Phase Change in Li[sub x]FePO[sub 4]

A series of was synthesized by full chemical oxidation of the stoichiometric to with 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 and . The deviation can be explained by assuming a spinodal-type model, where the intermediate region consists of and phases, where and at room temperature. This is consistent with Srinivasan and Newman's prediction [ J. Electrochem. Soc. , 151 , A1517 (2004) ] for the narrow monophase region ( and ) close to the stoichiometric end members of and at room temperature. The nonstoichiometric parameters α and β 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 and its derivatives, because they are directly linked with the density of lithium defect, mixed valence state, and hence the hopping probability of both of lithium ions and polarons.