Roger Frech

Raman and FTIR Spectroscopic Study of Li x FePO4 ( 0 ⩽ x ⩽ 1 )

A series of compounds, Li x FePO 4 (0 ≤ x ≤ 1), was prepared by chemical delithiation and investigated with Fourier transform infrared (FTIR) and Raman spectroscopy for the first time. Sample homogeneity with respect to Fe and P was analyzed with an electron microprobe. The imaging capabilities of the electron microprobe reveal larger-sized particles (10-20 μm diam) and fine intergranular particles (≤1 μm). Elemental analysis with the electron microprobe demonstrates that the Li x FePO 4 particles are relatively homogeneous. Thus, the electron microprobe may be used in conjunction with other instruments that are more commonly used to investigate particle morphology, such as scanning or transmission electron microscopes. Isotopic substitution experiments show that the symmetric and antisymmetric bending vibrations of the PO 3- 4 anion are highly mixed with Li + translatory vibrations. Consequently, no band in the IR spectrum of LiFePO 4 may be assigned solely as a lithium ion cage mode. Spectroscopic measurements of the Li x FePO 4 series demonstrate that the intramolecular modes of PO 3- 4 are particularly sensitive to the extraction of Li + ions from LiFePO 4 and the accompanying oxidation of Fe 2+ to Fe 3+ . Spectroscopic data suggest that the PO 3- 4 effective force constants (band frequencies), dipole moment derivatives (FTIR intensities), and polarizability derivatives (Raman intensities) change as LiFePO 4 is converted into FePO 4 , Together, the data strongly support the two-phase shell model of LiFePO 4 delithiation.

Vibrational spectroscopic and electrochemical studies of the low and high temperature phases of LiCo1−x MxO2 (M = Ni or Ti)

A spinel structure with the space group Fd3m is unambiguously assigned to LiCoO2 synthesized at 400 °C, according to Raman and infra-red spectra of LT-LiCoO2 and factor group analysis of the vibrational modes for the layered (R3m) and the spinel (Fd3m) structure models. The vibrational modes involving significant motion of lithium atoms are observed at 536 and 271 cm−1 for HT-LiCoO2, and at 451 and 267 cm−1 for LT-LiCoO2 in the infra-red spectra. Although solid solutions could be prepared for HT-LiCo1−xMxO2 (M = Ni, O < x < 1; M = Ti, x < 0.2) and LT-LiCo1−xNixO2 (x < 0.2) some impurities were observed in LT-LiCo1−xTixO2. The nickel or titanium-doped LT-LiCo1−x MxO2 (M = Ni and Ti, x < 0.2) cathodes exhibit smaller capacities than does their parent compound LT-LiCoO2.

Coordination and conformation in PEO, PEGM and PEG systems containing lithium or lanthanum triflate

The effect of chain length, cation type and polymer end group on the coordination and conformation in polymer electrolytes has been studied using high molecular weight poly(ethylene oxide) (PEO), poly(ethylene glycol) (PEG(400)) and poly(ethylene glycol)dimethyl ether (PEGM(400)) as polymers, with LiCF3SO3 and La(CF3SO3)3 as salts. The experimental technique used has been Fourier transform infra-red spectroscopy. A coordination of nine to ten ether oxygens to the lanthanum ions has been found for the PEGM system by increasing the concentration of lanthanum triflate in the polymers and following the disappearance of the COC stretching vibration of non-coordinated ether oxygens. The same coordination seems to be present also for the PEG and PEO systems. Conformational changes have been observed on addition of salt to the polymer, by following the disappearance of an absorption band at about 990 cm−1, indicative of a trans conformation around the CH2CH2 bond. For all three polymer types the gauche conformation is dominant at high salt concentrations. The OH end groups present in the PEG samples are important for the coordination in this system and both the cations and the triflate anions are coordinated by the OH groups to some extent.

Effect of propylene carbonate as a plasticizer in high molecular weight PEOLiCF3SO3 electrolytes

The effect of propylene carbonate on the PEO-LiCF 3 SO 3 system was studied using infrared spectroscopy and ionic conductivity measurements. It has been observed that large amounts of propylene carbonate tend to decrease the degree of ionic association in polymer electrolytes, while the ionic conductivity increases by several orders of magnitude. Propylene carbonate interacts preferentially with the crystalline PEO phase relative to the (PEO) 3 LiCF 3 SO 3 compound present in the PEO-LiCF 3 SO 3 salt complexes.

