EELS compositional and valence mapping in iron fluoride–carbon nanocomposites

Iron fluoride–carbon nanocomposites (FeF2–C and FeF3–C) are newly developed materials for positive electrodes in Li–ion batteries. These new composites represent high-capacity and reversible conversion electrode materials with capacity close to theoretical value and four times that of lithium cobalt oxide; the most widely used compounds as positive electrode. In this paper, we report detailed chemical and structural analysis of FeF2–C nanocomposite electrode at different discharge states. The FeF2–C nanocomposite was prepared by high-energy ball milling at ambient conditions and further processed in helium environment. The as synthesized electrode consists of FeF2 nanodomains with crystallite size ranging from 2 to 18 nm in an amorphous carbon matrix. The electrode material was analyzed by the electron energy loss spectroscopy (EELS) compositional imaging with nanometer resolution, and selected area electron diffraction (SAED) at 36 %, 75 %, and fully discharged states. The high-resolution EELS elemental maps and intensity line profiles of Fe and F were obtained to investigate phase transformation during discharge. Valence maps of Fe were also produced based on the Fe L 3/L 2 white line intensity ratios. The 2-nanometer scale EELS elemental maps reveal that Fe in the as processed electrode consists of a mixed valence states (2+ and 3+). The discharged samples at 36 and 75 % demonstrate compositional modulation with F-rich and F-depleted areas with the fully discharged material consisting of nanoscale regions of LiF and Fe in agreement with the cathodic conversion reaction.