Structural stability and Li-ion transport property of LiFePO4 under high-pressure

LiFePO4 with olivine structure is an intensively investigated cathode material for lithium ion batteries. However, the relationship between lattice parameters and Li-ion transport property has not been concerned previously. In this work, pristine LiFePO4 nanoparticle was synthesized by hydrothermal method. In-situ high-pressure synchrotron X-ray diffraction (XRD), in-situ Raman spectroscopy and first-principles calculations were used to characterize the structure evolution of LiFePO4 from ambient pressure to 21.5 GPa and obtain its equation of state and bulk modulus. Unlike previously reported phase transition of LiFePO4 from olivine structure to β′ phase (symmetry group Cmcm) under high pressure and high temperature, we did not observe any phase transition at pressure below 21.5 GPa. Here, the lattice parameters show an anisotropic decreasing as pressure increases, and LiO bonds are much more compressible than FeO and PO bonds during compression. The Li+ migration barrier energy of LiFePO4 under pressure was further investigated by first-principles calculations. The barrier energy along [010] and [001] directions increases with applied pressure, and the one-dimensional ionic diffusion property of LiFePO4 remains at the pressure below 28.2 GPa. These findings will enhance our understanding on this important cathode material and provide hints on materials synthesis and modification by high-pressure technology.