High-Voltage and Noncorrosive Ionic Liquid Electrolyte Used in

Oct 3, 2016 - As a promising post-lithium battery, rechargeable aluminum battery has the potential to achieve a three-electron reaction with fully use...
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High-Voltage and Noncorrosive Ionic Liquid Electrolyte Used in Rechargeable Aluminum Battery Huali Wang,† Sichen Gu,† Ying Bai,*,† Shi Chen,† Feng Wu,†,‡ and Chuan Wu*,†,‡ †

Beijing Key Laboratory of Environmental Science and Engineering, School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, China ‡ Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing 100081, China S Supporting Information *

ABSTRACT: As a promising post-lithium battery, rechargeable aluminum battery has the potential to achieve a threeelectron reaction with fully use of metal aluminum. Alternative electrolytes are strongly needed for further development of rechargeable aluminum batteries, because typical AlCl3contained imidazole-based ionic liquids are moisture sensitive, corrosive, and with low oxidation voltage. In this letter, a kind of noncorrosive and water-stable ionic liquid obtained by mixing 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIM]OTF) with the corresponding aluminum salt (Al(OTF)3) is studied. This ionic liquid electrolyte has a high oxidation voltage (3.25 V vs Al3+/Al) and high ionic conductivity, and a good electrochemical performance is also achieved. A new strategy, which first used corrosive AlCl3-based electrolyte to construct a suitable passageway on the Al anode for Al3+, and then use noncorrosive Al(OTF)3-based electrolyte to get stable Al/electrolyte interface, is put forward. KEYWORDS: rechargeable aluminum battery, electrolyte, [BMIM]OTF ionic liquids, high voltage, electrochemical performance

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Al(OTF)3 is added to the [BMIM]OTF ionic liquid under stirring at a series molar concentration (0, 0.05, 0.1, 0.5, and 1 mol/L) to prepare Al(OTF)3/[BMIM]OTF ionic liquid electrolytes. Infrared spectra is adopted to study the interactions between Al salt and [BMIM]OTF. Figure 1a displays the spectra in the range of 400−4000 cm−1, whereas Figure 1b−d shows some enlarged regions. Frequencies and assignments for ionic liquids are shown in Table S1. An obvious decrease in peak intensity and a slightly blue-shift of peak position are observed in C−H stretching vibration on imidazole ring (3154 and 3115 cm−1) and alkyl groups (2966, 2939, and 2878 cm −1 ) with the increasing Al(OTF) 3 concentration. The broad peak at 1262 cm−1 is attributed to SO3 asymmetric stretching, which blue-shifts to 1289 cm−1 in 1 mol/L Al(OTF)3/[BMIM]OTF ionic liquid. And a slightly blue-shift of SO3-symmetric stretching is also found with the increasing Al(OTF)3 concentration. These strong intermolecular interactions between Al(OTF)3 and ionic liquid were induced by electron-withdrawing group OTF−. The position and width of peak corresponding to SO3-stretching are sensitive, which can reflect the interaction with neighboring molecules.20 Therefore, ionic liquids with high Al salt concentration show higher vibration frequency, which indicates that the OTF− anion has higher aggregates with the Al3+.

ith the wide use of renewable energy in electric power generation, energy storage technology has become even more important. To meet with the demand of low-cost, highenergy-density power storage system, researchers have widely investigated new battery systems inrecent years. Rechargeable aluminum batteries involving a three electron redox reaction have a higher volumetric energy density than lithium-ion batteries, which attract more and more attention of researchers.1−12 Cathode materials in rechargeable aluminum batteries have been tremendously developed, including transition metal oxides (V2O5, VO2, MnO2, TiO2),1,2,4,9,10 metal sulfides (Mo 6 S 8 ), 1 3 copper hexacyanoferrate (CuHCF),14,15 carbon materials (graphite3,6 and carbon paper16) and sulfur.17 However, investigations of electrolytes are at a very early stage.18,19 So far, the most commonly used electrolytes in rechargeable aluminum batteries are ionic liquids composed of AlCl3 and imidazolium halide (for example, 1-ethyl-3-methylimidazolium chloride). However, regardless of the relatively narrow electrochemical window, AlCl3 based ionic liquids require a strict separation from atmospheric moisture and have a severe corrosion on cell parts (such as collector, stainless steel cell case). Therefore, the development of noncorrosive electrolytes is highly desired in rechargeable aluminum batteries. In this work, a noncorrosive and water-stable ionic liquid electrolyte obtained by mixing 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIM]OTF) with the corresponding aluminum salt (Al(OTF)3) is studied. © 2016 American Chemical Society

