Article pubs.acs.org/JPCC
Haloaluminate-Free Cationic Aluminum Complexes: Structural Characterization and Physicochemical Properties Toshihiko Mandai* and Patrik Johansson Department of Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden S Supporting Information *
ABSTRACT: The large electrochemical activities of haloaluminate anions [AlnX3n+1]−, anionic complexes derived from AlX3 and Lewis basic fertilizers, have significantly contributed to the development of industrial coatings and more recently also to electrochemical energy storage. In contrast, cationic metal complexes have just emerged as a class of species interesting as multivalent main charge carriers for Mg, Ca, and especially here Al batteries. Despite the potential of such complexes to efficiently deliver Al3+ cations at the electrode| electrolyte interfaces, very few cationic aluminum complexes that do not contain moisture sensitive [AlnX3n+1]− counteranions have been reported due to the few, and difficult to synthesize, commercially available parent aluminum salts. Here a range of cationic aluminum complexes with different ligands and anionic structures were successfully synthesized by complexation of AlCl3 with certain ligands to create fully solvated [Al(L)6]Cl3 complexes and subsequent application of anion metathesis reactions. X-ray crystallography aided by vibrational spectroscopy corroborates the formation of discrete complexes with hexacoordinated octahedral Al3+ cations balanced by three isolated anions. The resulting physicochemical properties are strongly dependent on the constituent ions, and one special choice of ligand and anion results in a novel design of a room temperature quasi-ionic liquid having high ionic conductivity. Although the high-melting complexes with DMSO ligands are inactive, the molten complex exhibits both cathodic and anodic currents. This is the first electrolyte that allows quasi-reversible electrochemical plating/stripping of Al metal without any fragile anion being present.
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INTRODUCTION
The recent successes of reversible electroplating/stripping of mono- and multivalent metals from coordination metal complexes,10−18 where the active species are cationic, have inspired us to investigate similar aluminum complexes. Electrochemically active aluminum complexes have been reported previously, but all of them incorporate the [AlCl2(L)m]+ cation (L denotes ligand) and the chloroaluminate [AlnCl3n+1]− anion.3−6,19−27 Indeed, covering the top layer of the electrolyte by a water-immiscible hydrophobic insulator has enabled reversible electroplating/stripping of Al metal under ambient conditions.4 In addition, electroactive cationic aluminum species, formed via complexation of AlCl3 with neutral 4-propylpyridine and dipropyl sulfide,28,29 have recently been reported to show enhanced stability toward moisture, but in general many issues due to the usage of AlCl3 still need to be addressed. Another possible pathway is developing novel electrolytes free from AlX3. Unfortunately, aluminum ionic complexes without any presence of very complicated ligands and/or fragile aluminate anions are quite rare,30−33 mainly due to the very
Post-lithium-ion batteries are a collection of various promising battery technologies utterly needed to respond to the rapidly growing demands for electric power sources, today especially notable in the electromobility field, but increasing in importance also for large-scale energy storage applications for renewable energy. While some strategies focus on enabling the use of high energy cathode materials such as S and O2 with Li metal anodes, e.g., Li−S and Li−air batteries, others turn away from Li altogether and focus on multivalent metal-based batteries, e.g., Mg,1 Ca,2 and Al batteries.3 A rechargeable Al battery offers a very large theoretical negative electrode volumetric capacity, 8046 mAh cm−3, much owing to the three-electron transfer possible by Al3+. Safety issues, however, arising mainly from the existing aluminum conductive electrolytes, have hampered a materialization of any practical Al battery, as has problems of plating/stripping. As of today, almost all nonaqueous Al-conducting electrolytes and batteries incorporate AlCl3 as the electroactive ingredient,3−8 which results in electrolytes very reactive with moisture and highly corrosive even toward stainless steel. Therefore, a development of other kinds of electrochemically active aluminum compounds is urged for.9 © XXXX American Chemical Society
Received: July 19, 2016 Revised: September 6, 2016
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DOI: 10.1021/acs.jpcc.6b07235 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
procedures and the analytic data for [Al(L)6]X3 are both provided in the Supporting Information. Methods. The thermal properties of the synthesized [Al(L)6]X3 aluminum complexes were determined by thermogravimetry (TG) and differential scanning calorimetry (DSC) using a TG 209 F1 Iris (NETZSCH) and a DSC Q1000 (TA Instruments), respectively. TG was performed to estimate the thermal stabilities of the complexes. The samples were heated from room temperature to 500 °C at a heating rate of 10 °C min−1 under a dry nitrogen atmosphere. The thermal transitions were evaluated by DSC. The samples were hermetically sealed in aluminum pans inside the Ar-filled glovebox. A scanning rate of 10 °C min−1 and an appropriate temperature range depending on the thermal stability of each complex were employed. The liquid density and dynamic (absolute) viscosity were measured using an