell operation.
Dr Genoveva Burca, the I12-Jeep Principal Beamline Scientist says:
“The high energy X-ray I12-JEEP beamline plays a powerful and unique role at Diamond, offering both X-ray imaging and diffraction techniques in one experiment for in situ and/or operando studies. The high-energy X-rays available on I12, along with the high-resolution setup, result in shorter scan times compared to the standard lab equipment. This allows for more observations to be gathered at various stages of failure, in real time.”
The combination of synchrotron X-ray tomography and diffraction offered by Diamond’s I12-JEEP beamline allowed the scientists to spatially co-ordinate where they collect X-ray diffraction patterns.
This involved focussing the X-ray beam on a small spatial area and collecting the diffraction patterns. Observing these precise regions of the cell provided further insights about the material’s crystal structure and strains in those regions.
Bingkun Hu, from the University of Oxford, is the lead author of the paper. He explains why X-ray imaging using synchrotron light was beneficial for this research:
“In the past, we relied on failing the cell and then looking at it afterwards, which is quite destructive. X-ray imaging using synchrotron light, however, is non-destructive and can be done in situ. X-ray tomography at Diamond allowed us to capture very high-resolution images in real time of what's going on inside a solid-state battery whilst we're operating it.
We obtained very clear insights characterising both cell failure and the mitigation strategies under investigation. These insights are invaluable when defining criteria and refining strategies.”
The research compared the use of three different solid electrolyte materials as inner layers to a tri-layered electrolyte design, namely Lithium Scandium Chloride (Li3ScCl6) and two sulphide materials: LGPS (Li10GeP2S12) and LPS (Li3PS4). In all cases, the same outer material was used: a sulphide argyrodite-type solid electrolyte (Li6PS5Cl).
The aim was to understand whether these designs could actually inhibit lithium dendrites, and if so whether a criteria for the necessary microstructural and mechanical design could be elucidated by comparing the three different cases.
The experiments revealed that a significant difference in mechanics (such as stiffness) between the outer layer and the inner layer can enable crack deflection to occur at the interface, this newly discovered requirement highlights that only certain combinations of materials can be effective for these layered designs.
They noted that certain inner layer materials not only deflect the crack at the interface but can also deflect the crack within their layer. This can happen when the inner layer material contains large, dense regions with preferred orientation.
Additionally, inners layers which react spontaneously with lithium metal can decompose when the dendrite grows into them. In some cases, the resultant reaction products can plug the crack, helping to stop the dendrite growing further.
In summary, the results demonstrated that even if dendrite initiation cannot be avoided, dendrite growth can be inhibited/deflected in multi-layered solid electrolytes by exploiting the differences in elastic properties (stiffness) between layers or by utilising the preferred orientation of particles within the inner layer.
Professor Sir Peter Bruce, a co-author of the paper from the University of Oxford, explains the significance of these findings:
“What makes this study stand out is that we’ve identified important criteria for the design of multi-layered electrolytes which hadn’t been identified previously. We've brought attention to the fact that the mechanical properties of each of the electrolytes are key, and that their differences best predict when and how the crack will change direction and deflect.”
Looking forward, Sir Peter says:
“There's a lot of work underway to get working batteries with lithium metal anodes. The big motivation is that solid-state batteries are seen as one of the most promising routes. If solid-state batteries with lithium metal anodes work, they would be safer, very high energy batteries. We will continue to research solid-state batteries and bring our mechanistic understandings to help develop a practical device/battery cell in the future. Our aim is to complement our understanding and help accelerate any kind of progress towards the ultimate aim, which is a working practical device.”
[1] Hu et al., Deflecting lithium dendritic cracks in multi-layered solid electrolytes, Joule (2024), https://doi.org/10.1016/j.joule.2024.06.024
[2] Ning, Z., Li, G., Melvin, D.L.R. et al. Dendrite initiation and propagation in lithium metal solid-state batteries. Nature 618, 287–293 (2023). https://doi.org/10.1038/s41586-023-05970-4
[3] Ning, Z., Jolly, D.S., Li, G. et al. Visualizing plating-induced cracking in lithium-anode solid-electrolyte cells. Nat. Mater. 20, 1121–1129 (2021). https://doi.org/10.1038/s41563-021-00967-8
[4] I12: the Joint Engineering, Environment and Processing (JEEP) beamline at Diamond Light Source [Drakopoulos et al. (2015). J. Synchrotron Rad. 22, 828-838, https://doi.org/10.1107/S1600577515003513] accessed on the 20.10.2024; See also: https://www.diamond.ac.uk/Instruments/Imaging-and-Microscopy/I12.html accessed on the 20.10.2024
[5] I13-2: the Diamond Manchester Imaging Branchline at Diamond Light Source. https://www.diamond.ac.uk/Instruments/Imaging-and-Microscopy/I13-2.html accessed on the 23.10.2024
To find out more about the I12 JEEP beamline, or to discuss potential applications, please contact Principal Beamline Scientist, Genoveva Burca: [email protected]
To find out more about the I13-2 beamline, or to discuss potential applications, please contact Principal Beamline Scientist, Christoph Rau: [email protected]
Diamond Light Source is the UK's national synchrotron science facility, located at the Harwell Science and Innovation Campus in Oxfordshire.
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