Received: August 23, 2016 Accepted: October 3, 2016 Published: October 3, 2016 27444

DOI: 10.1021/acsami.6b10579 ACS Appl. Mater. Interfaces 2016, 8, 27444−27448

Letter

ACS Applied Materials & Interfaces

Figure 1. FTIR spectra of Al(OTF)3/[BMIM]OTF ionic liquids with different concentration: (a) full range, (b) 3200−2800, (c) 1300−1000, and (d) 800−500 cm−1.

Figure 2. (a) Electrochemical window, (b) conductivity−temperature, (c) Arrhenius plot, and (d) Vogel−Tammann−Fulcher plot of Al(OTF)3/ [BMIM]OTF ionic liquids.

containing Al(OTF)3 are found more stable than pure [BMIM]OTF ionic liquid. The cathodic reduction potential is limited by reduction of 1, 3-dialkylimidazolium, which shifted to a more negtive position with adding of Al(OTF)3. The oxidation decompositions of all ionic liquids start at the potential after 3.25 V vs Al/Al3+, which is much higher than

Pairing and cluster phenomenon occurs in the liquids with high Al(OTF)3 concentration.21 Cyclic voltammetry profiles of Al(OTF)3/[BMIM]OTF ionic liquids measured vs Al quasi-reference electrode are shown in Figure 2a. The working electrode is glassy carbon (GC) disk electrode and the scan rate is 10 mV/s. Ionic liquids 27445

DOI: 10.1021/acsami.6b10579 ACS Appl. Mater. Interfaces 2016, 8, 27444−27448

Letter

ACS Applied Materials & Interfaces

Figure 3. (a) Cycling performance of rechargeable aluminum batteries using Al(OTF)3/[BMIM]OTF ionic liquids; charge/discharge profiles of (b) 0.1, (c) 0.5, and (d) 1 mol/L.

high voltage electrolyte of rechargeable aluminum battery. Coulombic efficiency is shown in Figure S1, and it is found increased with Al(OTF)3 concentration (about 50, 70, and 80% for 0.1, 0.5, and 1.0 mol/L). Charge/discharge profiles of batteries are shown in Figure 3b−d, except the sample with 0.05 mol/L ionic liquid electrolyte, which exhibits no electrochemical activity. All curves show a discharge voltage platform at about 0.25 V, and two charge voltage platform at 1.2 and 2.7 V, respectively. The huge potential difference between charge and discharge profile indicates the large polarization in battery, which may be caused by concentration polarization on electrode/electrolyte interface and electrochemical polarization in electrode. The concentration polarization is impacted by Al ion transportation, while Al ion transportation is influenced by the concentration of Al salt. It can be seen that battery with 0.5 mol/L ionic liquid electrolyte shows smaller potential difference than that of 0.1 mol/L, due to the higher Al3+ concentration. However, for 1 mol/L ionic liquid, larger polarization is observed, caused by pairing and cluster of ions in ionic liquid. Impedance of battery using 0.5 mol/L ionic liquid was measured (Figure S2). The resistance is found to be increased after cycling, indicating an increased electrochemical polarization, which is consistent with the trend of charge/discharge profiles. It is worth noting that Al foil was used as current collector of cathode in batteries with Al(OTF)3/[BMIM]OTF ionic liquid electrolyte, whereas the stainless steel collector was used in batteries with AlCl3 containing ionic liquid electrolytes.2,7 The oxide film on the Al surface cannot be removed by Al(OTF)3/ [BMIM]OTF ionic liquids, and thus can protect the Al collector from reacting with the electrolyte. That is why the metal Al anode must be specially treated, namely, by immersing in acidic AlCl3/[BMIM]Cl = 1.1:1 ionic liquids for 24 h, to partially remove the oxide film on its surface, so that the Al deposition/dissolution reaction can be carried out efficiently.

