A XANES study of LiVPO4F: a factor analysis approach.

Evolving factor analysis (EFA) of X-ray absorption near-edge spectroscopy (XANES) data is shown to be a useful tool to understand the phase relationships and compositional ranges of stability in the LiVPO4F-VPO4F system. EFA was used to calculate the concentration of phases versus state-of-charge in a lithium-ion battery and true XANES spectra. The results of EFA showed that, indeed, three phases were present during cycling of a LiVPO4F∥Li cell: LiVPO4F, LixVPO4F, and VPO4F. In contrast to what was reported by others, the second phase was not a fixed composition with x = 0.67, but, instead, existed over a range of lithium stoichiometry, x = 0.25 to 0.80. EFA results also showed that the reactions leading to these phases are reversible.