Atomic-Scale Visualization of Antisite Defects inLiFePO4

Abstract

We visualize the antisite exchange defects in LiFePO4 crystals with an ordered olivine structure by using annular dark-field scanning transmission electron microscopy (STEM). A recognizable bright contrast is observed in some of the Li columns of STEM images in a sample annealed at a lower temperature, which directly demonstrates the disordered occupations by Fe atoms. Furthermore, such exchange defects appear to be locally aggregated rather than homogeneously dispersed in the lattice, although their overall concentration is fairly low. The present study emphasizes the significance of atomic-level observations for the defect distribution that cannot be predicted by macroscopic analytical methods.

Thermally-induced cation disorder in LiFePO4

“…This appears not to be the case, however when cation site exchange is found in samples prepared by low temperature synthesis [7][8][9] .…”

“…In the specific case of LiFePO 4 , several diffraction studies on materials prepared *Manuscript Click here to view linked References by low temperature routes have shown a small amount of Li, Fe cation exchange in samples prepared hydrothermally and using wet chemistry 7,8 . A small concentration of Fe on Li sites was visualised in LiFePO 4 crystals using dark field scanning transmission electron microscopy, STEM 9 . Above ~180 °C, a fully ordered structure occurred in samples that were prepared hydrothermally as well as by direct solid state reaction.…”

Tailoring Native Defects in LiFePO4: Insights from First-Principles Calculations

“…This is consistent with the reported experimental data showing the presence of various native defects in LiFePO 4 samples. [12][13][14][15][16][17][18][19] We note that there are several limitations inherent in our calculations. The first set of limitations comes from standard methodological uncertainties contained in the calculated formation enthalpies and phase diagrams as discussed in Ref.…”

“…Experimental reports on the defects have, however, painted different pictures. Some authors reported evidence of some iron and lithium atoms exchanging sites and forming the antisite pair Fe Li -Li Fe , 17,19 while others determined that Fe Li is formed in association with lithium vacancies (V Li ). 15,18 These conflicting reports suggest that the results may be sensitive to the actual synthesis conditions, and indicate that a better understanding of the formation of native defects in LiFePO 4 is needed in order to produce samples with controlled defect concentrations.…”

“…11,12 Iron antisites (Fe Li ) have also been reported to be present in LiFePO 4 samples. [14][15][16][17][18][19] This native defect is believed to be responsible for the loss of electrochemical activity in LiFePO 4 due to the blockage of lithium channels caused by its low mobility. 18,19 Clearly, native defects have strong effects on the material's performance.…”