those of AlCl3 contained ionic liquids. Therefore, a much wider charge−discharge range can be achieved in aluminum batteries using this kind of ionic liquids. Relationship between conductivity and temperature of Al(OTF)3/[BMIM]OTF ionic liquids is shown in Figure 2b. Conductivities of ionic liquids are found increase with temperature for all the samples and the conductivity− temperature plots follow both the Arrhenius Formula (Figure 2c) and Vogel−Tammann−Fulcher (VTF) Equation (Figure 2d). With Al(OTF)3 concentration increasing, conductivity decreases after a slightly increase at 0.05 mol/L. It is probably because the ionic concentration is increased by adding of Al(OTF)3. When concentration continues to increase, pairing and cluster phenomenon occurs and leads to decreased dissociation degree, resulting in decreased conductivity and increased viscosity of ionic liquids. Apparent activation energy (Ea) calculated from Arrhenius and VTF plot is shown in Table S2. Both fitting methods show the same trend of Ea. Charge/discharge performances of rechargeable aluminum batteries with Al(OTF)3/[BMIM]OTF ionic liquid electrolytes were investigated, where V2O5 nanowire was used as cathode material. Synthesis of V2O5 nanowire, preparation of cathode and assembly of cell were all similar to our previous work,9 except that the collector of cathode was Al foil and Al anode was treated by immersing in acidic AlCl3/[BMIM]Cl = 1.1:1 ionic liquids for 24 h to remove the oxide film on its surface. Detailed experiment is shown in Supporting Information. All batteries were tested at a potential window of 3−0.02 V, and the current density was10 mA/g. As shown in Figure 3a, battery performances are improved with the concentration of Al(OTF)3/[BMIM]OTF ionic liquid electrolyte at first, then declined. The optimized concentration is 0.5 mol/L. The battery delivers an 87 mAh/g initial capacity and stabilized at 40 mAh/g. Nevertheless, it shows the feasibility of Al(OTF)3/[BMIM]OTF ionic liquid used as 27446

DOI: 10.1021/acsami.6b10579 ACS Appl. Mater. Interfaces 2016, 8, 27444−27448

Letter

ACS Applied Materials & Interfaces

Figure 4. Above: schematic diagram of Al deposition/dissolution on surface of untreated and treated Al anode; below: SEM images of Al foils before and after immersion in 0.5 mol/L Al(OTF)3/[BMIM]OTF and AlCl3/[BMIM]Cl = 1.1:1 for 24 h.

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As the schematic diagram (Figure 4) shows, Al3+ deposition/ dissolution process is blocked by Al2O3 film on surface of untreated Al anode, while after removal of the oxide film, the reaction proceeds unimpeded. SEM images of Al foil after immersed in 0.5 mol/L Al(OTF)3/[BMIM]OTF and AlCl3/ [BMIM]Cl = 1.1:1 ionic liquid are also shown in Figure 4. No change is found on Al foil immersed in Al(OTF)3/[BMIM]OTF ionic liquid, while pitting corrosion occurs on Al foil immersed in AlCl3/[BMIM]Cl = 1.1:1 ionic liquid. It demonstrates that efficient electrochemical reaction can only take place on Al anode surface with pretreatment, by which the dense oxide film is damaged and active metal Al is exposed. Actually, blank rechargeable aluminum battery using Al(OTF)3/[BMIM]OTF ionic liquid electrolyte with untreated Al anode is also fabricated and tested, which exhibits no electrochemical activity, as shown in Figure S3. These results show that interface between Al anode and the electrolyte is a key factor that impact the battery performance. In summary, Al(OTF)3/[BMIM]OTF ionic liquids with high oxidation decomposition potential (3.25 V vs Al/Al3+) and conductivity are promising to be used as high-voltage electrolytes for rechargeable aluminum battery. Stable cycling performances are achieved in rechargeable aluminum batteries with Al(OTF)3/[BMIM]OTF ionic liquid electrolyte, on condition that the Al anode with fresh and unoxidized surface is used. The state of Al foil surface and the interface of Al/ electrolyte are found have a great impact on performance of rechargeable aluminum batteries. The strategy here, namely, design and construct suitable interface for ion diffusion and deposition, can turn the materials that were previously deemed to be inactive into highly active materials for advanced secondary batteries.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b10579. Experimental details of ionic liquid preparation and characterization, electrode preparation and testing, IR vibration assignment, calculated Ea, Coulombic efficiency, impedance of rechargeable aluminum battery, and performance of battery with untreated Al anode (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work is supported by the National 973 project of China (2015CB251100), and the Program for New Century Excellent Talents in University (Grant NCET-13-0033).

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DOI: 10.1021/acsami.6b10579 ACS Appl. Mater. Interfaces 2016, 8, 27444−27448