Substituent-Effect of Imidazolium Ionic Liquid: A Potential Strategy for

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Substituent-Effect of Imidazolium Ionic Liquid: A Potential Strategy for High Coulombic Efficiency Al Battery Congrong Yang, Suli Wang, Xiaoming Zhang, Qiang Zhang, Wenjia Ma, Shansheng Yu, and Gongquan Sun J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02318 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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The Journal of Physical Chemistry

Substituent-effect of Imidazolium Ionic Liquid: a Potential Strategy for High Coulombic Efficiency Al Battery Congrong Yang, Suli Wang,* Xiaoming Zhang, Qiang Zhang, Wenjia Ma, Shansheng Yu and Gongquan Sun* Division of Fuel Cell & Battery, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR China Corresponding Authors *[email protected]; [email protected]

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ABSTRACT

The chemical structure-effect relationship between imidazolium ion liquid and the properties of electrolyte or Al battery is firstly completely revealed. First, it is proved that there indeed exsits an interaction between imidazolium cation and chloroaluminate anions (active ions, Al2Cl7- and AlCl4-), blocking their reaction in the anode or positive electrode. Second, by introducing the substituent-effect, which could enhance the steric hindrance and change detailed electronic distribution of imidazolium cations, the intermolecular force between imidazolium cations and chloroaluminate anions is effectively weakened. As a result, the electrodeposition achievement and electrodeposition/electrostripping reversibility of aluminium, intercalation/deintercalation capacity of AlCl4- in the positive electrode are significantly improved. Therefore, the as-assembled battery with AlCl3/[MPIM]Cl electrolyte shows a ~100% CE. This work provides a potential approach on how to enhance the performance of Al battery by designing the ionic liquid structure.


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Introduction In recent years, impressive advances in harvesting renewable energy have led to a pressing demand for the complimentary energy storage technology. Owing to the high volumetric capacity, the natural abundance and low cost of Al, Al batteries have become one of the candidates for the energy storage devices.1-8 Before 2010, their applications were hindered by the fatal drawbacks resulting from the use of aqueous electrolytes, such as anode corrosion, hydrogen evolution side reactions, and passive oxide film formation.9-11 Afterwards, organic solvents were investigated in electrolytes, but there was no charge/discharge performance of a battery using this electrolyte.12 Ionic liquids with low vapor pressure, relatively high conductivity and wide electrochemical windows have been adopted as electrolytes in rechargeable Al battery and shown an electrochemical behavior.13-16 However, for improvement AlCl3/imidazolium ionic liquid electrolyte systems as the parameter space for the Al battery still remains largely unexplored. The Coulombic efficiency (CE) of this battery is 97-98%, which is much lower than that of state-of-the-art LIBs (99.98% CE).17-18 Moreover, the real output capacity of Al battery is just one-tenth or worse over the theoretical specific anode capacity of 2,980 mAh/g.5-6, 19-21 As a critical material, electrolyte plays a key role on the performance of this battery. Thus, the comprehension and further design of electrolyte is very important for a successful battery technology.3, 22-24 In the common AlCl3/[MEIM]Cl (acidic) electrolyte, AlCl3 or Al2Cl6 would react with [MEIM]Cl, giving Al2Cl7-, AlCl4- and [MEIM]+ (to maintain neutrality) ions. During charging, Al2Cl7- is reduced to aluminum metal, and AlCl4- intercalates in the positive electrode. During discharge, aluminum is oxidized and integrates with AlCl4- to form Al2Cl7-, and AlCl4- deintercalates from positive electrode. So the deposition/stripping reversibility of aluminum at the Al negative electrode and intercalation/deintercalation capability of AlCl4- in the positive electrode would significantly


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influence the capacity and CE of Al battery. Here, it should be noted that [MEIM]+, a dominating cation, could form static electricity with Al2Cl7- or/and AlCl4-,13 which would affect the deposition/stripping reversibility of aluminum and the intercalation/deintercalation capability of AlCl4-. So weakening the intermolecular force between imidazolium cation and chloroaluminate anions is considered a potential strategy for improving the performance of Al battery. However, up to date, no research reveals the effect aroused by the chemical structure of imidazolium, nor gave a clear interpretation on the relationship between the chemical structure of imidazolium and the property of electrolyte. In this work, to complete understand the structure-effect relationship and design an optimal electrolyte for Al battery, the substituent-effect is brought into the imidazolium ionic liquids, and the property of electrolyte and the performance of battery are investigated in detail.

