Batteries are a critical technology for the transition to a sustainable energy economy. Rechargeable lithium ion (Li ion) batteries power our electronic devices and electric cars and are needed to store energy generated from renewable sources. The design and discovery of new materials underpins the development of high performing and reliable rechargeable batteries that are long-lasting, cost-effective, fast charging, safe and sustainable. Most Li-ion batteries rely on a liquid electrolyte to conduct ions between the anode and cathode. However, liquid electrolytes can leak and are flammable, which can lead to fires. One solution to this issue is to use a solid electrolyte, and researchers at the University of Liverpool have discovered a solid material with high enough Li ion conductivity to replace the liquid electrolytes in current Li ion battery technology, improving safety and energy capacity. Their work, recently published in Science, used a collaborative computational and experimental workflow, synthesising the material in the laboratory, using synchrotron techniques to determine its structure, and demonstrating it in a battery cell. Their disruptive design approach offers a new route to discover more high-performance materials that rely on the fast motion of ions in solids.
If you’re looking for a new material for battery electrolytes, then you want something with high ionic conductivity and good chemical compatibility between the solid electrolyte and lithium metal is required. The existing high-performance solid-state electrolytes come from a small number of structural families with transport paths that minimize changes in cation coordination. With the assumption that this is what gives them their high conductivities, the search for new materials has continued along the same lines – emphasising anion packings that provide a single type of Li coordination environment
However, the team at the University of Liverpool has taken a different approach, opting for a design strategy using multiple anions to construct suitable pathways, supported by AI and physics-based calculations. The material they synthesised, Li7Si2S7I, is a pure lithium-ion conductor created by an ordering of sulphide and iodide with many different cation coordination environments that combine to create superionic conductivity. Created from non-toxic earth-abundant elements, the new material operates in a new way and achieves a high enough Li ion conductivity to replace liquid electrolytes.
At the start of the project, computational exploration of the Li-Si-S-I phase field offered up a number of candidate compositions, which were synthesised in carbon crucibles in the lab. Using X-ray Diffraction to identify the materials formed highlighted a novel phase. After suitable crystals for single-crystal diffraction were grown, the team used high resolution single-crystal XRD on Diamond’s I19 beamline to solve the crystal structure.
“We made a material that wouldn’t normally be considered an interesting potential ionic conductor,” said Dr John Claridge.
It doesn't really match the criteria most people would look at. It's quite low symmetry, and people tend to think high symmetry is the thing. Measurements on our initial crystal samples - using XRD on the I19 beamline, and X-ray Powder Diffraction (XPD) on I11 - showed that the material has a lot of disordered lithium sites. They seemed to change as a function of temperature, which indicated they're mobile.
The team has regular (BAG)access to the I19 and I11 beamlines, allowing research to progress more quickly. After the initial measurements, lead author Dr Guopeng Han was able to develop the ceramic formulation of the material that was needed for in operando studies.
These confirmed that the high conductivity of Li7Si2S7I arises from a combination of Li+ sites of widely varying geometry and anion coordination. This diversity offers multiple rapid transport pathways by providing many different low-barrier site-to-site connections.
Professor Matt Rosseinsky explained in a press release:
This research demonstrates the design and discovery of a material that is both new and functional, The structure of this material changes previous understanding of what a high-performance solid-state electrolyte looks like. Specifically, solids with many different environments for the mobile ions can perform very well, not just the small number of solids where there is a very narrow range of ionic environments. This dramatically opens up the chemical space available for further discoveries.
The team are now exploring ways to optimise the chemistry of Li7Si2S7I to further enhance the properties of the material. The new understanding provided by the study will also allow the identification of other materials that are well suited to exploratory synthesis.
To find out more about the beamlines or discuss potential applications, please contact I11 Principal Beamline Scientist [email protected] or I19 Principal Beamline Scientist Dave Allan: [email protected].
Han G et al. Superionic lithium transport via multiple coordination environments defined by two-anion packing.Science 383.6684 (2024): 739-745. DOI:10.1126/science.adh5115
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