Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 4732−4737
pubs.acs.org/JPCL
Solvation-Structure Modification by Concentrating Mg(TFSA)2− MgCl2−Triglyme Ternary Electrolyte Kohei Shimokawa,† Hajime Matsumoto,‡ and Tetsu Ichitsubo*,† †
Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Research Institute of Electrochemical Energy, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31, Midorigaoka, Ikeda, Osaka 563-8577, Japan
‡
J. Phys. Chem. Lett. Downloaded from pubs.acs.org by DURHAM UNIV on 08/06/18. For personal use only.
S Supporting Information *
ABSTRACT: Mg(TFSA)2/triglyme(G3)-based electrolytes (TFSA: bis (trifluoromethanesulfonyl) amide) are one of candidates for magnesium rechargeable batteries, but the passivation of Mg-metal anode due to the TFSA anion is fatal in practical use. In this work we show that at elevated temperatures around 150 °C a comparable amount of MgCl2 salt can be dissolved in concentrated Mg(TFSA)2/G3 solutions, and the passivation of Mg metal is markedly suppressed in such highly concentrated solutions (1 ≤ G3/Mg-salts ≤ 2) in comparison with in the dilute solutions (G3/Mg-salts ≫ 2). By decreasing the amount of G3 solvent, the solvation structure of Mg2+ ions is modified in that free TFSA anions are drastically lowered, which would consequently decrease the reactivity of TFSA anions. We also demonstrate that a full-cell using MgCo2O4 cathode with the electrolyte of Mg(TFSA)2/MgCl2/G3 1:1:2 at 150 °C delivers a cell voltage of ∼2 V versus Mg-metal anode.
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discuss the origin of the improved electrochemical performance. Finally, we corroborate the feasibility of the highly concentrated Mg(TFSA)2−MgCl2/G3 solution as the electrolyte for high-temperature MRBs. The solubility of MgCl2 is dramatically enhanced by mixing with the comparable amount of Mg(TFSA)2 at elevated temperatures around 150 °C. In particular, almost all MgCl2 salt can be dissolved when mixing in the molar ratio of Mg(TFSA)2/MgCl2/G3 1:1:2 at 150 °C. Interestingly, the mixture exhibits enhanced thermal stability even at 150 °C, and it is crystallized without any phase separation by cooling to room temperature (Figure 1a). In addition, in the X-ray diffraction (XRD) profile of the obtained crystals (Figure 1b), there are no peaks identifying the crystals of the Mg(TFSA)2/ G3 1:1 or the pure salts. This indicates that the three components reacted with each other to form a certain complex. It was reported that in the equimolar mixtures of Mg(TFSA)2 and long-chain glymes such as G3 and tetraglyme (G4) all glyme molecules coordinate to Mg2+ ions to form stable equimolar complexes (e.g., denoted as Mg(G3)1) due to their strong chelating ability, which increases the melting temperature and enhances the thermal stability.14,15 Therefore, in the mixture of Mg(TFSA)2/MgCl2/G3 1:1:2 (G3/Mg-salts = 1), it is considered that most of the G3 molecules coordinate to Mg2+ ions originating from both salts, resultingly, to form Mg(G3)1. In addition, on the basis of the fact that the inorganic MgCl2 salt is hardly soluble in pure G3 solvent, MgCl2 can be dissolved with Mg(TFSA)2 in a certain kind of
