The effect of FEC as a co-solvent on the electrochemical performance and surface chemistry of silicon nanowire (SiNW) anodes was thoroughly investigated. Enhanced electrochemical performance was observed for SiNW anodes in alkyl carbonates electrolyte solutions containing fluoroethylene carbonate (FEC). Reduced irreversible capacity losses accompanied by enhanced and stable reversible capacities over prolonged cycling were achieved with FEC-containing electrolyte solutions. TEM studies provided evidence for the complete and incomplete lithiation of SiNW's in FEC-containing and FEC-free electrolyte solutions, respectively. Scanning electron microscopy (SEM) results proved the formation of much thinner and compact surface films on SiNW's in FEC-containing solutions. However, thicker surface films were identified for SiNW electrodes cycled in FEC-free solutions. SiNW electrodes develop lower impedance in electrolyte solutions containing FEC in contrast to standard (FEC-free) solutions. The surface chemistry of SiNW electrodes cycled in FEC-modified and standard electrolytes were investigated using X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy. The impact of FEC as a co-solvent on the electrochemical behavior of SiNW electrodes is discussed herein in light of the spectroscopic and microscopic studies.
We report herein on a rigorous analysis of unique electrolyte solutions for novel rechargeable magnesium
batteries and nonaqueous magnesium electrochemistry, which contain organometallic complex electrolyte
species, by Raman spectroscopy. These solutions comprise ethereal solvents and products of reactions
between R2Mg Lewis base species and AlCl2R Lewis acid species that exist in solution in dynamic
multiple equilibria. The reactions involve the exchange of ligands between the magnesium and the
aluminum to form ions such as MgCl+, Mg2Cl3
+, and AlCl4-
n
R-
n
(n ≤ 4), stabilized by the ether molecules.
The Raman peak assignments were based on a rigorous study of solutions containing reference compounds
and some quantum-mechanical calculations. Raman spectroscopy enabled a quantitative analysis of the
various species in solution.
This study examined the aging mechanisms of layered cathode materials for lithium batteries upon exposure to air and the influence of this aging on the thermal stability and electrochemical performance of these materials composed of solid solutions of
LiMO2
(
M=[MnNi]
or [MnNiCo]) in Li cells. A unique methodology for the quantitative characterization of surface carbonates on
LiMO2
compounds based on differential scanning calorimeter (DSC) measurements was developed. Correlations were made between the formation of
Li2CO3
and other carbonates on the surface of the lithiated metal oxide powders and the changes in the structure and electrochemical performance of the cathode materials. The techniques used included solid-state NMR, X-ray photelectron spectroscopy, Fourier transform IR, high resolution scanning electron microscopy, high resolution transmission electron microscopy and the thermal analysis, DSC, and accelerating rate calorimetry in conjunction with electrochemical measurements.
The formation, degree of crystallinity, and adherence of dense titania (TiO 2 ) thin film coatings on a high-temperature polyimide resin (PMR-15) can be influenced by the chemical composition of the polymer surface. Furthermore, solution deposition conditions can be adjusted to provide additional control over the morphology and crystallinity of the titania films. Recipes for solution-based titania deposition that used a slowly hydrolyzing titanium fluoride salt in the presence of boric acid as a fluoride scavenger allowed growth of films up to 750 nm thick in 22 h. By adjusting solution pH and temperature, either amorphous titania or oriented crystalline anatase films could be formed. Surface sulfonate groups enhance the adhesion of solution-deposited oxide thin film coatings. While most sulfonation procedures severely damaged the PMR-15 surface, the use of chlorosulfonic acid followed by hydrolysis of the installed chlorosulfonyl groups provided effective surface sulfonation without significant surface damage. In some cases, the oxide deposition solution caused partial hydrolysis of the polymer surface, which itself was sufficient to allow adhesion of the titania film through chelation of titanium ions by exposed benzoic acid groups on the polymer surface.
In this paper, we report on attempts to use differential scanning calorimetric measurements of aged Li electrodes for the study of the kinetics of the growth of surface films on the active metal. Standard, commonly used alkyl carbonate solutions such as ethylene and di-methyl carbonates with LiPF6 were explored. Heating Li samples in solutions after aging by DSC, resulted in well-resolved curves of reaction heats vs temperature. Exothermic reactions occurring at temperatures below 150 degrees C could be attributed to changes related to the surface films and their heat evolved, increased as a function of storage time, and hence these heats represent the thickness of the surface films that grow upon storage. Scanning electron microscopy of the Li surfaces as a function of storage and heating to different temperatures confirmed that the thermal reactions of Li surfaces in these solutions up to 150 degrees C relate to the surface films only. XPS studies revealed that these processes of the surface films change the metastable organic Li salts to more stable inorganic compounds such as LiF and Li2O. Massive red-ox reactions, between the salt anion and the solvents and between the solution species and the active metal, occur at temperatures higher than 150 degrees C. The kinetics of growth of the surface films on Li show an inverse logarithmic behavior, expected for thin surface films with which the rate-limiting step of their growth depends on ions transport across the film.
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