Mapping Lithium Ion Distribution in Iron Phosphate Cathodes

Researchers at TU Graz have successfully pinpointed the exact location of capacity loss in a lithium iron phosphate cathode. Their findings have been published in the journal Advanced Materials Science.

In real-world applications, batteries often perform significantly below their theoretical capacity.

Lithium iron phosphate, a crucial component for batteries used in tools, stationary energy storage devices, and electric vehicles, is valued for its affordability, long service life, and resistance to spontaneous combustion. While progress is being made in energy density, experts are still perplexed by the fact that the practical electricity storage capacity of lithium-iron phosphate batteries can be up to 25 % less than their theoretical capacity.

To unlock this latent capacity, it is vital to understand precisely where and how lithium ions are stored and released within the battery material during charging and discharging cycles. Scientists at Graz University of Technology have made a significant breakthrough in this area.

Using transmission electron microscopes, they were able to map the arrangement of lithium ions in the crystal lattice of a lithium iron phosphate cathode with unprecedented resolution. They precisely quantified the distribution of lithium ions within the crystal and systematically tracked their movement through the battery material.

Key Clue for Increasing the Capacity of Batteries Further

Our investigations have shown that even when the test battery cells are fully charged, lithium ions remain in the crystal lattice of the cathode instead of migrating to the anode. These immobile ions incur a cost in capacity.

Daniel Knez, Institute of Electron Microscopy and Nanoanalysis, Graz University of Technology

The researchers discovered that immobile lithium ions are unevenly distributed within the cathode. They successfully identified and mapped these areas of varying lithium enrichment with precision, separating them down to just a few nanometers. In the transition areas between these regions, they observed distortions and deformations in the crystal lattice of the cathode.

These details provide important information on physical effects that have so far counteracted battery efficiency and which we can take into account in the further development of the materials.

Ilie Hanzu, Study Researcher, Institute of Chemistry and Technology of Materials, Graz University of Technology

Methods Also Transferable to Other Battery Materials

The researchers utilized TU Graz's atomic-resolution ASTEM microscope to analyze material samples prepared from the electrodes of charged and discharged batteries. They integrated electron diffraction measurements, atomic-level imaging, and electron energy loss spectroscopy.

By combining different examination methods, we were able to determine where the lithium is positioned in the crystal channels and how it gets there. The methods we have developed and the knowledge we have gained about ion diffusion can be transferred to other battery materials with only minor adjustments in order to characterize them even more precisely and develop them further.

Nikola Šimić, Study First Author, Institute of Electron Microscopy and Nanoanalysis, Graz University of Technology

Journal Reference:

Šimić, N., et al. (2024) Phase Transitions and Ion Transport in Lithium Iron Phosphate by Atomic-Scale Analysis to Elucidate Insertion and Extraction Processes in Li-Ion Batteries. Advanced Energy Materials. doi.org/10.1002/aenm.202304381