agnesium rechargeable batteries (MRBs) have been one of candidates for post-Li batteries in the following points: (i) Mg metal possesses high specific capacity (2205 mAh g−1) and low electrode potential (−2.38 V vs SHE), (ii) Mg metal tends to be deposited in nondendritic form unlike Li,1,2 and (iii) Mg is one of abundant chemical elements. However, a serious problem is that there are few cathode materials that can accommodate Mg2+ ions at high electrode potentials. High-temperature operation is a way to overcome this problem because it facilitates the bulk diffusion of Mg2+ ions inside the cathode materials.3−5 To construct hightemperature MRBs, the electrolyte must possess the capability that facilitates Mg deposition/dissolution with low overpotential at elevated temperatures. Mg(TFSA)2-based electrolytes (TFSA: bis(trifluoromethanesulfonyl)amide) are promising candidates in terms of thermal stability and electrochemical window.6,7 However, one drawback is that Mg-metal anode is readily passivated, probably due to the decomposition of TFSA, which causes a large overpotential for Mg dissolution.8−10 The recent works have demonstrated that the overpotential for Mg dissolution can be fairly suppressed by adding MgCl2 in several ether-based Mg(TFSA)2 solutions,11−13 but the passivation is still not completely inhibited.10 In this work we show that the passivation of Mg metal is markedly suppressed in highly concentrated Mg(TFSA)2− MgCl2/G3 solutions. Although MgCl2 is hardly soluble in pure G3 solvent, a large amount of MgCl2 salt can be dissolved under the coexistence of suitable amounts of Mg(TFSA)2 and G3 at elevated temperatures around 150 °C. By means of Raman spectroscopy, we investigate the solvation structure of Mg2+ ions depending on the electrolyte concentration and © XXXX American Chemical Society
Received: July 16, 2018 Accepted: August 2, 2018
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DOI: 10.1021/acs.jpclett.8b02209 J. Phys. Chem. Lett. 2018, 9, 4732−4737
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The Journal of Physical Chemistry Letters
mixture of Mg(TFSA)2/MgCl2/G3 1:1:2, the following reaction is considered Mg(TFSA)2 + MgCl2 + 2G3 → 2TFSA[Mg(G3)1]Cl
The solvation structure of Mg2+ ions depending on the concentration was investigated by the Raman spectroscopy in the systems of Mg(TFSA)2/G3 and Mg(TFSA)2−MgCl2/G3. It is known that the Raman bands in the range of 800−900 cm−1 arise from the combination of CH2 rocking and COC stretching modes of glymes, which are sensitive to the change of the solvation structure. As shown in Figure 2a (left), a broad Raman band was observed at 850 cm−1 in the spectra for the dilute Mg(TFSA)2/G3 solutions, which can be assigned to the vibrational mode of free G3 solvent.17,18 On the contrary, in the spectrum for Mg(TFSA)2/G3 1:2, remarkable Raman bands were observed at 875.5 and 883.5 cm−1, whereas the Raman band at 850 cm−1 was absent. Such Raman bands in the range of ca. 870−890 cm−1 can be assigned to the “breathing mode” of the G3 molecules coordinating to Mg2+ ions.14,15,19 Thus the spectrum for Mg(TFSA)2/G3 1:2 indicates that most of G3 molecules coordinate to Mg2+ ions to form the complex of Mg(G3)2, which is consistent with a previous report.19 It is expected that there also exists Mg(G3)2 in the dilute solution of 0.5 M Mg(TFSA)2/G3 (i.e., Mg(TFSA)2/G3 1:11 by molar ratio) because the same breathing modes were observed in the spectrum. However, a different breathing mode was observed in the spectrum for Mg(TFSA)2/G3 1:1, which indicates that the solvation structure was modified from Mg(G3)2 to Mg(G3)1.14 Interactions between TFSA anions and Mg2+ ions also alter depending on the concentration. Figure 2a (right) shows the Raman band associated with the expansion/contraction of the entire TFSA anions. It is known that the Raman band appears at ca. 740 cm−1 when there is no direct coordination of TFSA
Figure 1. (a) Before and after crystallization of the mixture of Mg(TFSA)2/MgCl2/G3 1:1:2 (molar ratio) by cooling to room temperature and (b) powder-XRD profiles measured for the obtained crystals.
