Atomic scale insight into the fundamental mechanism of Mn doped LiFePO4†
A systematical and atomic scale investigation on the fundamental mechanism of Mn doped LiFePO4 is conducted in this work. For the first time, it is found that the doping depth of Mn on the surface of LiFePO4 is 10–15 nm. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) results further on the atomic scale demonstrate that Mn doping could effectively protect the crystal structure of LiFePO4 from being corroded by the electrolyte during electrochemical cycling. Density functional theory (DFT) calculations suggest that the Mn doped LiFePO4 could be regarded as a composite with LiFePO4 bulk as the core and LiMnxFe1−xPO4 as the outer layers. Unlike pure LiFePO4, the Mn doped olivine LiFePO4 (LiMnxFe1−xPO4) is more stable and less susceptible to phase transition related amorphization, and thus could serve as a protective shell against LiFePO4 degradation during electrochemical cycling.