Article pubs.acs.org/JPCC
Influence of Hydrofluoric Acid Formation on Lithium Ion Insertion in Nanostructured V2O5 Jing Wu,†,⊥ Nellymar Membreno,† Wen-Yueh Yu,‡ Jaclyn D. Wiggins-Camacho,†,# David W. Flaherty,‡,∇ C. Buddie Mullins,*,†,‡,§,∥ and Keith J. Stevenson*,†,§,∥ †
Department of Chemistry and Biochemistry (1 University Station A5300), ‡Department of Chemical Engineering (1 University Station C0400), §Center for Electrochemistry, and ∥Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *
ABSTRACT: Vanadium oxide (V2O5) is a multifaceted material possessing desirable redox properties, including accessibility to multiple valence states, which make it attractive as a cathode for lithium ion batteries and microbatteries. Studies show that performance of this electrode material is dependent on the electrolyte employed and that solid electrolyte interphase (SEI) layer formation is responsible for the fade in capacity with multiple cycling. Nanostructured V2O5 thin films synthesized through reactive ballistic deposition (RBD) were studied with electrochemical methods, ex situ Raman and ex situ XPS in two widely used electrolytes: LiClO4/propylene carbonate (PC) and LiPF6/ diethyl carbonate (DEC) + ethylene carbonate (EC). Films cycled in LiPF6/DEC+EC experienced a 32% greater capacity fade between the first and second lithiathion/delithiation cycles than those cycled in LiClO4/PC, due to a redox-induced change in the surface morphology and composition and an irreversible transformation into an amorphous state as monitored by ex situ Raman. From X-ray photoelectron spectroscopy (XPS), it was shown that V2O5 cycled in LiPF6/DEC+EC contained a high atomic concentration percentage of fluoride (16.18%) in comparison with V2O5 electrodes cycled in LiClO4/PC (3.94%). No significant amounts of carbonates, oxalates, or oxyfluorophosphates typically associated with SEI formation were found when V2O5 was cycled in either electrolyte. The results obtained suggest instead that HF, formed upon water contamination of the electrolyte, reacts with V2O5 through a self-catalyzed process both at open circuit and under applied potential. The formation of vanadium oxyfluorides causes active mass loss and severe capacity fade upon discharging/charging. prepared by atomic layer deposition (ALD),15 chemical vapor deposition (CVD),16 physical vapor deposition (PVD),17,18 and other similar deposition techniques has also been investigated for microbattery applications. In this work, glancing angle reactive ballistic deposition (RBD) was used to prepare nanostructured V2O5 films of high surface area. This method, which is a variant of PVD, is based on a “hit-and-stick” or ballistic deposition growth process.19 During glancing angle ballistic deposition, the substrate is positioned at an angle toward the vaporization source. Initially, the deposition is stochastic and produces a random deposition of elevated points. However, at oblique deposition angles, the topographically elevated points, created randomly during the initial deposition, preferentially intercept the incoming flux while shadowing lower regions from incoming atoms. This selfshadowing growth process results in porous, columnar films, which are ideal for thin film lithium ion batteries.19−22 RBD takes this deposition process further by allowing control of the
1. INTRODUCTION V2O5 is a widely studied material offering a rich literature that dates back to over a century ago. Interest in this material can be partly attributed to the high valency of vanadium, which offers versatile redox state-dependent properties, finding use in a broad range of technologies including catalysis, electrochromism, optical switching devices, and as electrodes for lithium ion batteries.1 In the most recent decades, extensive studies have been conducted on V2O5 as a cathode material for lithium ion batteries due to its already mentioned high valence state as well as its layered structure that offers a relatively high theoretical gravimetric capacity of 295 mAh/g upon the reversible intercalation/deintercalation of Li+.1 V2O5 ambigels have shown some of the most remarkable gravimetric capacities upward of 600 mAh/g.1 However, moderate electrical conductivity2,3and Li+ diffusion4,5 in the lattice limit its energy capacity as well as the discharging and charging rates. To overcome these disadvantages, nanostructures of V2O5 have been developed to provide a large surface area and short Li+ diffusion path including nanowires,6−8 nanosheets,9 nanobelts,10,11 nanobeams,12 and nanotubes.13,14 Thin film V2O5 © 2012 American Chemical Society
Received: June 16, 2012 Revised: September 12, 2012 Published: September 12, 2012 21208
dx.doi.org/10.1021/jp305937b | J. Phys. Chem. C 2012, 116, 21208−21215
The Journal of Physical Chemistry C
Article
V2O5 has been shown to produce oxyfluorides and in the presence of water is a self-catalyzed process.29−33 Our aim in this work was to elucidate the effects of HF and its direct reaction with V2O5 with specific interest in studying the amplified destructive effects of this reaction in nanostructured V2O5 made through RBD because its porosity presents greater contact between the electrode and electrolyte.