oscillating U-tube densitometer DMA 4500M and a falling ball viscometer Lovis 2000ME, respectively (Anton Paar). The ionic conductivity was determined by dielectric spectroscopy using a Novocontrol broadband dielectric spectrometer in the frequency range of 10−1−107 Hz. The liquid sample was placed between stainless steel electrodes, fixed at 13.6 mm in diameter and 1.45 mm in thickness with the use of a Teflon spacer. The conductivity cell was assembled in the glovebox and transferred into a cryofurnace allowing the measurements to be carried out under a steady flow of nitrogen gas. The dc conductivity was defined as the low-frequency plateau in the frequency-dependent (ac) conductivity plot. All characterizations above were made as functions of temperature. Raman spectra were collected with a Bruker MultiRam FTRaman spectrometer equipped with a liquid nitrogen cooled germanium detector, excited by the 1064 nm line of a Nd:YAG laser. The spectra were recorded in the range 100−3600 cm−1 with a spectral resolution of 2 cm−1. Each sample was put in a glass vial with a screw lid under Ar atmosphere and transferred to the Raman setup to avoid exposure to air. Fourier-transform infrared (FT-IR) spectroscopy was carried out using a Bruker ALPHA spectrometer and a diamond ATR crystal, with a spectral resolution of 4 cm−1. To avoid contamination, all ATRFT-IR measurements were conducted inside the Ar-filled glovebox. All spectroscopic measurements were performed at ambient temperature (∼20 °C). The electrochemical properties were characterized using a three-electrode cell with a Gamry Reference 600 potentiostat/ galvanostat/ZRA in an Ar-filled glovebox. A Pt disk (1.58 mm radius) working electrode and an Al wire as both counter and quasi-reference electrodes were used. The pretreatment procedure of the electrodes is described elsewhere.9 Cyclic voltammetry (CV) was conducted at 80 °C with a scan rate of 20 mV s−1. The bulk electrolysis was carried out on a Pt electrode by applying a constant potential of −0.5 V vs quasi Al3+/Al at 80 °C. Scanning electron microscopy (SEM) images were taken by using a JSM-6490A (JEOL) with an energy dispersive X-ray spectroscopy (EDX) attachment. The deposit was scratched and mounted on a sample holder with a conducting carbon paper and then transferred to the SEM instrument without any exposure to air. The EDX measurement was subsequently performed on the same sample as imaged. Crystal Structures. Single crystals of [Al(DMSO)6][TFSI]3, [Al(DMSO)6][TfO]3, and [Al(MIm)6][TFSI]3 suitable for X-ray crystallography were obtained by recrystallization from ACN. A single crystal was mounted on a glass pin with a
limited choices of aluminum salts commercially available. AlCl3 is, however, known to react with various organic compounds; combining AlCl3 with most organic compounds, including O, S, and P donor ligands, results in the [AlCl4]− anion.4,5,26,34,35 The reactions are based on an asymmetric cleavage of polymerized bulk AlCl3; 2AlCl3 → AlCl2+ + AlCl4−. Some amines are prone to form adducts [AlCl3(L)m] and/or [AlCl2(L)m][AlCl4] with AlCl3 (dependent on the molar ratio),36 but [AlR2(L)2]X complexes when using AlR2X.23,27 In contrast to the organic compounds above, dimethyl sulfoxide (DMSO) allows the formation of fully dissociated complexes [Al(DMSO)6]Cl3 through a symmetric cleavage of bulk AlCl3,31,37 indicating that chloroaluminate-free cationic aluminum complexes indeed are achievable from AlCl3 under certain circumstances and chemistries. Although most haloaluminate anion-free aluminum complexes possess high melting points, much above ambient temperatures, the physicochemical properties of the ionic complexes can be tailored in detail by modifying the constituent ionsa strategy much used in order to create ionic liquids (ILs), often by just a slight increase in the ligand size or decreased symmetry.38−40 The same is true for metalcoordination compounds, e.g., by incorporating the glyme and N-alkylimidazole ligands without loss of its electrochemical activity.10,17,41−43 Such (low-melting) liquid coordination compounds are now categorized as quasi-ILs and liquid metal salts or more recently as solvate ILs. The pioneering work by Dai’s group demonstrated the overall strategy to make cationic metal-containing ionic liquids (ILs) for metal deposition.44 This same strategy should be applicable to aluminum with the target to obtain a novel coordination compound with desired properties. We herein have synthesized a range of cationic aluminum coordination complexes by complexation of AlCl3 with certain ligands and subsequent application of anion metathesis reactions. Their structural characteristics were determined by X-ray crystallography and vibrational spectroscopy. Furthermore, the transport and electrochemical properties of a single resulting aluminum quasi-IL were investigated to assess its fundamental promises as an Al-battery electrolyte.
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EXPERIMENTAL SECTION Materials and Synthesis. AlCl3 (anhydrous, 99.99% trace metals basis), toluene (anhydrous, 99.8%), acetonitrile (anhydrous, 99.8%), dimethyl sulfoxide (DMSO; anhydrous, 99.9%), 1-methylimidazole (MIm; 99%), 1-butylimidazole (BIm; 98%), sulfolane (99%), 1-ethyl-3-methylimidazolium chloride (EMImCl; 98%), and sodium trifluoromethanesulfonate (NaTfO; 98%) were all purchased from SigmaAldrich. Battery grade lithium bis(trifluoromethanesulfonyl)imide (LiTFSI; 99.9%) was purchased from Solvionic. EMImCl and NaTfO were dried under high vacuum at 60 °C for 48 h and then stored in an Ar-filled glovebox (
