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A systematic study of LiFePO 4 with cyclic voltammetry ͑CV͒ was conducted using thin electrodes with a loading of 4 mg/cm 2 . Peak current of the CV profile was proportional to the square root of scan rate under 0.2 mV/s. Results were analyzed using a reversible reaction model with a resistive behavior. This resistance was consistent with other resistances obtained from electrochemical impedance spectroscopy and charge-discharge curves. Apparent Li diffusion constants of 2.2 ϫ 10 −14 and 1.4 ϫ 10 −14 cm 2 /s were obtained at 25°C for charging and discharging LiFePO 4 electrodes in 1 M LiPF 6 ethylene carbonate/diethyl carbonateϭ3:7 by volume, respectively. Activation energies of the apparent diffusion constants and electrode resistance are about 0.4 eV. These parameters are good indicators for assessing the effectiveness of material modifications such as surface coating and doping.

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Nitrogen-containing carbons N~C~_z have been made from different precursors at temperatures between 850 and I050~ Their composition and structure have been studied by chemical analysis, powder x-ray diffraction, x-ray absorption spectroscopy, and Auger electron spectroscopy. These techniques show that some nitrogen has been incorporated substitutionally for carbon. Nitrogen affects the behavior of Li/Li(NzC~ ~)8 electrochemical cells in two ways. First, the irreversible capacity observed during the first electrochemical reaction of Li with NzC~_~ (during the first discharge) increases with the nitrogen content of the samples. Second, the incorporated nitrogen causes a shift of the cell capacity to lower voltages compared to pure carbon electrodes. The first effect can be understood qualitatively using a model where Li reacts irreversibly with nitrogen-containing species (we call these nitrogen atoms chemical nitrogen) to form lithium-nitrogen-organic compounds. The second effect is caused by the nitrogen which has been substituted for carbon in the lattice (so-called lattice nitrogen). Such nitrogen-containing carbons are not considered useful as anodes for Li-ion cells.

Several porous silicon, siloxene (Si6H6O3), heat-treated siloxene, and layered polysilane (Si6H6) samples have been studied with K- and L-edge x-ray photoabsorption, photoemission, and powder x-ray diffraction. The x-ray absorption of layered polysilane and porous-Si are found to be remarkably similar. In particular, the K absorption edges of these samples shift by about 0.4–0.6 eV to higher energy relative to crystalline silicon. Siloxene samples heated to 400 °C in inert gas are best described as a mixture of SiO2 and amorphous-Si. When heat-treated siloxene is studied by photoelectron spectroscopy (surface sensitive) it resembles SiO2, when it is studied by x-ray absorption (bulk and surface) features from both SiO2 and amorphous-Si are observed and when it is studied by x-ray diffraction (bulk measurement) it resembles amorphous-Si. The SiO2 is therefore predominantly at the surface and heat-treated siloxene is very small amorphous-Si particles coated with SiO2. The Si L edge of heat-treated siloxene is not shifted significantly with respect to crystalline Si, unlike that of porous-Si, as-prepared siloxene, or layered polysilane. Taken together, these results suggest that heat-treated siloxene does not resemble electrochemically prepared porous-Si but that it might resemble rapid thermal annealed porous-Si. On the other hand, we believe that layered polysilane and unheated porous-Si may be related.

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