Experimental Section Imidazolium ionic liquid synthesis Imidazolium chloride ionic liquids were synthesized by the reaction of imidazole and chloralkane in an autoclave at 60 oC. The crudes were further purified by ethyl acetate and then immediately transferred to glove box after being completely dried. NMR spectra 27Al

NMR technology was used to determine the chemical exchange of Al in electrolytes. The

NMR spectra were obtained on a Bruker ACIII 400 spectrometer operating at 104.2 MHz using a 5 mm broad band probe operating at 298 K. The 27Al spectra were acquired with 64 K data points, 128 scans, 1 s relaxation delay and processed with 2 Hz line broadening function. Chemical shifts were referenced to a secondary standard 1 M Al(NO3)3 (δ = 0.0 ppm). DFT theoretical calculation


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The spatial sizes of imidazolium cations were calculated by DFT method. The spin-polarized DFT calculations were performed by the DMol3 code embedded in Materials Studio (Accelrys, SanDiego, CA)25 with long-range dispersion correction via Grimme’s scheme.26 The generalized gradient approximation with the Perdew–Burke–Ernzerhof (PBE) functional is employed to describe exchange and correlation effects.27 The All Electrons method is implemented for core treatment. The triple numerical atomic orbital augmented by a polarization function is chosen as the basis set (TNP).28 During geometrical optimization, the basis set cut-off was chosen to be 5.2Ǻ. The convergence tolerances for the geometry optimization were set to 10-5 Ha for the energy, 0.002 Ha/Ǻ for the force, and 0.005Ǻ for the displacement. The electronic SCF tolerance was set to 10-6 Ha. A Fermi smearing parameter of 0.003 Ha was used in the calculations. In all the structure optimization calculations, the atoms are fully relaxed. Electrolyte preparation and test Electrolytes were prepared in a dry argon-filled glove box (MBraunLabstar 1250/780) by slowly adding AlCl3 into different imidazolium ionic liquid synthesized above according to the designed molar ratio. Electrolytes were stirred overnight, obtaining homogeneous and clear liquids. The conductivities of different electrolytes were determined using a METTLER TOLEDO S7FIELD KIT conductivity meter. Cyclic Voltammetry (CV) measurements of electrolytes were performed on a CHI760E electrochemical workstation. In CV measurements, a Platinum disk electrode (diameter: 2 mm) was used as the working electrode, and Al flake was used as the counter and reference electrodes, at different scan rates. Battery assemblies and measurements


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To investigate the structure-effect relationship on the battery performance, rechargeable Al batteries (2032 coin-type cells) were assembled in an argon-filled glove box. The positive electrode was made with graphite: Super P: PVDF binder =8 : 1 : 1 (mass ratio), with ∼2.5 mg cm-2 loading of active graphite material. Whatman glass fiber (GF/C) was selected as the separator. Al metal foil (99.99% Al purity) was used as negative electrode, AlCl3/[MEIM]Cl or AlCl3/[MPIM]Cl was served as electrolyte. Galvanostatic electrochemical charge-discharge cycling of the coin cells was performed on a LAND CT2001A battery tester, under a potential window of 2.0-0.01 V, with a constant current density of 30 mAg-1. CV measurements of batteries were performed on a CHI760d electrochemical workstation, under a potential window of 2.5-0.6 V, with a scan rate of 0.5 mVs-1. CV measurements of batteries were performed on a CHI760d electrochemical workstation.

Results and discussion We have proved that electrolyte using imidazolium ionic liquid in Cl- form shows complete advantages, such as wide electrochemical window and good conductivity, over that of imidazolium in Br-, BF4- and TFSI- form (as shown in Figures S7-S9), so the further research on electrolyte is based on imidazolium ionic liquids in Cl- form. The substituent-effect, which could change the steric hindrance and symmetry of imidazolium cation and affect the intermolecular force between imidazolium cation and chloroaluminate anion, is brought into imidazolium cations to investigate the substituent-effect on the conductivity, electrodeposition/electrostripping reversibility of aluminium and intercalation/deintercalation capability of AlCl4- anion of electrolyte.


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To understand the substituent-effect on the electrochemical properties of electrolyte, six kinds of imidazolium ionic liquids (1-methyl-3-ethylimidazolium chloride ([MEIM]Cl), 1-methyl-3propylimidazolium chloride ([MPIM]Cl), 1-methyl-3-butylimidazolium chloride ([MBIM]Cl), 1methyl-3-isopropylimidazolium chloride ([MIPIM]Cl), 1,2-dimethyl-3-propylimidazolium chloride ([DMPIM]Cl) and 1,2-dimethyl-3-butylimidazolium chloride ([DMBIM]Cl)) are designed and investigated. The chemical structures of imidazolium cations are shown in Figure 1. The structure and purity of the imidazolium ionic liquids are confirmed by 1H NMR (as shown in Figures S1-S6, D2O as solvent).