material porosity, surface area, and composition. This is achieved by having the metal component deposited in a lowpressure environment of a nondirectional reactive gas. Materials such as TiO2,20,24 titanium carbide (TiC),21,22 and α-Fe2O323 have been made through RBD as electrodes for photoelectrochemical water oxidation and electrochemical energy storage. As lithium ion battery electrodes, these porous materials have multiple desirable features, which include increased electrode/electrolyte contact areas, thinner ion and electron conducting interfacial regions, and shorter Li+ diffusion paths that allow for better rates of ion-coupled electron transfer.19 Additionally, the nanosized RBD materials can better accommodate volume changes and lattice stresses that are induced by phase transitions during intercalation and deintercalation, as occurs in V2O5.19 Although nanostructured V2O5 (such as the RBD V2O5 film presented in this work) can improve the electrical properties for these cathodes, the material inherently presents the issue of thermodynamic instability due to increased surface energy associated with scaling down of particle size and increased surface-to-volume ratio. As a result, it is of utmost importance to understand and control the surface chemistry and electrode/ electrolyte interactions between nanostructured cathodes and anodes. For instance, even though the operating window for V2O5 falls within the electrochemical window of most electrolytes,25 a drastic change in the surface morphology, effective energy density, and cyclability of the cathode after discharging/charging has been reported by several groups.18,26−28 Cohen et al. investigated thin film V2O5 in LiPF6 and LiClO4 electrolytes, discovering through atomic force microscopy (AFM) that the PF6− anion was responsible for unique surface chemistry on the film. Deposits on the films, mainly thought to be LiF, were attributed to decomposition of ́ and others LiPF6 as seen on anodes.27 Swiatowska-Mrowiecka combined AFM and X-ray photoelectron spectroscopy (XPS) studies to investigate the SEI layer formation on V2O5 in LiClO4/PC. C and O core level XPS studies conducted after the intercalation and deintercalation steps showed that an irreversible surface contamination layer formed consisting of lithium carbonates, lithium alkyl carbonates, and lithium alkoxides. 28 They concluded from the XPS that the decomposition of propylene carbonate was responsible for the SEI layer formation, which partly contributed to surface exfoliation and amorphization of the material after multicycling.28 Most recently, Fleutot et al. performed similar AFM and XPS studies on thin film V2O5 with LiPF6 in a mixture of organic solvents as the electrolye.18 Fleutot and colleagues used the larger potential window of 3.7−1.5 V vs Li/Li+ with the intention of exacerbating the SEI layer phenomenon, noting the formation of mostly lithium carbonates. Their main finding was that, although dissolution of lithium carbonate occurred, a thin film of other SEI products that included lithium alkoxides, polyethylene oxide, and oxalates was left behind, explaining capacity loss during the first cycle and throughout subsequent cycles.18 The studies mentioned above emphasized the SEI layer formation on the V2O5 cathode as the primary cause for changes in its suface morphology, crystallinity, and capacity. These effects on the electrode were found to be more severe when using LiPF6 over LiClO4 as the supporting electrolyte, yet none of the above-mentioned studies considered the formation of HF upon water contamination of LiPF6 as a possible cause for active material loss and amorphization. Reaction of HF and
2. EXPERIMENTAL METHODS 2.1. V2O5 Film Preparation. Vanadium oxide (VOx) films were prepared using the RBD scheme described elsewhere.19−24 VOx films were deposited onto a 1 cm × 1 cm stainless steel foil (25 μg/cm2 in mass loading) from a vanadium rod (ESPI Metals) using an electron-beam evaporator (EMF 3s, Omicron) in an O2 atmosphere (99.98%, Matheson) under a pressure of 10−6 Torr. The deposition rate when the substrate was held at normal incidence was approximately 0.02 μg/cm2·s as determined by a quartz crystal microbalance (QCM, Maxtek). Prior to deposition, the substrate was cooled to −196 °C by liquid nitrogen and the films were deposited with a glancing angle of 80°. Following deposition, the films were annealed in air at 240 °C for 3 h. 2.2. Electrochemical Measurements. Galvanostatic experiments were performed in an Ar-filled glovebox (MBraun) at room temperature, using a three-electrode system connected to a CHI 660 workstation. RBD V2O5 thin films served as the working electrode, while lithium foil (Sigma-Aldrich) was used as both the counter and the reference electrode. The current density applied was 0.243 A/g. The LiClO4 in PC electrolyte was prepared by vacuum drying LiClO4 (Sigma-Aldrich) at 180 °C for a 48 h period. Separately, PC (Acros) was dried over activated molecular sieves (3 Å) for more than 3 days. The two were then mixed to form a 1 M LiClO4/PC solution. The 1 M LiPF6 in 1:1 DEC and EC electrolyte was obtained from Novolyte. 2.3. Raman, XPS, SEM, and HRTEM Characterization. Ex situ Raman spectroscopy was performed with a Renishaw In Via microscope system using the backscattering configuration. A 514.5 nm Ar laser was used as the excitation source, maintained at