Schlenk equilibrium as well as in the Mg(HMDS)2 (HMDS: hexamethyldisilazide) and MgCl2 electrolyte.16 Namely, in the
Figure 2. Raman spectra for (a) Mg(TFSA)2/G3 mixtures in the range of 775−1000 (left) and 725−770 cm−1 (right) and for (b) Mg(TFSA)2− MgCl2/G3 mixtures in the range of 775−1000 (left), 725−770 (middle), and 175−500 cm−1 (right). (c) Schematic illustration showing the typical solvation structures of Mg2+ ions in the mixture of Mg(TFSA)2/MgCl2/G3 1:1:2 (molar ratio). 4733
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Figure 3. (a) Cyclic voltammograms measured for the solution of Mg(TFSA)2/MgCl2/G3 1:1:10 at 100 °C with various scan rates. Mg deposition/dissolution behavior on the working electrodes (Pt plates) and the counter electrodes (Mg ribbons) at 5.0 mV s−1 in the Mg(TFSA)2− MgCl2/G3 solutions: Mg(TFSA)2/MgCl2/G3 = (b) 1:1:30 at 100 °C, (c) 1:1:10 at 130 °C, (d) 1:1:4 at 130 °C, (e) 1:1:3 at 130 °C, and (f) 1:1:2 at 150 °C.
suggests that the solvation structure is formed mainly based on [MgCl]+. On the contrary, a remarkable Raman band appeared at 877 cm−1 in the spectrum for Mg(TFSA)2/MgCl2/G3 1:1:2, which can be assigned to a breathing mode of Mg(G3)1 because the Raman bands associated with free G3 solvent were not observed. The vibrational mode of TFSA anions appeared at ca. 741.5 cm−1 in the spectrum for Mg(TFSA)2/MgCl2/G3 1:1:2, which is lower than that (ca. 745 cm−1) for Mg(TFSA)2/G3 1:1 possibly because the interaction between TFSA anions and Mg2+ ions is weakened by the repulsive force from Cl−. Nevertheless, it is expected that most of the TFSA anions form CIPs or AGGs in the range of ca. 1 ≤ G3/Mg-salts ≤ 2 because the Raman band was clearly shifted from 739.5 cm−1. Considering that G3 has four oxygen atoms in the molecule and Mg2+ ions prefer the coordination number of six (or seven) in glyme-based solutions,15,19,21,25,26 the typical solvation structure of Mg2+ ions in the mixture of Mg(TFSA)2/ MgCl2/G3 1:1:2 can be depicted as Figure 2c. Passivation of Mg metal due to the TFSA anion is a fatal problem because it leads to a large overpotential for Mg dissolution in Mg(TFSA)2 solutions.8−10 Although the addition of MgCl2 is effective to decrease the overpotential,11−13 the passivation was still not completely inhibited in the dilute Mg(TFSA)2−MgCl2/G3 solutions. Figure 3a shows the cyclic voltammograms obtained for the solution of Mg(TFSA)2/MgCl2/G3 1:1:10 at 100 °C with various scan rates. While Mg dissolution occurs with a low overpotential at high scan rates, the overpotential increases with the decrease in the scan rate. This indicates that deposited Mg is readily passivated within a time, especially for relatively low scan rates such as 5.0 mV s−1. Figure 3b−f shows Mg deposition/ dissolution behavior at 5.0 mV s−1 in the Mg(TFSA)2−MgCl2/ G3 solutions with various concentrations, where the behavior
anions with cations, and it shifts to higher frequency in the case that TFSA anions form contact ion pairs (CIPs) or aggregates (AGGs) with cations.18,20,21 In the dilute Mg(TFSA)2/G3 solutions, the Raman band was observed at 739.5 cm−1, which indicates that TFSA anions exist as free or solvent-separated ion pairs (SSIPs).19 On the contrary, in the spectrum for Mg(TFSA)2/G3 1:1, the Raman band clearly shifted to higher frequencies, which indicates the formation of CIPs (or AGGs) with Mg2+ ions.14 As for the Mg(TFSA)2−MgCl2/G3 system, such solvation structure modification also