Real-time synchrotron powder X-ray diffraction study of the antisite defect formation during sub- and supercritical synthesis of LiFePO4and LiFe1−xMnxPO4nanoparticles

In situ synchrotron powder X-ray diffraction (PXRD) is used to study the formation of LiFePO 4 nanoparticles during hydrothermal synthesis from LiOH, H 3 PO 4 , and two different iron precursors, FeSO 4 and (NH 4 ) 2 Fe(SO 4 ) 2 . Furthermore, the synthesis of Li(Fe 1− x Mn x )PO 4 ( x = 0.25, 0.50 and 0.75) from LiOH, H 3 PO 4 and FeSO 4 /MnSO 4 is studied. The reactions involve an unknown intermediate phase, which is not the previously observed intermediate NH 4 FePO 4 ·H 2 O. The intermediate phase quickly transforms into LiFePO 4 and Li(Fe 1− x Mn x )PO 4 even at rather low temperatures. The presence of ammonium enhances the formation of LiFePO 4 , 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 LiFePO 4 and Li(Fe 1− x Mn x )PO 4 nanoparticles challenging.