Direct evidence of antisite defects in LiFe0.5Mn0.5PO4via atomic-level HAADF-EELS

Abstract

Using comprehensive transmission electron microscopy (TEM) techniques, the associations between the Mn dopant content, microstructure and improved rate performance of LiFe_(1−x)Mn_xPO₄ (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)Mn_xPO₄ (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)Mn_xPO₄ 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)Mn_xPO₄ rather than in pure LiFePO₄ by HAADF-EELS acquired from a single-atom column at high spatial resolution. The rate performance of LiFe_0.9Mn_0.1PO₄ and LiFe_0.8Mn_0.2PO₄ 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.