In Situ Atomic-Scale Imaging of Phase Boundary Migration in FePO

Orthorhombic Li x FePO 4 (0 ≤ x ≤ 1) system has attracted much attention for its application as a high power cathode material in lithium ion batteries. [ 1 ] Although the performance of this material has been greatly improved by cation doping, surface coating and size reduction, [ 2–4 ] the fundamental phase transformation mechanisms accompanying lithiation/delithiation are still controversial. [ 5–19 ] Theoretical calculations have revealed that the lithiation/delithiation in LiFePO 4 is highly anisotropic with lithium ion diffusion being mainly confi ned into the channels along the b -axis (space group Pnma ). [ 20–22 ] Ex situ high resolution transmission electron microscopy (HRTEM) study of microsized LiFePO 4 demonstrated that the phase boundary (PB) between LiFePO 4 and FePO 4 tends to lie on the bc plane. [ 23 ] Several variants of the PB migration mechanism have been proposed, such as the “domino-cascade” model and the “new core-shell” model. [ 10 , 11 ] Singh et al. developed a comprehensive surface-reaction-limited intercalation theory and fi rst predicted that the LiFePO 4 /FePO 4 PB migrated perpendicular to the lithium ion insertion direction like a “travelling wave”. [ 12 ]