Review of LiFePO₄Phase Transition Mechanisms and New Observations from X-ray Absorption Spectroscopy

The high rate capability and reversibility of lithium iron phosphate battery cathodes is attributed to the highly reversible transition between its LiFePO 4 and FePO 4 phases. Conflicting models exist for the phase transition mechanism, such as the shrinking or expanding core models, surface reaction limited model, and the single-phase kinetic model. A literature review suggests that the subscribed theories depend upon the experimental methods and also the geometry and size of the LiFePO 4 particles. We study the electronic structure and disorder during the two-phase transition of commercial oval-shaped LiFePO 4 particles using in situ XAS (X-ray absorption spectroscopy) with the Δμ XANES (X-ray absorption near edge structure) difference technique along with traditional EXAFS (extended X-ray absorption fine structure) to track the Debye-Waller (DW) factor. The Δμ XANES magnitude, |Δμ|, tracks changes in the electronic structure which does not follow a synchronous path with lithium content. The magnitude of the DW shows the degree of structural disorder reaches a maximum near the middle of the charge/discharge cycle. The combined |Δμ| and EXAFS results suggests the LiFePO 4 /FePO 4 transition occurs through a "collective mosaic" or an unrelaxed "sequential" single-phase kinetic model.