Batteries often fall short of their expected capacity, sometimes by a significant amount. Researchers at TU Graz have now pinpointed where this capacity loss occurs in lithium iron phosphate cathodes.
Lithium iron phosphate is a crucial material used in batteries for electric vehicles, stationary energy storage, and various tools. It offers longevity, is relatively cost-effective, and has a low risk of catching fire. Recent advancements have also improved its energy density. Nonetheless, specialists are still trying to understand why these batteries can miss their theoretical electricity storage capacity by as much as 25%. To tap into this unused capacity, it is essential to identify exactly how and where lithium ions are stored and released within the battery material during charging and discharging. Researchers from Graz University of Technology (TU Graz) have made a crucial stride in this area. Utilizing transmission electron microscopes, they meticulously traced the movement of lithium ions through the battery material, mapped their arrangement within the crystal structure of an iron phosphate cathode at an unprecedented level of detail, and accurately quantified their distribution in the crystal.
Important insights for enhancing battery capacity
“Our research has revealed that even when the test battery cells are fully charged, some lithium ions remain trapped in the crystal structure of the cathode rather than moving to the anode. These stationary ions contribute to a reduction in overall capacity,” explains Daniel Knez from the Institute of Electron Microscopy and Nanoanalysis at TU Graz. The researchers found that these immobilized lithium ions are unevenly spread throughout the cathode. They successfully identified these differing lithium concentration areas, differentiating them with precision down to a few nanometers. Distortions and deformations were discovered in the crystal lattice at these transitional zones. “These findings provide valuable insights into the physical phenomena that have so far hindered battery efficiency; we can factor these into future material development,” remarks Ilie Hanzu from the Institute of Chemistry and Technology of Materials, who played a key role in this research.
Techniques adaptable to other battery materials
For their study, the researchers analyzed material samples from both charged and discharged battery electrodes using the atomic-resolution ASTEM microscope at TU Graz. They employed a combination of electron energy loss spectroscopy, electron diffraction measurements, and atomic-resolution imaging. “By integrating various examination techniques, we were able to ascertain the specific locations of lithium within the crystal channels and the pathways it follows to get there,” says Nikola Å imić from the Institute of Electron Microscopy and Nanoanalysis, who is the lead author of the recently published research paper in the journal Advanced Energy Materials. “The techniques we developed and the insights gained regarding ion movement can also be applied to other battery materials, requiring only slight modifications to enhance their characterization and development.”