Figure 1. Chemical structures of imidazolium cations. Substituent-effect on intermolecular force of electrolytes 27Al

NMR is used to judge the chemical exchange of Al (in AlCl4- and/or Al2Cl7-) resulted

from the substituent-effect. Figure 2 shows 27A1 NMR spectra obtained from AlCl3/different imidazolium ionic liquids electrolytes with mole ratio of 1.3:1. At 25 oC, the resonances at about 100 ppm are corresponding to isolated AlCl4- without Al2Cl7-, which is in agreement with others’ work.1 However, at the local atomic-electronic structure level, the chemical shift trends of Al2Cl7- is similar to that found in AlCl4-, so the chemical shift changes of AlCl4- stemmed from imidazolium cations is could be also used to analyse the effect of imidazolium cations on Al2Cl7-. It is shown that the substituents on imidazolium cathions have a significant effect on the chemical shift of AlCl4-. Compared to [MEIM]+, [MPIM]+ possesses a stronger steric hindrance, which would block the interaction between AlCl4- and imidazolioum cations and increase the chemical shift of AlCl4-. Similarly, the introduction of methyl onto C2-position of imidazolium


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could greatly enhance steric hindrance and lessen the intermolecular force between AlCl4- and imidazolioum cations, which would result in a higher chemical shift of AlCl4-.1, 29-30 However, when N3-position is substituted by butyl, the symmetry and detailed electronic distribution of imidazolium cations would greatly change and the intermolecular force between AlCl4- and imidazolioum cations would be intensified. Isopropyl, with greater steric hindrance and better symmetry than butyl (confirmed by DFT results, as shown in Figure S10), makes a weaker intermolecular force between AlCl4- and imidazolioum cations.1 Therefore, the chemical shift order of AlCl4- in electrolytes with different imidazolium ionic liquids is [MPIM]Cl ≈ [DMPIM]Cl > [MEIM]Cl > [DMBIM]Cl > [MIPIM]Cl > [MBIM]Cl. From 27A1 NMR spectra of electrolytes with different imidazolium ionic liquids, it can be seen that the substituent-effect would significantly influence the chemical exchange of AlCl4- and (or) Al2Cl7-, which can be attributed to the electrostatic interaction between imidazolium cations and chloroaluminate anions. Compared to [MEIM]+, [MPIM]+ and [DMPIM]+ cations possess a stronger steric hindrance, effectively weakening the intermolecular force between imidazolium cations and chloroaluminate anions. However, the detailed electronic distribution changes of [MBIM]+ would cause a serious intermolecular force between imidazolioum cations and chloroaluminate anions.


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Figure 2. 27Al NMR spectra obtained fromAlCl3/imidazolium ionic liquids electrolytes with mole ratio of 1.3. Chemical shifts are referenced to a secondary standard 1 M Al(NO3)3 (δ = 0.0 ppm) and the temperature is fixed at 25 oC.


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Substituent-effect on electrodeposition/electrostripping reversibility of aluminium in electrolyte 27Al

NMR results have preliminarily revealed the electrostatic force of chloroaluminate anions

stemmed from different imidazolium cations. Then the influence of substituent-effect on electrodeposition/electrostripping reversibility of aluminium in electrolytes is further studied. The voltage range in CV measurement is set at -0.5-2 V (scan rate: 20 mVs-1) due to the anodic limiting potential of all electrolytes (nearly at 2.1 V, as shown in Figure S11). In theory, the onset potential according to the reduction of Al2Cl7- should be at 0 V.8 Therefore, the overpotential could be used to determine the difficulty for the reduction of Al2Cl7-. In addition, the potential difference between electrodeposition and electrostripping current peak of aluminium in CV measurement could judge the electrodeposition/electrostripping reversibility of aluminium of electrolytes. From Figure 3, it can be seen that the onset potential order of the initial reduction current of Al2Cl7- in electrolytes is AlCl3/[MPIM]Cl>AlCl3/[MEIM]Cl >AlCl3/[MIPIM]Cl>AlCl3/[MBIM]Cl, indicating an easier Al2Cl7- reduction in AlCl3/[MPIM]Cl electrolyte. Moreover, potential difference between electrodeposition and electrostripping current peak of aluminium in AlCl3/[MPIM]Cl electrolyte is the minimum (0.46 V, as shown in Figure S12), showing a best electrodeposition/electrostripping reversibility of aluminium. The substituent-effect on electrodeposition/electrostripping reversibility of aluminium exists in electrolyte as it does for the intermolecular force between imidazolium cation and chloroaluminate anions. This result indicates that the weaker interaction between [MPIM]+ cation and Al2Cl7- benefits for Al2Cl7- reduction and the electrodeposition/electrostripping reversibility of aluminium in electrolyte.


