Optimizing the capacity of lithium iron phosphate batteries

Battery technology Optimizing the electricity storage capacity of lithium iron phosphate batteries

2024-08-26 Source: Press release | Translated by AI 2 min Reading Time

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Lithium iron phosphate is one of the most important materials for electric vehicle batteries. However, in practice they sometimes significantly undercut their theoretical capacity. Researchers at Technical University Graz (Austria) have now observed where the capacity loss occurs in a lithium iron phosphate cathode.

Graz University of Technology is working on an explanation as to why lithium iron phosphate batteries undercut their theoretical electricity storage capacity by up to 25 percent in practice. Here, lithium-rich (bottom right) and lithium-poor (top left) areas of the sample material are shown. For easier comparison, both areas are also shown in images of simulations.(Image: Felmi | TU Graz) — Graz University of Technology is working on an explanation as to why lithium iron phosphate batteries undercut their theoretical electricity storage capacity by up to 25 percent in practice. Here, lithium-rich (bottom right) and lithium-poor (top left) areas of the sample material are shown. For easier comparison, both areas are also shown in images of simulations.
(Image: Felmi | TU Graz)

Lithium iron phosphate is durable, comparatively inexpensive and does not tend to spontaneously combust. Energy density is also making progress. A disadvantage is the fact that lithium iron phosphate batteries undercut their theoretical electricity storage capacity by up to 25 percent in practice. In order to utilize this reserve capacity, it is necessary to know where and how lithium ions are stored and released in the battery material during the charging and discharging cycles. Graz University of Technology has now succeeded in taking a significant step in this direction: In investigations using transmission electron microscopes, researchers were able to systematically track the lithium ions on their way through the battery material, image their arrangement in the crystal lattice of an iron phosphate cathode with unprecedented resolution and precisely quantify their distribution in the crystal.

Capacity losses due to immobile ions

"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 cost capacity," explains Daniel Knez from the Institute for Electron Microscopy and Nanoanalytics at the university. The immobile lithium ions are unevenly distributed in the cathode. The researchers have succeeded in precisely determining these areas, which are enriched with lithium to varying degrees, and separating them from each other down to a few nanometers. Distortions and deformations were found in the crystal lattice of the cathode in the transition areas.

Gerald Kothleitner, Werner Grogger, Nicola Šimić, Daniel Knez (all from the Institute of Electron Microscopy and Nanoanalytics) and Anna Jodlbauer from the Institute of Chemistry and Technology of Materials next to a scanning electron microscope on which some of the investigations were carried out (from left to right).
(Image:Helmut Lunghammer | TU Graz)

Methods can be used for other battery materials

For the investigations, TU Graz prepared material samples from the electrodes of charged and discharged batteries and examined them under the atomic-resolution ASTEM microscope. They combined electron energy loss spectroscopy with electron diffraction measurements and imaging at atomic level. "By combining different methods of investigation, we were able to determine where the lithium is positioned in the crystal channels and how it gets there," explains Nikola Šimić from the Institute of Electron Microscopy and Nanoanalytics. "The methods we have developed and the knowledge we have gained about ion diffusion can also be transferred to other battery materials with only minor adjustments in order to characterize and further develop them even more precisely." (se)

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The individual cell of the solid-state battery, which was developed as material for the next generation of CeraCharge. (Image:TDK Europe)

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