Search

Understanding the reaction mechanism of olivine compounds as electrode materials for lithium lithium-ion batteries have has received much attention recently. The question whether olivine LiFePO4 undergoes two-phase or non-nonequilibrium single-phase reaction during electrochemical processes has taken center stage in the understanding of the faster reaction kinetics observed in this material. Here, the lithiation/delithiation mechanism of Mg Mg-substituted LiFePO4 using high high-resolution X-ray diffraction(XRD), transmission electron microscopy(TEM), and electrochemical measurements is reported. Ex situ partially (de)lithiated olivine- LiMg0.2Fe0.8PO4 show the existence of stable equilibrium intermediate phases as characterized by the presence of more than two phases and broadness of diffraction peaks. Electron energy loss spectroscopy profiles across individual nanoparticles further confirm uniform lithiation with a constant Fe-L3 energy measured across each nanoparticle, suggestive of solid solution behavior in individual particles. In addition, a continuous shift in the diffraction peak position is observed even in the 'two-phase' region in the ex situ electrochemical (de)lithiated electrodes. [ABSTRACT FROM AUTHOR]

Iron fluoride-carbon nanocomposites (FeF-C and FeF-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 FeF-C nanocomposite electrode at different discharge states. The FeF-C nanocomposite was prepared by high-energy ball milling at ambient conditions and further processed in helium environment. The as synthesized electrode consists of FeF 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/ L 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Understanding the reaction mechanism of olivine compounds as electrode materials for lithium lithium-ion batteries have has received much attention recently. The question whether olivine LiFePO4 undergoes two-phase or non-nonequilibrium single-phase reaction during electrochemical processes has taken center stage in the understanding of the faster reaction kinetics observed in this material. Here, the lithiation/delithiation mechanism of Mg Mg-substituted LiFePO4 using high high-resolution X-ray diffraction(XRD), transmission electron microscopy(TEM), and electrochemical measurements is reported. Ex situ partially (de)lithiated olivine- LiMg0.2Fe0.8PO4 show the existence of stable equilibrium intermediate phases as characterized by the presence of more than two phases and broadness of diffraction peaks. Electron energy loss spectroscopy profiles across individual nanoparticles further confirm uniform lithiation with a constant Fe-L3 energy measured across each nanoparticle, suggestive of solid solution behavior in individual particles. In addition, a continuous shift in the diffraction peak position is observed even in the 'two-phase' region in the ex situ electrochemical (de)lithiated electrodes. [ABSTRACT FROM AUTHOR]