Exceptional Electrochemical Performance of Si-Nanowires in 1,3- Dioxolane Solutions: A Surface Chemical Investigation Vinodkumar Etacheri, † Uzi Geiger, † Yossi Gofer, † Gregory A. Roberts, ‡ Ionel C. Stefan, ‡ Rainier Fasching, ‡ and Doron Aurbach* ,† † Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel ‡ Amprius, Inc., 1430 O’Brien Drive, Suite C, Menlo Park, California 94025, United States * S Supporting Information ABSTRACT: 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. 1. INTRODUCTION Rechargeable Li-ion batteries have attracted significant interest due to their wide range of applications in portable electronic devices, implantable medical devices, electric vehicles, and so forth. 1−3 Although Li-ion batteries have been commercialized, the improvement of their specific charge capacities and energy density is one of the major challenges in battery research. 2 Graphite was selected as the main substitute for Li-metal anode material in rechargeable Li-ion batteries due to the reversible intercalation of Li-ions without significantly changing its morphology and volume. The main drawback of graphite anodes is the relatively low specific capacity (372 mAh/g compared to 3800 mAh/g for Li metal), which limits the capacity of Li-ion batteries. 4,5 Therefore, replacing the anode material is essential for improving the performance of Li-ion batteries. As a result, intensive work has been devoted in recent years to the development of high capacity anode materials. 6−15 Recently, silicon (Si) has been reported as an important Li-M alloying anode material for replacing graphite-based intercala- tion anodes. 16,17 Silicon has attracted significant interest due to its excellent theoretical capacity (Li 3.75 Si, 4000 mAh/g). 18,19 Moreover, silicon is the second most abundant element on earth, which makes it attractive for commercial battery applications. 5 Nanostructured Si anodes composed of spherical particles, pillars, wires, and rods have been also reported by previous researchers. 16,17,20−30 However, the critical disadvant- age of Si is its lack of capacity retention under charge− discharge (lithiation−delithiation) cycling due to disintegration of the active mass caused by large volume change (∼300%). 3,14,16,31 This huge volume change of Si also results in the pulverization of surface films during the charge− discharge process. As a result, it is very difficult to develop stable passivating surface films on Si anodes. 3 Both the solvent and salt of the electrolyte solution undergo reduction on the anode, which operates at low potentials close to metallic lithium. 32,33 This results in the formation of surface films on the anode surface that work as a solid electrolyte interphase (SEI). 32 These surface films passivates the anode surface and prevents further decomposition of the electrolyte solution. 34 The surface film formation mechanism and its composition and effect on the electrochemical properties of graphite anodes have been systematically investigated by previous researchers. 34−37 It was found that surface film formation on graphite anodes is a critical factor responsible Received: January 20, 2012 Revised: March 3, 2012 Published: March 19, 2012 Article pubs.acs.org/Langmuir © 2012 American Chemical Society 6175 dx.doi.org/10.1021/la300306v | Langmuir 2012, 28, 6175−6184