Article pubs.acs.org/Langmuir
Phenomenological Transition of an Aluminum Surface in an Ionic Liquid and Its Beneficial Implementation in Batteries B. Shvartsev,† D. Gelman,†,‡ D. Amram,† and Y. Ein-Eli*,†,‡ †
Department of Materials Science and Engineering and ‡The Nancy and Stephen Grand Technion Energy Program, Technion - Israel Institute of Technology, Haifa, Israel 3200003 ABSTRACT: Aluminum (Al) electrochemical dissolution in organic nonaqueous media and room temperature ionic liquids (RTILs) is partially hampered by the presence of a native oxide. In this work, Al activation in EMIm(HF)2.3F RTIL is reported. It was confirmed that as a result of the interaction of Al with the RTIL, a new film is formed instead of the pristine oxide layer. Aluminum surface modifications result in a transformation from a passive state to the active behavior of the metal. This was confirmed via the employment of electrochemical methods and characterization by XPS, AFM, and TEM. It was shown that the pristine oxide surface film dissolves in EMIm(HF)2.3F, allowing an Al−O−F layer to be formed instead. This newly built up layer dramatically restricts Al corrosion while enabling high rates of Al anodic dissolution. These beneficial features allow the implementation of Al as an anode in advanced portable power sources, such as aluminum−air batteries.
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INTRODUCTION In order to design and produce a battery utilizing Al as an anode, some crucial challenges must be met, such as the need to obtain high anodic currents at relatively low overpotentials and the desire to have extremely low Al corrosion rates in the electrolytic media. In an attempt to meet these criteria, employing room temperature ionic liquids (RTILs) as the electrolyte of choice in Al-based batteries is, therefore, a sound approach.1−4 Ionic liquids have a major advantage in comparison to aqueous electrolytes because Al immersed in RTIL media is less prone to parasitic corrosion reactions that generate hydrogen.5 Several researchers explored the possible use of chloroaluminate ionic liquids as electrolytes in Al-based batteries.6,7 A drawback of chloroaluminate ionic liquids is the difficulty of producing them, as the reaction between 1-ethyl-3methylimidazolium chloride (EMImCl) and AlCl3 is highly exothermic.6−8 Additionally, chloroaluminate ionic liquids react rapidly and exothermally with moisture.8,9 Air- and water-stable ionic liquids are an alternative to chloroaluminate. Although various RTILs were already synthesized and applied (for example, 1-ethyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)imide, 1-butyl-1-methylpyrrolidinium-bis(trifluoromethylsulfonyl)imide, trihexl-tetradecyl-phosphonium-bis(trifluoromethylsulfonyl)imide,9 and 1-butyl-3-methylimidazolium tetrafluoroborate10), in Al power source applications the results suggest that Al activation in nonaqueous media is the most challenging issue.11,12 One of the major obstacles of applying these RTILs in Albased power sources is related to the stability of the pristine aluminum oxide covering the Al surface because it is quite difficult to dissolve it.11,12 The native aluminum oxide layer prevents an effective electrochemical dissolution of the Al anode and hinders the efficiency of any battery-related applications. The low solubility of the aluminum oxide in © XXXX American Chemical Society
RTIL media stems from the fact that most of the RTILs described in the literature contain weakly coordinating anions such as tetrafluoroborate (BF4−),13,14 hexafluorophosphate ( P F 6 − ) , 1 5 , 1 6 o r b is ( t r i flu o r o m et h y l s u l f o n y l ) i m i d e ((CF3SO2)2N−).17 A different approach to increasing the solubility of metal oxides is to use RTIL with appending coordinating groups, for example, betainium bis(trifluoromethylsulfonyl)imide, [Hbet][TFSI].18,19Even though a variety of oxides were found to be soluble in this RTIL, iron, cobalt, aluminum, and silicon oxides were insoluble or very poorly soluble in it.19 The high intrinsic ionic conductivity of 100 mS/cm, nonflammability, and thermal and chemical stability with considerable electrochemical window (3.2 V on a glassy carbon electrode,20 being quite comparable to other RTILs) makes EMIm(HF)2.3F a good candidate as an electrolyte in high-end electrochemical power sources (the anion structure is presented in Figure 1).20−25 In a previous report, we described the application of 1-ethyl3-methylimidazolium oligo-fluoro-hydrogenate [EMIm(HF)2.3F] ionic liquid in an aluminum−air battery system.23
Figure 1. (HF)2.3F− anion structure: (HF)2F− (a) and (HF)3F− (b). Received: September 6, 2015 Revised: December 2, 2015
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DOI: 10.1021/acs.langmuir.5b03362 Langmuir XXXX, XXX, XXX−XXX
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Langmuir
(1 mL) were added to the cells, and then they were allowed to rest for 24 h prior to their discharge at a current density of 0.5 mA·cm−2. Surface Analysis. Scanning transmission electron microscopy (STEM; FEI Titan 80−300 keV S/TEM) was used to analyze crosssection samples, prepared by the lift-out method in a focused ion beam (FIB; FEI Strata 400-S).27 The FIB lamellae were positioned on a Ti grid and further thinned by low-kV milling using Ar+ ions at an energy of 500 eV (Linda GentleMill3). Elemental analysis was performed with energy-dispersive X-ray spectroscopy (EDS) in the TEM. Surface analysis was also performed with X-ray photoelectron spectroscopy (XPS; Thermo VG Scientific, Sigma probe, GB) with a monochromatized Al Kα (1486.6 eV) X-ray source at a base pressure of
