Atomic scale insight into the fundamental mechanism of Mn doped LiFePO4

Abstract

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 LiMn_xFe_1−xPO₄ as the outer layers. Unlike pure LiFePO₄, the Mn doped olivine LiFePO₄ (LiMn_xFe_1−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.