Review of LiFePO4 Phase Transition Mechanisms and New Observations from X-ray Absorption Spectroscopy

Abstract

The high rate capability and reversibility of lithium iron phosphate battery cathodes is attributed to the highly reversible transition between its LiFePO4 and FePO4 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 LiFePO4 particles. We study the electronic structure and disorder during the two-phase transition of commercial oval-shaped LiFePO4 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 LiFePO4/FePO4 transition occurs through a “collective mosaic” or an unrelaxed “sequential” single-phase kinetic model.