Effect of plasticizers on high molecular weight PEO-LiCF3SO3 complexes

Abstract An infrared spectroscopic study on the effect of plasticizers such as ethylene carbonate and propylene carbonate on the PEO-LiCF 3 SO 3 system is reported. It has been observed that the additives seem to favor less associated ionic species when present in large amounts. A few differences between ethylene carbonate and propylene carbonate as plasticizers have been noted and discussed. The ionic conductivity of the plasticized polymer complexes are about 3–4 magnitudes higher than that of (PEO) 9 LiCF 3 SO 3 . The plasticizers seem to preferentially interact with the crystalline PEO phase, thereby rendering the electrolytes amorphous.

Mesoporous tin oxides as lithium intercalation anode materials

Mesoporous tin oxides have been prepared by a surfactant templating method. Characterization with transmission electron microscopy techniques showed that the compounds formed as hexagonal mesoporous phases, retaining their mesostructural nature with a pore size of about 4 nm after calcination at 350 °C. These materials showed good electrochemical performance as anode materials for lithium ion batteries, with over 400 mAh/g charge capacity and good reversibility.

Dependence of ionic association on polymer chain length in poly(ethylene oxide)-lithium triflate complexes

Abstract Ionic association in complexes of lithium trifluoromethanesulfonate with low-molecular-weight poly(ethylene oxide) dimethyl ethers was studied using Raman and infra-red vibrational spectroscopy. The relative concentrations of ion pairs and more highly associated ionic species are quite dependent on chain length. This dependence was measured in both the CF 3 symmetric deformation mode and the SO 3 symmetric stretching mode. The effect of the terminating group was considered by comparing the dimethyl ethers with hydroxy-terminated poly(ethylene oxide) at comparable chain lengths. Hydrogen bonding effects in the hydroxy-terminated poly(ethylene oxide) compounds were also found to depend on chain length.

Plasticizer interactions with polymer and salt in propylene carbonate-poly(acrylonitrile)-lithium triflate

Abstract A comparative infrared spectroscopic study has been undertaken in a system of poly(acrylonitrile)-propylene carbonate-lithium triflate to examine the plasticizer interactions with the polymer and salt as well as ionic association. Lithium triflate is highly associated and the lithium ion interaction is stronger with propylene carbonate (PC) than with poly(acrylonitrile) (PAN). It is hypothesized that the local structure about the lithium ion is characterized by coordination with the three oxygen atoms of the PC molecule and a triflate anion oxygen, with a weak interaction with the CN group of PAN.

In Situ Roman Studies of Graphite Surface Structures during Lithium Electrochemical Intercalation

Surface structural changes were investigated using in situ Raman microspectroscopy during lithium intercalation into Lonza KS-44 and KS-6 graphite materials in LiClO 4 solutions of ethylene carbonate (EC)/dimethyl carbonate (DMC) or EC/1,2-dimethoxyethane (DME). In the 1 M LiClO 4 EC/DMC solution, the Raman spectral changes indicate that graphite undergoes a series of transitions during the electrochemical lithium intercalation process from initial formation of the dilute stage 1 GIC (graphite intercalation compound) to a stage 4 GIC, then through a stage 3 to stage 2 and finally to a stage 1 GIC. The reversibility of the Raman spectral changes indicates that the top layers of the graphite are not damaged during the first charge and discharge cycle. Intercalation-induced strain causes the E 2g2 (i) mode of the unbound graphite layers to shift toward lower frequency in the stage 4 and stage 3 GICs. In the 1 M LiCiO 4 EC/DME solution, the Raman spectra changed dramatically in the high-potential range (0.9-0.5 V) during the discharge and charge processes, indicating that the graphite surface structure is altered even at the high potentials where major lithium intercalation is not expected to occur. Irreversible spectral changes in the E 2g2 band shoulders suggest that the surface structural changes are also irreversible. Raman spectral changes of the E 2g2 band in the potential range 0.9-0.5 V are attributed to extensive surface graphite exfoliation caused by solvent cointercalation with lithium ions. This is in accordance with the large irreversible capacity consumption in the potential range 0.9-0.5 V observed during the first discharge process. In the low-potential range, the Raman spectral changes of the graphite in LiClO 4 EC/DME solutions are similar to those in LiClO 4 EC/DMC solution.

The structure of associated ionic species in poly(propylene oxide)-alkali metal trifluoromethanesulphonate complexes

Abstract This paper presents the results of Raman scattering and i.r. transmission investigations of poly(propylene oxide) ( M w = ∼ 3000 ) complexed with various concentrations of lithium, sodium and potassium trifluoromethanesulphonate. Analysis of the i.r. and polarized Raman spectra of the SO3 stretching region, in conjunction with complementary data from other spectral regions, suggests the existence of three associated ionic species: two differently co-ordinated ion pairs and a triple ion. It is argued that the free ions which were previously postulated to exist in these polymer-salt complexes are really cation-anion pairs weakly interacting through the CF3 end of the anion.