A first-principles study of bulk and surface Sn-doped LiFePO4: The role of intermediate valence component in the multivalent doping

Doping can be employed to enhance the electrical conductivity and electrochemical performance of LiFePO4, a promising material for Li-ion batteries. However, the microscopic mechanism of doping is not fully understood. In this study, ab initio density functional theory (DFT) with the generalized gradient approximation (GGA) + U calculations was performed on both bulk and surface Sn-doped LiFePO4. Our results indicate that surface doping is preferred over bulk or subsurface doping because it shows a lower doping energy and surface energy. The doping effect appears to be local, and the effect of the Li vacancy (V-Li) distribution was examined. The multivalent Sn doping may facilitate the formation of an Fe2+/Fe3+ complex with the involvement of an effective intermediate Sn3+ component, which complements the existing charge transfer model for LiFePO4. The effective Sn3+-Fe3+/2V(Li) complex may exist on the LiFePO4 surfaces, providing possible surface design schemes to control charge transfer. The results suggest that the Sn dopant could modulate band gap and local charge transfer, and improve the electrochemical performance at the last stage of the charging process with no capacity loss. However, an optimized doping concentration may exist for electrochemically inactive doping with an unfavorable doping energy. (C) 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim