Thermodynamic and Kinetic Stability of the Solid Solution Phase in Nanocrystalline LixFePO4

Samples of nanostructured Li x FePO 4 with characteristic crystal sizes of 26 nm, and compositions of x = 0.35 and 0.65, were synthesized by ball-milling and chemical delithiation. X-ray powder diffraction showed that the solid solution phase started to form whenever two-phase materials were heated above 200 °C. The solid solution phase of nanocrystalline Li 0.65 FePO 4 was quick to form above 200 °C but did not unmix at lower temperatures. Unmixing below 200 °C was found after long-time annealing of nanocrystalline Li 0.35 FePO 4, however, consistent with the equilibrium phase diagram of bulk Li x FePO 4 . The stability of the solid solution of nanocrystalline Li x FePO 4 is kinetic in origin, perhaps originating with effects of crystal surfaces.