The new “genomic” method reveals the atomic arrangements of the battery material

UPTON, NY – Scientists at the United States Department of Energy (DOE) Brookhaven National Laboratory, Stony Brook University (SBU), the materials project at DOE’s Lawrence Berkeley National Laboratory (Berkeley Lab), the University of California, Berkeley and European collaborators have developed a new way to decipher the atomic-level structure of materials based on data collected from ground dust samples. They describe their approach and demonstrate its ability to solve the structure of a material that shows the promise of transporting ions through sodium ion batteries in an article just published in the journal Chemistry of materials.

“Our approach combines experiment, theory and modern computational tools to deliver the high-quality structural data needed to understand important functional materials, even when only dust samples are available,” said corresponding author Peter Khalifah, who holds an appointment. jointly with Brookhaven Lab and SBU.

The technique is somewhat a form of reverse engineering. Instead of solving the structure directly from the experimental data measured on the dust sample – a problem too complex to be possible for many materials – it uses computer algorithms to construct and evaluate all plausible structures of a material. By analyzing the “genome” associated with a material in this way, it is possible to find the correct structure even when this structure is so complex that conventional methods of solving the structure fail.

For the study described in the paper, powder X-ray diffraction experiments were performed at the ALBA synchrotron in Barcelona, ​​Spain, by European collaborators Matteo Bianchini and Francois Fauth, part of a team led by Christian Masquelier. Scientists used the bright X-rays of that structure to study the atomic arrangement of a cathode material of a sodium ion battery known as NVPF at a variety of temperatures ranging from ambient to very low cryogenic temperatures at which gases atmospheric agents liquefy. This work is necessary because the disturbance in the ambient temperature structure of NVPF disappears when it is cooled to cryogenic temperatures. And while batteries operate near room temperature, deciphering the cryogenic structure of the material is still critically important because only this noise-free, low-temperature structure can provide scientists with a clear understanding of the true chemical bond that is present at room temperature. This chemical bonding environment strongly affects the way ions move through the structure at room temperature and thus affects the performance of NVPF as a battery material.

“The bonding environment around sodium atoms – how many neighbors each has – is essentially the same at low temperature as at room temperature,” Khalifah explained, but trying to capture those details at room temperature is like trying to convince children. stand still for a photo. “Everything becomes blurry because the ions are moving too fast for a picture to be taken.” For this reason, some of the bonding environments derived from the ambient temperature data are not correct. Conversely, cryogenic temperatures freeze the motion of sodium ions to provide a true picture of the local environment sodium ions are in when they are not moving.

“When the material is cooled, twenty-four nearby sodium ions are forced to choose one of two possible sites and their preferred low-energy ‘sorting’ pattern can be solved,” Khalifah said.

A preliminary analysis of Bianchini powder X-ray diffraction data indicated that the sorting model is very complex. For materials with such complex orders, it is typically not possible to solve their three-dimensional atomic structure using the powder diffraction data.

“The powder diffraction data is flattened to one dimension, so a lot of information is lost,” Khalifah said.

But materials composed of many different types of elements, as in the case of NVPF – which is built from sodium, vanadium, phosphorus, fluorine and oxygen atoms with an overall chemical formula of Na3V2 (PO4) 2F3 – are too hard to grow in. larger crystals for more conventional 3-D X-ray crystallography.

Then, the Brookhaven group collaborated with John Dagdelen and other researchers at Lawrence Berkeley National Laboratory to develop a new “genomic” approach capable of solving very complex structures using only powder diffraction data. The collaborative work was carried out within the Materials Project, a DOE-funded research team led by Kristin Persson at LBNL that is developing innovative computational approaches to accelerate the discovery of new functional materials.

“Instead of using the powder diffraction data to directly solve the structure, we took an alternative approach,” Khalifah said. “We asked ‘what are all the plausible arrangements of the sodium ions in the structure’, and then we tested each of them in an automated way to compare it with the experimental data to understand what the structure was.”

