Researchers Shed Light on Workings of Lithium Iron Phosphate Cathode Materials

1 August 2008

The lithium iron phosphate nanoparticles (100 nm) making up the agglomerate are individually transformed though the “domino-cascade process” as the battery is charged.

Lithium iron phosphate is one of the promising cathode materials for use in next-generation lithium-ion batteries, given its environmentally friendly properties, lower cost and good thermal stability. With these batteries, lithium intercalation (the extraction and insertion of lithium ions from and into the electrode materials that is the basis for charge/discharge cycles) proceeds through a two-phase reaction between compositions very close to LiFePO₄ and FePO₄.

Both of these end member phases are very poor electronic and ionic conductors, however, raising the question of how these batteries manage to conduct electricity effectively.

Researchers at France’s CNRS (Centre national de la recherche scientifique), in collaboration with a team from CEA-Liten, have shed light on this seeming paradox and experimentally verified a “domino-cascade” model that shows local stresses within the material allow electrical and ionic conduction to spread from one area of the cathode material to the next, making the battery function.

The domino-cascade process identified by the researchers occurs as soon as stresses are present at the interface between the discharging material and the material in the discharged state. Electrical and ionic conduction is then extremely rapid in the interfacial zone, propagating from one spot to the next like dominos as the interface moves. The model has been verified by microscopic measurements.

This novel reaction process, resembling a wave sweeping through the crystal, explains how two insulating materials (one in the charged state and the other in the discharged state) can nonetheless make lithium-ion batteries function.

The results, published in the August issue of the journal Nature Materials, open new perspectives in the search for new electrode materials even with poor ionic and electronic conductivities, according to the researchers.

Resources

• C. Delmas, M. Maccario, L. Croguennec, F. Le Cras & F. Weill (2008) Lithium deintercalation in LiFePO₄ nanoparticles via a domino-cascade model. Nature Materials 7, 665 - 671 doi: 10.1038/nmat2230