Article pubs.acs.org/Langmuir
Exceptional Electrochemical Performance of Si-Nanowires in 1,3Dioxolane 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-LiNO3) 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/ LiPF6). 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-LiNO3 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 DOLbased 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 intercalation anodes.16,17 Silicon has attracted significant interest due to its excellent theoretical capacity (Li3.75Si, 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 © 2012 American Chemical Society
particles, pillars, wires, and rods have been also reported by previous researchers.16,17,20−30 However, the critical disadvantage 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 6175
dx.doi.org/10.1021/la300306v | Langmuir 2012, 28, 6175−6184
Langmuir
Article
for reversible cycling and long-term stability.38,39 Many polar aprotic solvents and electrolyte additives were successfully employed for enhancing the electrochemical performance and surface chemistry of graphite anodes for Li-ion batteries.35,40,41 Some compositions were found to be useful for reducing the irreversible capacity loss and improving the cycle life by surface film modification. From the relevant literature reports, it is clear that the nature of the surface films formed on electrodes plays an important role in the electrochemical performance, longterm stability and safety of Li-ion batteries. Among the various polar aprotic solvents studied, 1,3dioxolane (DOL) has been considered as one of the most promising solvent for secondary Li-batteries, even with a salt such as LiClO4.42−45 This was mainly due to the very high conductivity and cycling efficiency of Li-electrodes in DOL. A rechargeable Li (metal) battery system containing DOL solutions was even commercialized during the mid-nineties.46 The cycling efficiency of lithium electrodes and Li-TiS2 cells with DOL-based electrolytes was among the highest measured, as compared to other polar aprotic solvents.39 It has been reported that DOL pretreatment increases the interfacial characteristics of Li-anodes due to the formation of stable passivating surface films.47 The use of DOL as a cosolvent has been reported for enhancing the electrochemical performance of Li−S batteries and double-layer capacitors.48−51 In addition to this, polymer electrolytes comprising poly(1,3-dioxolane) complexes of LiClO4, LiAsF6, and LiTFSI were reported in the literature.52−54 Since the application of SiNWs as Li-ion battery anodes started only recently, little attention has been directed toward understanding the effect of DOL solutions on the electrochemical performance of SiNWs. Some researchers reported that surface films are not formed on SiNW electrodes during charge−discharge processes.55 Nevertheless, we should expect surface film formation on Si electrodes in all polar aprotic Lisalt solutions due to the fact that these electrodes operate at potentials where the organic electrolyte solutions are not thermodynamically stable.56 In addition, higher reduction rates of polar aprotic electrolyte solutions should be expected for high surface area materials such as SiNWs.32 Although DOL was addressed as an important component for several Li-battery systems, the mechanism by which the presence of DOL affects their electrochemical performance requires further investigation. To the best of our knowledge, there are no systematic studies reported on the performance and surface chemistry of SiNW electrodes in DOL based electrolyte solutions. The work reported herein explores the effect of DOL based electrolyte solutions (DOL/LiTFSI and DOL/LiTFSI-LiNO3) on the electrochemical performance, morphology, and surface chemistry of SiNW anodes. Measurements were carried out at 30 and 60 °C. However, for the sake of brevity, and in order to emphasize performance level and the impact of the electrolyte solutions, we present herein the results obtained at 60 °C. It should be noted that testing at elevated temperatures is mandatory for any evaluation of Li-ion battery materials.
the counter electrode. Standard alkyl carbonate electrolyte solutions, EC-DMC (1:1)/LiPF6 (1 M) (Ube Industries, Japan) were used as received. 1,3-Dioxolane (Merck) was mixed separately with LiTFSI (Merck) and LiNO3 (Sigma) to form DOL/LiTFSI (1 M) and DOL/ LiTFSI (1 M) -LiNO3 (0.1M) electrolyte solutions. The H2O and HF contents in these electrolyte solutions were not more than 10 and 30 ppm, respectively. LITFSI was selected as the Li salt for DOL solutions due to its high solubility without causing DOL polymerization and its superior thermal stability in contrast to other Li salts. Conductivities of different electrolyte solutions (at 30 and 60 °C) employed in this study is provided in Table S1 in the Supporting Information. The glovebox (VAC Inc.) used for preparing the electrolyte solutions and electrochemical cells was filled with highpurity argon (99.9995%). In addition to this, the glove boxes were equipped with O2 and H2O absorbers to reduce the oxygen and moisture content to the ppm level. Galvanostatic charge−discharge cycling tests of the Li−Si half cells were performed at a 6 C rate in the potential range of 0.05−1.0 V (vs Li/Li+) using a Maccor 2000 multichannel battery tester. The decision to work in such high charging-discharging rates was made in order to investigate the performance of SiNWs in DOL based electrolyte solutions under extreme conditions. The capacity of these SiNW electrodes was limited to 2200 mAh/g during galvanostatic cycling. A battery test unit model 1470, coupled with a Solartron 1255 frequency response analyzer (driven by Corrware and Zplot software from Scribner Associates, Inc.), was used for the cyclic voltammetry and impedance spectroscopy studies (in the 50 kHz to 50 mHz range). A Technai G2 TEM system from FEI Inc. working at an accelerating voltage of 200 KV was used for transmission electron microscopy (TEM) measurements. An Inspect FEI microscope working at an accelerating voltage of 15 KV and magnification of 10 000× was employed for the scanning electron microscopy (SEM) studies of pristine and cycled SiNW electrodes. A Kratos Axis HS spectrometer (England) equipped with an Al Kα X-ray radiation source (photon energy 1486.6 eV) was used for the X-ray photoelectron spectroscopic (XPS) analysis of cycled SiNWs. These highly sensitive SiNW electrodes were transferred from the highly pure argon atmosphere of the glovebox to the XPS system using a homemade transfer system fitted with a gate valve and a magnetic manipulator. The binding energies of all elements present were determined by setting the CC/CH component of the C 1s peak at 284.5 eV. After the removal of a nonlinear Shirley background, highresolution, core level spectra were used for quantitative surface chemical analysis. Vision software (Kratos) was used for deconvoluting the XPS spectra into Gaussian/Lorenzian-shaped components. A Nicolet 6700 FTIR spectrometer placed in a glovebox (Ravona Inc., Israel) under moisture and CO2-free air (purified by Balston Inc. Air purifier) was used for characterizing the surface films formed on cycled electrodes. The FTIR spectra (in the range of 4000−400 cm−1) of these electrodes were recorded with a diffuse reflectance accessory from Pike Technologies.
3. RESULTS AND DISCUSSION 3.1. Electrochemical and Microscopic Analysis. Chrono-potentiometry and cyclic voltammetry (vs Li counter and reference electrodes) techniques were employed for analyzing the basic electrochemical behavior of SiNW electrodes in three electrolyte solutions, EC-DMC/LiPF6, DOL/LiTFSI, and DOL/LiTFSI-LiNO3. The voltage profiles corresponding to the first discharge (first Li-intercalation of SiNWs) of these half-cells containing DOL and alkyl carbonate based electrolyte solutions possess almost similar shape (Figure 1). This reflects Li−Si alloying and surface film formation due to the decomposition of electrolyte solutions. The discharge profile indicates a very fast potential drop to the low potentials, where Li−Si alloying occurs (
