Abstract
A new type of positive electrode for Li-ion batteries has been
developed recently based on FeF3/C and
FeF2/C nanocomposites. The microstructural and redox
evolution during discharge and recharge processes was followed by electron
energy loss spectroscopy (EELS) to determine the valence state of Fe by
measuring the Fe L3 line energy shift and from Fe
L3/L2 line intensity ratios. In addition,
transition metal fluorides were found to be electron beam sensitive, and
the effect of beam exposure on EELS spectra was also investigated. The
EELS results indicate that for both FeF3/C and
FeF2/C nanocomposite systems, a complete reduction of iron
to FeO is observed upon discharge to 1.5 V with the formation
of a finer FeO/LiF subnanocomposite (∼7 nm). Upon
complete recharging to 4.5 V, EELS data reveal a reoxidation process to a
Fe2+ state with the formation of a carbon metal fluoride
nanocomposite related to the FeF2 structure.