The NVPF structure is one of the most complex ever solved for a material that uses only powder diffraction data.

“We could not have done this science without modern computational tools – the enumeration methods used to generate the chemically plausible structures and the sophisticated automated scripts to refine those structures using the pymatgen (Python Materials Genomics) software library,” Khalifah said.

Based on the structural knowledge available for NVPF and a set of basic chemical rules for bonding, there are more than half a million plausible ordering schemes for sodium atoms in the NVPF. Even after applying computational algorithms to identify equivalent structures generated through different sorting choices, there are nearly 3,000 possible unique sortings left.

“These 3,000 test structures are more than can reasonably be tested by hand, but their correctness could be assessed by a single computer running non-stop for about two days,” Khalifah said.

The correctness of each test structure was assessed using software to predict what its powder X-ray diffraction pattern would be, and then comparing the calculated results with the experimentally measured diffraction data, work done by Stony Brook Ph. D. the student Gerard Mattei. If the difference between the predicted and observed diffraction patterns is relatively small, the software can optimize any test structure by changing the positions of its constituent atoms to improve the agreement between the calculated and observed patterns.

But even after that modification, nearly 2,500 of the optimized structures could be used to fit well with the experimental diffraction data.

“We weren’t expecting to get so many good fits,” Khalifah said. “So, we had a second challenge in determining which of those many possible structures was correct by looking at which one had the correct symmetry.”

Crystallographic symmetry provides the rules that constrain how atoms can be arranged in a material, so you need to fully understand the symmetry of a structure to describe it correctly, Khalifah noted.

The team had generated each of the test structures with a specific set of symmetry constraints. And while it was very difficult to determine the true symmetry of any test structure after its optimization, a comparison of all 2,500 optimized structures allowed the researchers to determine which symmetry elements were needed to correctly describe the true structure of NVPF.

The ability to compare results in many trials allows for a greater degree of confidence in the final solution and is an additional advantage that the new method used in this work has over traditional approaches. Furthermore, the theoretical calculations carried out by LBNL researchers John Dagdelen and Alex Ganose indicated that the final solution is stable against distortions, confirming the validity of this result.

The resolved structure revealed that there is much greater diversity in the bonding of sodium atoms than previously recognized.

“From the room temperature data, it appeared misleading that all the sodium atoms were bonded to six or seven neighboring atoms,” Khalifah said. “Conversely, the low temperature data clearly indicated that some sodium atoms have only four neighbors. One result of this is that sodium atoms with fewer neighbors are much less locked into place and therefore are expected to have a longer time. easy to move around the structure – an essential property for battery operation “.

The authors believe that this new approach should be widely applicable for solving the complex structures that commonly occur in battery materials when ions are removed during charging. This is particularly relevant in the materials used in sodium and potassium ion batteries, which are being developed as cheaper and more abundant alternatives to lithium ion battery materials. This research should therefore play an important role in unlocking the potential of earth-abundant materials that can be used to increase energy storage capacities to meet societal needs such as network-scale storage.

This research was funded by the DOE Office of Science.

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Brookhaven National Laboratory is supported by the US Department of Energy’s Office of Science. The Office of Science is the largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit https://www.energy.gov/science/

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One of ten national laboratories supervised and funded primarily by the Office of Science of the United States Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical and environmental sciences, as well as energy technologies and homeland security. Brookhaven Lab also builds and operates major scientific facilities available to university, industrial and government researchers. Brookhaven is managed and operated for the DOE Office of Science by Brookhaven Science Associates, a limited liability company founded by the Research Foundation for the State University of New York on behalf of Stony Brook University, the largest academic user of laboratory facilities , and Battelle, a non-profit scientific and technological organization.

Scientific article: Enumeration as a tool for structure solution – a materials genomic approach to solving the cation-order structure of Na3V2 (PO4) 2F3 [https://pubs.acs.org/doi/10.1021/acs.chemmater.0c03190]