The team at the US Department of Energy’s (DOE) Argonne National Laboratory created and tested the single crystal electrode with researchers from Northwestern University and the University of Illinois at Chicago.
“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,” said Sanja Tepavcevic, assistant scientist in Argonne’s Materials Science division.
The team chose a sodium-ion battery already under development to compete with current lithium-ion batteries as sodium is far more abundant than lithium. A single crystal electrode of sodium-iridium oxide (Na2IrO3) was used as the cathode material in a small test cell. For comparison, the researchers also tested similar cells with polycrystalline cathodes. Using the Advanced Photon Source (APS) at Argonne they could determine the precise position of every atom in the crystal structure for different states of cell charge and discharge in the crystal.
“This project simply would not have been possible without the extraordinary material characterization resources of the APS,” said Tepavcevic. “We also greatly benefitted from the expertise of team member Jennifer Hong Zheng in her world-class capability at growing single crystals to precise specifications.”
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 extra capacity derives from surface reactions, not the bulk of the material as previously thought.
Important to improved battery design is knowing how and why material changes occur during cycling. From the test results, the team determined the chemical structure of the three distinct phases that form during charge, 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 on charge, which persisted during discharge and grew in size with cycle number.