Lithium iron phosphates as cathode materials in lithium batteries

Sammanfattning: Iron-based oxides are perhaps the only realistic cathode materials for electric and electric-hybrid vehicle applications. Two types of lithium iron phosphate, LiFePO4 andR-Li3Fe2(PO4)3, have been investigated for use within the lithium/lithium-ion polymer batteryconcept. The main techniques exploited have been: electrochemical cycling; X-ray andneutron diffraction - to study structural changes of the host material during lithiuminsertion/extraction; Mössbauer spectroscopy - to probe changes in the local structure of theFe atoms.Lithium extraction/insertion for LiFePO4, (olivine-type structure) has been followed by acombination of in situ X-ray diffraction and in situ Mössbauer spectroscopy. QuantitativeRietveld analysis of diffraction data has given good agreement with electrochemical andMössbauer spectroscopy data: ca. 20% of the LiFePO4 phase remains after charging. Possiblemechanisms to describe the capacity loss are proposed. High-temperature cycling (at 40oC and60oC) has shown the specific capacity to be markedly enhanced at higher temperatures.In situ Mössbauer spectroscopy has also been used to study the insertion of lithium into R-Li3Fe2(PO4)3. The two non-equivalent Fe3+ ions cannot be distinguished, whereas two typesof Fe2+ site appear on lithium insertion. Lithium appears to enter a site closer to one of theFe atoms. Elevated-temperature cycling shows the temperature dependence of R-Li3Fe2(PO4)3to be less marked than for the LiFePO4 system.The magnetic properties of R-Li3Fe2(PO4)3 have been investigated for the first time: a para- to antiferromagnetic transition is seen at 28 ~ K. Neutron powder diffraction shows that the Fe magnetic dipoles lie close to the ab-plane and are ferromagnetically aligned within an Fesheet perpendicular to the c-axis, but are antiferromagnetically coupled to adjacent sheets.