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.

The effect of 1,3-dioxolane (DOL) based electrolyte solutions (DOL/LiTFSI and DOL/LiTFSI-LiNO(3)) on the electrochemical performance and surface chemistry of silicon nanowire (SiNW) anodes was systematically investigated. SiNWs exhibited an exceptional electrochemical performance in DOL solutions in contrast to standard alkyl carbonate solutions (EC-DMC/LiPF(6)). Reduced irreversible capacity losses, enhanced and stable reversible capacities over prolonged cycling, and lower impedance were identified with DOL solutions. After 1000 charge-discharge cycles (at 60 °C and a 6 C rate), SiNWs in DOL/LiTFSI-LiNO(3) solution exhibited a reversible capacity of 1275 mAh/g, whereas only 575 and 20 mAh/g were identified in DOL/LiTFSI and EC-DMC solutions, respectively. Transmission electron microscopy (TEM) studies demonstrated the complete and uniform lithiation of SiNWs in DOL-based electrolyte solutions and incomplete, nonuniform lithiation in EC-DMC solutions. In addition, the formation of compact and uniform surface films on SiNWs cycled in DOL-based electrolyte solutions was identified by scanning electron microscopic (SEM) imaging, while the surface films formed in EC-DMC based solutions were thick and nonuniform. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy were employed to analyze the surface chemistry of SiNWs cycled in EC-DMC and DOL based electrolyte solutions. The distinctive surface chemistry of SiNWs cycled in DOL based electrolyte solutions was found to be responsible for their enhanced electrochemical performances.

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Electrochemically active thin films of Mg 2 Si in various film thicknesses of 30-380 nm have been prepared with the pulsed laser deposition technique. The thinnest film of 30 nm showed a highly stable cycling behavior at 0.1-1.0 V vs. Li, delivering capacity greater than 2000 mAh/g for more than 100 cycles. Though the film morphology became remarkably rougher with cycling, the films showed good stability. However, the first cycle irreversible capacity loss increased with film thickness. Therefore, lithium adsorption/desorption reaction forming Li-Si alloy at the Si-rich film surface is suggested as one of the sources of the large capacity of the 30 nm film. The superior capacity retention, when compared to porous electrodes of this alloy, may be attributed to a limited structural volume change in the two-dimensional film, shorter lithium diffusion path and enhanced conductivity from stainless steel substrate. The goals of this study are to promote the emerging need of thin film anodes for all solid-state microbatteries and clarify the capacity failure of powder intermetallic anodes for rechargeable lithium batteries.The search for anode materials to replace graphite in rechargeable lithium batteries has intensified over recent years due to concerns about safety during overcharge in the presence of organic electrolyte and a highly oxidizing cathode material such as LiCoO 2 . 1 Considerable effort has been devoted to searching for suitable alternative anodes among lithium binary alloys 2-4 and intermetallics 5-10 which operate a few hundred millivolts above metallic lithium. Magnesium silicide alloys have been studied for this application because Si has a good affinity for lithium. 11-16 Consensus regarding the electrochemical reaction between Li and Mg 2 Si has not been reached. Part of the reason may be due to the fact that Mg 2 Si is a semiconductor whose performance will be dependent on particle morphology and current density, and porous electrodes from alloyed powders show rapid capacity fade over the first ten cycles. It has been generally recognized that lithiation of the alloying metal elements takes place by significant volume change causing electrochemical-mechanical disintegration of particles during cycling. 5-15 Many attempts have been made to improve the electrochemical performance of the alloy anode materials by means of controlling the crystal structure to achieve a small lattice volume change and a particle morphology subject to a minimum mechanical stress, based on the study of capacity failure mechanisms of those alloy anodes. 17-20 Much can be learned regarding the performance of low conductivity materials when they are in thin film form. 16,[21][22][23]

The reaction mechanism for electrochemical lithium insertion into magnesium silicide was studies by in situ and ex situ X-ray diffraction ͑XRD͒. Lithium intercalation during the initial stages of insertion was indicated by a slight shift toward lower angles in the diffraction patterns. Upon further insertion, Li 2 MgSi was formed. This was indicated by the appearance of magnesium peaks, the disappearance of Mg 2 Si peaks, and the appearance of Li 2 MgSi peaks in the diffraction patterns. Magnesium XRD peaks that accumulated during cycling indicated that the formation of Mg 2 Si during lithium removal was not entirely reversible. Li 2 MgSi was synthesized by mechanical alloying. XRD patterns and nuclear magnetic resonance spectra of the mechanically alloyed and electrochemically produced phases are compared.