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Figure 3. Substituent-effects on CV measurements of electrolytes with 20 mVs-1 of scan rate. The CVs are carried at room temperature. The ratio of AlCl3 to different imidazolium ionic liquids are fixed at 1.3:1. Substituent-effect on the conductivity of electrolyte Conductivity of electrolyte is another vital parameter for its application in battery, so substituent-effect on the conductivity of electrolyte is investigated to display a proper electrolyte with high conductivity. Figure 4 shows the conductivities of different electrolytes. It can be seen that the substituents on N3 or C2 of imidazolium show a significant effect on the conductivity of electrolyte. According to the formula of conductivity σ=FqC⁄6πηr (σ is conductivity, η is viscosity and r is ion radius), it can be concluded that the conductivity of electrolyte is directly influenced by viscosity and ion radius. η is partly influenced by the intermolecular force between


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imidazolium cation and chloroaluminate anions.31-32 Thus, the conductivity of electrolyte could be optimized by reducing imidazolium cation size and weakening the intermolecular force between imidazolium cation and chloroaluminate anions.

Figure 4. Conductivities of different electrolytes. DFT calculations are used to simulate imidazolium cation, the optimum chemical structure and the size results are shown in Figure S10 and Table 1.25-28 The substituents have no effect on the imidazolium cation size in X direction, but significantly influence its size in Y and Z directions, which would determine the mobility of ion. Distinctly, when C2-H is substituted by methyl group, the imidazolium ionic liquids own a larger molecular size (in Y direction), and a stronger intermolecular force between cation and anions (consistent with 27Al NMR result).13, 33 As a result, the conductivity of AlCl3/[DMPIM]Cl or AlCl3/[DMBIM]Cl is much lower than that of C2-H imidazolium based electrolytes. In C2-H imidazolium based electrolytes, a much larger ion radius(more than three times in Z direction) of [MPIM]+ than that of [MEIM]+ hasn’t caused a big gap of conductivity between AlCl3/[MPIM]Cl and AlCl3/[MEIM]Cl (12.8 mScm-1 vs 14.3 mScm-1, at room temperature), which may be due to the weaker intermolecular force between [MPIM]+ and chloroaluminate anions. Strikingly, alkyl chain ramification (from propyl to


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isopropyl), shows a poor conductivity (9.0 mScm-1, nearly equal to AlCl3/[MBIM]Cl). This can be explained to the strong steric effect of isopropyl and the bigger size in Y direction. The substituents would greatly affect the imidazolium cation size and the intermolecular force between imidazolium cations and chloroaluminate anions. As a result, the conductivity of electrolyte would be significantly influenced. The electrolyte containing [MPIM]+, a larger size but a weaker intermolecular force with chloroaluminate anions, shows a relatively high conductivity. Table 1 Different imidazolium cation sizes calculated by DFT. X(Å) Y (Å) Z (Å) [MEIM]+

6.01

2.17

0.57

[MPIM]+

6.06

2.18

1.88

[MBIM]+

6.01

2.17

2.88

[MIPIM]+

5.94

3.15

0.86

[DMPIM]+

6.20

3.56

1.40

[DMBIM]+

6.02

3.43

2.70

Substituent-effect on the properties of batteries It has been clear that AlCl3/[MPIM]Cl electrolyte possesses easier Al2Cl7- reduction ability, better electrodeposition/electrostripping reversibility of aluminium and a relatively high conductivity. As a proof of concept, full batteries are fabricated to investigate the practicality or advantage of AlCl3/[MPIM]Cl electrolyte. AlCl3/[MPIM]Cl with ratio of 1.2:1or 1.3:1, showing high conductivity and good electrodeposition/electrostripping reversibility of aluminium (as shown in Figure S13), and AlCl3/[MEIM]Cl are used as the electrolytes with Al anode and graphite positive electrode.


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Considering the electrochemical widow of electrolyte, the cut-off charge voltage of Galvanostatic charge and discharge is set at 2.0 V (as shown in Figure 5A).

