This composite image shows the voltage curve when charging a sodium-iridium oxide cathode, the microstructures formed at different voltages and the single crystals grown for the test cells. Image: Argonne National Laboratory.Scientists at the US Department of Energy (DOE)’s Argonne National Laboratory have created and tested a single-crystal electrode that promises to yield pivotal discoveries for advanced batteries under development worldwide for electric vehicles, consumer electronics and other applications. They collaborated on the project with researchers from Northwestern University and the University of Illinois at Chicago, and report their findings in a paper in Advanced Energy Materials.
The electrode materials in advanced batteries are ‘polycrystalline’, meaning they have numerous differently oriented crystalline regions. Because polycrystalline electrodes are relatively simple to fabricate, scientists have continued to utilize these materials when developing advanced batteries. But polycrystalline electrodes are full of different kinds of defects within the ordered structures that often can affect their performance.
"We recognized that single crystals can play a vital role in identifying promising new ways to understand, at atomic and molecular levels, the chemistries that control charge-discharge processes in batteries with polycrystalline electrodes," explained Sanja Tepavcevic, assistant scientist in Argonne's Materials Science division.
As a model system to investigate a single-crystal cathode, the scientists chose a sodium-ion battery being developed at Argonne to compete with current lithium-ion batteries. The main attraction of these batteries is that sodium is a far more abundant element than lithium.
The team prepared single crystals of a sodium-iridium oxide (Na2IrO3) and used them as the cathode material in small test cells. For comparison, they also tested similar cells with polycrystalline cathodes. By drawing upon the scientific facilities at Argonne – in particular, the Advanced Photon Source (APS), a DOE Office of Science User Facility – the scientists could determine the precise position of every atom in the crystal structure for different states of cell charge and discharge.
"This project simply would not have been possible without the extraordinary material characterization resources of the APS," said Tepavcevic. "We also greatly benefited from the expertise of team member Jennifer Hong Zheng in her world-class capability at growing single crystals to precise specifications."
Much was learned about the cathode chemistry during charge-discharge cycling of the test cells. In particular, the team investigated the origin of the extra capacity beyond that expected for the NaIrO3 endpoint structure.
"With our single crystals, we could separate surface from bulk effects that were not apparent in earlier work with polycrystalline materials alone," said Tepavcevic. The team demonstrated that the extra capacity derives from surface reactions, not the bulk of the material as previously thought.
Important for improved battery design is knowing how and why material changes occur during cycling. From their test results, the team determined the chemical structure of three distinct phases that form during charging, two of which were not known before. They also found that cell capacity faded with cycling because of the formation of a new detrimental phase during charging; this phase persisted during discharge and grew in size with cycle number.
"We learned more about sodium-ion batteries with our single-crystal electrodes than we ever thought possible at the project start," said John Mitchell, a distinguished fellow in Argonne’s Materials Science division. "Clearly, single crystals open the window to a far better understanding of the chemical and electronic transformations that control energy storage and release in all battery types, as well as their degradation mechanisms with cycling." With such knowledge, future battery researchers will be able to develop design rules for synthesizing new and improved polycrystalline materials with desired functionality.
This story is adapted from material from Argonne National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.