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

Samples of nanostructured LixFePO4 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 Li0.65FePO4 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 Li0.35FePO4, however, consistent with the equilibrium phase diagram of bulk LixFePO4. The stability of the solid solution of nanocrystalline LixFePO4 is kinetic in origin, perhaps originating with effects of crystal surfaces.