Figure 5. Galvanostatic charge and discharge curves and CV measurements of an Al/ graphite batteries with AlCl3/[MEIM]Cl and AlCl3/[MPIM]Cl electrolytes. (A) Galvanostatic charge and discharge with current density of 30 mAg-1; (B) CV measurements with 0.5 mVs-1 of scan rate. Initial cycling at 30 mAg-1 required several cycles for stabilization of the capacity and CE. When using AlCl3/[MPIM]Cl, the optimal ratio is 1.3, which might be accounted for a better ratio of [AlCl4-]/[Al2Cl7-] existing in AlCl3/[MPIM]Cl with a ratio of 1.3:1, benefitting for the intercalation and deintercalation of AlCl4-.2 CV measurements of Al and graphite electrodes in the AlCl3/[MEIM]Cl or AlCl3/[MPIM]Cl electrolyte are used to analyse the charge or discharge characteristic peak. From Figure 5B, it can be seen that the Al batteries exhibit a clear discharge voltage plateaus in the range of 1.9-1.5 V (vs. Al) and graphite oxidation peaks are at about 2.0 V (vs. Al) when using either AlCl3/[MEIM]Cl or AlCl3/[MPIM]Cl electrolyte, which is contributed to the graphite positive electrode. The batteries using AlCl3/[MEIM]Cl and AlCl3/[MPIM]Cl electrolytes both have charge specific capacities of about 20 mAhg-1, which is similar to other’s work.2 Strikingly, discharge specific capacities of Al battery using AlCl3/[MPIM]Cl electrolyte is about 20 mAhg-1, exhibiting a nearly 100% CE, which is much higher than that of battery using AlCl3/[MEIM]Cl electrolyte (95% CE). To identify the


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contribution to the high CE, the interface resistance of the batteries using different electrolytes is further investigated. Figure S17 shows the Nyquist profiles of the batteries using different electrolytes before charges. The charge transfer resistance Rct is depicted by the middle semicircle. From EIS measurements of the Al and graphite positive electrodes in the AlCl3/[MEIM]Cl and AlCl3/[MPIM]Cl electrolytes, it can be seen that the interface resistance of the battery using AlCl3/[MPIM]Cl is 82 Ω, which is much lower than that of the battery using AlCl3/[MEIM]Cl (112 Ω). It clearly reveals that AlCl4- could more easily intercalate and deintercalate over the interface of the positive electrode and AlCl3/[MPIM]Cl electrolyte.

Conclusions In conclusion, substituent-effect of imidazolium cations significantly influences the intermolecular force between imidazolium cation and chloroaluminate anions, the electrodeposition/electrostripping reversibility of Al in electrolyte and the intercalation/deintercalation behavior of AlCl4- into the positive electrode. By designing the substituents, steric hindrance and detailed electronic distribution of imidazolium cation could be enhanced, and the intermolecular force between imidazolium cation and chloroaluminate anions would be effectively weakened. As a result,the reduction ability of Al2Cl7-, the electrodeposition/electrostripping reversibility of Al and the intercalation/deintercalation capability of AlCl4- into the positive electrode have been greatly improved. Contributing to the above, the battery of Al and graphite electrodes in AlCl3/[MPIM]Cl electrolyte shows a nearly 100% Coulombic efficiency. This work: (1) clearly presents the structure-effect relationship between the substituents of imidazolium ion liquid and the performance of electrolyte or Al battery; (2) provides a potential guidance on how to design ionic liquids and optimize the performance of electrolytes for Al battery.


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ASSOCIATED CONTENT Supporting Information. Synthesis and 1H NMR of imidazolium ionic liquids, electrolyte preparation, 27Al NMR technology operation process, conductivities and CV measurement of AlCl3/[MEIM]Br and AlCl3/[MEIM]Cl, electrochemical properties of AlCl3/[MEIM]BF4 and AlCl3/[MEIM]TFSI, CV measurements of electrolytes, substituent-effects on potential difference, DFT calculation, ratio effect on the properties of AlCl3/[MPIM]Cl electrolyte, electrochemical stability of AlCl3/[MPIM]Cl, battery assembly process, Galvanostatic electrochemical charge-discharge cycling and CV measurements, EIS of Al batteries using different electrolytes. AUTHOR INFORMATION Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is financially supported by the Key Program of the Chinese Academy of Sciences (KGZD-EW-T08), the National Natural Science Foundation (21706251), National Defense Science and Technology Innovation Special Zone (18-163-B-ZD-008-002-02), the National Natural Science Foundation (21606225) and National Key Research and Development Plan (2016YFB0101204), the DFT calculations utilized resources at the High Performance Computing Center of Jilin University. REFERENCES


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