Direct evidence of antisite defects in LiFe0.5Mn0.5PO4 via atomic-level HAADF-EELS

Using comprehensive transmission electron microscopy (TEM) techniques, the associations between the Mn dopant content, microstructure and improved rate performance of LiFe(1-x)MnxPO4 (0 <= x <= 0.5) were well established. Via the synergistic mechanism including both templating and chelating effects contributed by cetyltrimethyl ammonium bromide (CTAB) and citric acid, a series of LiFe(1-x)MnxPO4 (0 <= x <= 0.5) olivine crystals with adjustable Mn doping content were synthesized. No impurity phase was detected. Accidentally, a novel type of roughness phenomenon at the particle boundaries of LiFe(1-x)MnxPO4 particles was observed, which depended on citric acid chelation. At the atomic level, the Mn ions were confirmed to be homogeneously substituted at the iron sites, which were furthermore examined by the combined analysis of electron energy loss spectroscopy (EELS), high angle annular dark-field (HAADF) imaging, magnetic susceptibility measurements and X-ray diffraction (XRD). Li/Fe antisite defects were found in the doped LiFe(1-x)MnxPO4 rather than in pure LiFePO4 by HAADF-EELS acquired from a single-atom column at high spatial resolution. The rate performance of LiFe0.9Mn0.1PO4 and LiFe0.8Mn0.2PO4 was improved compared to that of LiFePO4. Our findings might provide new insights into the understanding of Li-ion battery cathode materials with Mn dopant from a microstructural point of view.