Phase change in LixFePO4

A series of LixFePO4 (0 <= x <= 1) was synthesized by full chemical oxidation of the stoichiometric LiFePO4 to FePO4 with NO2BF4 followed by the controlled chemical lithiation with Lil in acetonitrile. A careful X- ray powder diffraction investigation based on the Rietveld refinement revealed that the well-known biphase reaction proceeds but with slight deviation of the orthorhombic lattice constants from those of the stoichiometric end members of LiFePO4 and FePO4. The deviation can be explained by assuming a spinodal-type model, where the intermediate LixFePO4 (alpha < x < - beta) region consists of Li alpha FePO4 and Li1-beta FePO4 phases, where alpha = ca. 0.032 and beta = ca. 0.038 at room temperature. This is consistent with Srinivasan and Newman's prediction for the narrow monophase region (0 < x < alpha and 1 - beta < x < 1) close to the stoichiometric end members of LiFePO4 and FePO4 at room temperature. The nonstoichiometric parameters a and b in the biphase region can be easily estimated by simply measuring the lattice constants. They appear to be an important predictor for the electrochemical activity of the olivine LixMPO(4) and its derivatives, because they are directly linked with the density of lithium defect, M3+/M2+ mixed valence state, and hence the hopping probability of both of lithium ions and polarons. (c) 2005 The Electrochemical Society.