Marten L. Denboer

X-ray absorption fine structure studies of FeS2 cathodes in lithium polymer electrolyte batteries

Abstract We have performed synchrotron X-ray absorption measurements on a series of sealed Li/composite polymer electrolyte (CPE)/FeS 2 cells charged or discharged to various potentials. The Fe K-edge measurements include both the near edge (NEXAFS) and extended (EXAFS) regions. The former provides information on the effective Fe valence while the latter reveals the coordination geometry. Six cells charged and discharged at different conditions were examined. Group A consists of: fully discharged (d-1.1 V), almost fully discharged (d-1.25 V), and partially charged (c-1.85 V) cathodes and group B includes fully charged (c-2.25), almost fully charged (c-2.05 V), and partially discharged (d-1.65 V) cathodes. There appear to be only two distinct Fe environments, one for group A and a dramatically different one for group B. The two different main absorption edge peak shapes observed in groups A and B also reflect these distinct Fe environments. The extended fine structure for group A samples reveals an ordered environment dominated by metallic Fe while the higher Li content group B cathodes are characterized by disorder with only a single Fe–S interatomic distance. Spectral fitting to the experimental data of the d-1.65 cell (in group B) suggests that the compound Li 2 FeS 2 is present, with no evidence of FeS. Original cathode material utilization is estimated to be only around 2/3 of full capacity, based on the amount of residual FeS 2 required for satisfactory spectral fitting. On the other hand, no metallic Fe (within detection limits) remains in the recharged cells.

X-RAY ABSORPTION AND MAGNETIC RESONANCE SPECTROSCOPIC STUDIES OF LIXV6O13

Polycrystalline LixV6O13 samples, 0.5⩽x⩽6, were prepared by chemical intercalation in n-butyl lithium and investigated spectroscopically by x-ray absorption, electron paramagnetic resonance (EPR), and 7Li solid state nuclear magnetic resonance (NMR). Both the EPR results and the vanadium K-edge x-ray absorption fine structure spectra show that the average oxidation state of the vanadium decreases with the addition of Li, and the x-ray results provide evidence of lithium deficient and oxygen deficient impurity phases. The local symmetry of the vanadium atoms first decreases with increasing x from 0⩽x⩽1 and then increases with increasing x as the vanadium octahedral environment becomes less distorted. These changes are revealed by both the intensity of the first V-O peak in the radial distribution function and by the decrease in the x-ray absorption pre-edge peak intensity. However, structural correlations beyond the nearest neighbor atoms rapidly decrease with increasing Li content above x=1.5, reflecting ...

Lithium-7 nuclear magnetic resonance and Ti K-edge X-ray absorption spectroscopic investigation of electrochemical lithium insertion in Li4/3+xTi5/3O4

Abstract The spinel compound Li4/3+xTi5/3O4 is known to undergo reversible lithium intercalation up to x=1 with almost no change in lattice parameters, hence its designation as a “zero strain” intercalation compound. Structural changes that accompany electrochemical Li intercalation into this compound were studied by both 7 Li nuclear magnetic resonance (NMR) and Ti K-edge X-ray absorption fine structure (XAFS). The NMR results demonstrate that Li occupancies do not follow a simple distribution between two possible sites, one tetrahedral and one octahedral. The presence of at least one additional octahedral site is suggested. Line width measurements show that the Li+ ions do not return to their original distribution after cycling. XAFS results indicate the presence of modest static disorder in Tiue5f8O and Tiue5f8Ti distances above x=0.5. Both methods thus reveal subtle structural details previously unobserved by X-ray diffraction (XRD).

Lithium-7 NMR Studies Of Li(1-x)CoO2 Battery Cathodes.

