A series of compounds, LixFePO4 false(0⩽x⩽1false), 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 LixFePO4 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 PO43− anion are highly mixed with Li+ translatory vibrations. Consequently, no band in the IR spectrum of LiFePO4 may be assigned solely as a lithium ion “cage” mode. Spectroscopic measurements of the LixFePO4 series demonstrate that the intramolecular modes of PO43− are particularly sensitive to the extraction of Li+ ions from LiFePO4 and the accompanying oxidation of Fe2+ to Fe3+. Spectroscopic data suggest that the PO43− effective force constants (band frequencies), dipole moment derivatives (FTIR intensities), and polarizability derivatives (Raman intensities) change as LiFePO4 is converted into FePO4. Together, the data strongly support the two-phase shell model of LiFePO4 delithiation. © 2004 The Electrochemical Society. All rights reserved.

Ionic association in nonaqueous electrolytes containing LiPF6 was investigated with infrared absorption spectroscopy. The spectral intensity of the nondegenerate nu1 mode of the PF6- anion was found to be sensitive to ion pairing. Although the nu(1) mode of an isolated PF6- anion is only Raman active, coordination of Li+ to PF6- destroys the octahedral symmetry of the anion and results in nu1 becoming simultaneously IR and Raman active. When the dielectric constant of the solvent is increased, the IR-intensity of the nu1 band decreases because ion pairing is not favored in high dielectric solvents. Spectroscopic studies of solutions containing LiPF6 in diglyme show that ion pairing is also affected by specific cation-solvent interactions. The diglyme-containing solutions contain significantly fewer ion pairs than expected on the basis of the solvent's dielectric constant. It is thought that diglyme:LiPF6 solutions consist mostly of "spectroscopically free" PF6- anions because the sixfold coordination of Li+ by two diglyme molecules hinders Li+...PF6- ion pairing.

NMR spectra were collected for cross-linked poly(N-isopropylacrylamide), poly(NIPAM), hydrogels in the presence of NaCl and CaCl2 aqueous solutions. Intensity variations in the 1H NMR signals of the polymer provide insight into the phase transition process. These data were used to observe a two-stage phase transition process. Thermodynamic quantities were obtained from a van't Hoff analysis of the temperature-dependent equilibrium constants, which were derived from the NMR data. The Delta H degrees and Delta S degrees values for the hydrogel in D2O are 3.4 kJ/mol and 11.2 J/mol.K for stage I, which is attributed to the formation of hydrophobic bonds between neighboring isopropyl groups. The formation of hydrogen bonds during stage II yielded Delta H degrees and Delta S degrees values of 14.8 kJ/mol and 48.4 J/mol.K in D2O. However, the corresponding Delta H degrees values in 150 mM NaCl and 150 mM CaCl2 are reduced to 1.5 and 1.8 kJ/mol for stage I of the dehydration process. This corresponds to the known effect of salts on hydrophobic bond energetics. The value of Delta S degrees also decreased to 4.9 and 5.9 J/mol.K in NaCl and CaCl2 solutions, respectively. However, the thermodynamic values during stage II were only slightly affected by the salts. The lower temperatures required to induce spontaneous precipitation implies that Delta G degrees of precipitation is reduced. With our measurement of equilibrium thermodynamics, we see that 150 mM NaCl and CaCl2 solutions have a greater effect on hydrophobic bond formation associated with the phase transition process. In this manner, these salts aid in solvent reorganization necessary to form the hydrophobic bond, and this suggests that the formation of hydrophobic bonds is a strong determining factor in the stability of poly(NIPAM) hydrogels in water.

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Raman spectroscopy is an excellent technique for probing lithium intercalation reactions of many diverse lithium ion battery electrode materials. The technique is especially useful for probing LiFePO 4 -based cathodes because the intramolecular vibrational modes of the PO 4 3− anions yield intense bands in the Raman spectrum, which are sensitive to the presence of Li + ions. However, the high power lasers typically used in Raman spectroscopy can induce phase transitions in solid-state materials. These phase transitions may appear as changes in the spectroscopic data and could lead to erroneous conclusions concerning the delithiation mechanism of LiFePO 4 . Therefore, we examine the effect of exposing olivine FePO 4 to a range of power settings of a 532-nm laser. Laser power settings higher than 1.3 W/mm 2 are sufficient to destroy the FePO 4 crystal structure and result in the formation of disordered FePO 4 . After the laser is turned off, the amorphous FePO 4 compound crystallizes in the electrochemically inactive α-FePO 4 phase. The present experimental results strongly suggest that the power setting of the excitation laser should be carefully controlled when using Raman spectroscopy to characterize fundamental lithium ion intercalation processes of olivine materials. In addition, Raman spectra of the amorphous intermediate might provide insight into the α-FePO 4 to olivine FePO 4 phase transition that is known to occur at temperatures higher than 450 • C.