Real-time synchrotron powder X-ray diffraction study of the antisite defect formation during sub- and supercritical synthesis of LiFePO4 and LiFe1-xMnx PO4 nanoparticles

In situ synchrotron powder X-ray diffraction (PXRD) is used to study the formation of LiFePO4 nanoparticles during hydrothermal synthesis from LiOH, H3PO4, and two different iron precursors, FeSO4 and (NH4)(2)Fe(SO4)(2). Furthermore, the synthesis of Li(Fe1-x Mn (x) )PO4 (x = 0.25, 0.50 and 0.75) from LiOH, H3PO4 and FeSO4/MnSO4 is studied. The reactions involve an unknown intermediate phase, which is not the previously observed intermediate NH4FePO4 center dot H2O. The intermediate phase quickly transforms into LiFePO4 and Li(Fe1-x Mn (x) )PO4 even at rather low temperatures. The presence of ammonium enhances the formation of LiFePO4, and it also leads to a significant reduction in the concentration of Li-Fe antisite defects. The in situ PXRD technique allows one to follow the influence of time, temperature and manganese doping on the antisite defect concentration, and it is shown that even under supercritical conditions a reaction time of several minutes is required to suppress the defects. This makes flow synthesis of defect-free LiFePO4 and Li(Fe1-x Mn (x) )PO4 nanoparticles challenging.