Lithium-deficient cathode materials Li(1-x)CoO2, where x = 0.1, 0.4 and 0.6 were prepared electrochemically from the stoichiometric parent compound (x = 0.0). The materials were observed to be air-stable, and x-ray diffraction characterization yielded good agreement with the in situ studies of Dahn and co-workers, regarding changes in lattice parameters. In addition to both static and magic angle spinning (MAS) 7Li NMR, measurements, the samples were investigated by EPR and cobalt K-edge NEXAFS. The removal of Li is accompanied by compensating electrons from the Co d-orbitals, as evidenced by both shifts in the NEXAFS peak and the observation of EPR signals due to spins localized on the Co ions. These spins, in turn, result in dramatic 7Li chemical shifts (89 ppm for x = 0.6) and line broadening. Whereas MAS analysis of Li0.9CoO2 indicates two magnetically inequivalent Li sites, the spectra become too broad to resolve different sites for higher values of x. Finally NMR linewidth and spin-lattice relaxation measurements as a function of temperature suggest a modest increase in Li+ ion mobility for Li-deficient samples as compared to the parent compound.

5p-5d resonance in Gd

Abstract Photoemission measurements of Gd films in the photon energy range 16 to 45 eV indicate that a 5p - 5d intershell interaction results in an enhancement of the 5d emission above the 5p 3 2 and 5p 1 2 core level thresholds. This resonant behavior has been previously observed in Sm, Yb, and Lu. A constant kinetic energy feature that appears only above the 5p 3 2 threshold is tentatively identified as being due to autoionization following the creation of a 5p 3 2 hole with a locally collapsed 5d- screening orbital as an intermediate step.

X-ray Absorption Spectroscopy Investigation of the Sub-Nanoscale Strain in Thin-Film Lithium Ion Battery Cathodes

LiCoO2 and LiNiO2, two important cathode materials for Li-ion batteries, were studied in their respective bulk and thin-film form. X-ray absorption spectroscopy (XAS) has been used to probe the local atomic structure and structural defects in the thin-film and bulk cathodes. Results comparing Li(Co,Ni)O2 in the bulk and thin-film forms suggests a correlation between intrinsic stress and local strain in the thin-film. This local strain is manifested by a collapse of the six-fold rotational symmetry within the metal-metal layer of the Li(Co,Ni)O2 system into a two fold one. The relationship between annealing conditions and the resulting local strain in these films is examined.

X-ray absorption and NMR studies of LiNiCoO{sub 2} cathodes prepared by a particulate sol-gel process

The authors have synthesized lithiated nickel oxide-based cathode materials containing stoichiometric and excess lithium using a new low temperature colloidal particulate sol-gel process. The process yields a xerogel precursor that transforms to the crystalline oxide at 800 C in 2h. They studied the Li environment with nuclear magnetic resonance (NMR) and that of the transition metal ions with x-ray absorption spectroscopy. They measured samples of LiNi{sub 1{minus}x}Co{sub x}O{sub 2}, with x = 0 and 0.25. The effect on each composition of the incorporation of 5 mol % Li was also examined. The precursor material appears to have no Ni{sup 3+}, as indicated x-ray absorption measurements, and is highly disordered, showing little sign of interatomic correlations beyond the nearest neighbor in extended x-ray absorption fine structure (EXAFS) spectra. The {sup 7}Li NMR line widths and spin-lattice relaxation (T{sub 1}) behavior are dominated by strong interactions with the paramagnetic Ni{sup 3+}. The presence of 5% excess Li causes almost no change in NMR line width or T{sub 1} in the mixed (Ni/Co) cathode, but does produce an almost 30% reduction in line width for the pure LiNiO{sub 2}, implying that Co stabilizes the structure. The near-edge x-ray absorption measurements show the localmorexa0» Ni environment is relatively unaffected by Co substitution, a result confirmed by EXAFS analysis. The heat-treated samples are highly ordered, and both the near-edge and extended analysis imply Co substitutes primarily for Ni{sup 2+}, not Ni{sup 3+}. The Jahn-Teller distortion is apparent in both the stoichiometric and Li-excess materials.«xa0less