Lithium iron phosphate cathode materials: investigating the structural changes caused by synthetic modifications and doping
In this project, lithium iron phosphate (LiFePO4) was synthesized by a hydrothermal method using homemade Teflon bombs as the reaction vessels. Changes in the LiFePO4 structure were investigated when, applying a series of synthetic modifications. The synthesis was optimized using the developed methodology and infrastructure and phase pure LiFePO4 was produced. Upon further characterization it was determined that although the products were phase pure, they were not defect free. This emphasized the importance of using a multi-technique approach to fully characterize and understand both the average and local structure, as well as identify the presence of amorphous phases. A series of synchrotron-based techniques were used, namely, X-ray absorption spectroscopy (XAS) using both X-ray near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), powder X-ray diffraction (PXRD) with Rietveld refinement, and to a lesser extent pair distribution function (PDF), together with laboratory instrumentation such as Mössbauer spectroscopy and Raman spectroscopy. The main synthetic challenge was the elimination of air that causes the rapid oxidation of Fe2+ to Fe3+. The presence of Fe3+ in the structure negatively influence the functioning of the material as a cathode, thus preventing oxidation is crucial and a large part of this project. Other factors that influenced the product was the lithium-ion addition rate during synthesis, where slower addition rates promote the formation of impurity phases as well as the oxidation to Fe3+ . To produce LiFePO4 with the lowest amount of defects, cation disorder and Fe3+ content, it was found that all the precursor solutions needed to be purged with nitrogen before and during mixing until they were sealed in the reaction vessels, longer reaction times were preferential, and a lithium addition rate of one drop every 3 seconds appeared optimal. After optimizing the synthetic route the possibility of doping (with Zn2+) and co-doping (with Zn2+ and Co2+) was investigated. It appeared that doping was possible using this method and there was no indication that either doping or co-doping specifically affected the Fe2+ content and no significant structural changes were noted.