occurs depending on the concentration (Figure 2b). The breathing mode of G3 in the spectrum for Mg(TFSA)2/MgCl2/G3 1:1:10 was red-shifted (at 867 cm−1) compared with that for the 0.5 M Mg(TFSA)2/ G3 solution (see blue curves in Figure 2). Namely, while the solvation structure of Mg(G3)2 is formed in the dilute Mg(TFSA)2/G3 electrolyte, this is not the case for the MgCl2-containing electrolyte. In addition, the vibrational mode of TFSA anions was observed at ca. 739.5 cm−1, which is very close to its Raman shift for Mg(TFSA)2/MgCl2/G3 1:1:30. These results indicate that G3 molecules would loosely coordinate to the Mg−Cl-based complexes, and TFSA anions are screened out to be SSIPs or free ions. Thus in the MgCl2containing electrolyte the Mg−Cl-based complexes tend to be formed preferentially because Mg−Cl bond is stronger than Mg−TFSA bond. As for the complex Mg−Cl vibrations such as MgCl2, [Mg2Cl3]+, and [Mg2Cl2]2+ in some ether solvents, the Raman bands were reported to appear in the range of ca. 210−240 cm−1, while [MgCl]+ was not detectable by the Raman spectroscopy.22−25 As shown in Figure 2b (right), although the Raman band at 226.5 cm−1 was slightly observed for the crystals of Mg(TFSA)2/MgCl2/G3 1:1:2, such Raman bands in the range of 210−240 cm−1 were substantially invisible for the Mg(TFSA)2−MgCl2/G3 solutions, which 4734
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controlling TFSA anions to form in-between structure of CIPs and SSIPs would be a key strategy to achieve reversible Mg deposition/dissolution with both high Coulombic efficiency and low overpotential. To evaluate the applicability of the electrolyte, we assessed the solution of Mg(TFSA)2/MgCl2/G3 1:1:2 as the electrolytes for high-temperature MRBs. Figure 4 shows the first
on the counter electrode is also shown in each graph. The selected cycle profile for each graph is after ∼10 times of sweepings. The temperature was set at a value in the range of 100−150 °C for each electrolyte, considering its thermal stability and melting temperature. As for the dilute solutions of Mg(TFSA)2/MgCl2/G3 1:1:30 and 1:1:10 (Figure 3b,c), the Mg deposition/dissolution behavior clearly shows that deposited Mg is readily passivated. It should be noted that Mg metal used for the reference electrode was also passivated, and its potential became higher than the equilibrium electrode potential of Mg2+/Mg in the solutions. The potential of Mg metal before the passivation can be estimated by tracing the behavior on the counter electrode just after Mg deposition. As shown in the Figures, the potential increase due to the passivation can be estimated to be 0.43 and 0.29 V for the solutions of Mg(TFSA)2/MgCl2/G3 1:1:30 and 1:1:10, respectively. Surprisingly, Mg dissolution behavior was drastically improved in the highly concentrated solutions in the range of ca. 1 ≤ G3/Mg-salts ≤ 2 (Figure 3d−f). The behavior on the working electrode shows that Mg dissolution occurs with low overpotential, unlike in the dilute solutions. Besides, the potential increases in Mg metal due to the passivation are lower than ∼0.1 V for these solutions. We also made cyclic voltammetry measurements for the mixtures of Mg(TFSA)2/ MgCl2/G3 1:2:10 and 1:2:4 (i.e., G3/Mg-salts = 3.3 and 1.3). Passivation of Mg was prevented in the mixture of Mg(TFSA)2/MgCl2/G3 1:2:4, whereas the overpotential for Mg dissolution was still observed for Mg(TFSA)2/MgCl2/G3 1:2:10 (Figure S4). Thus concentrating the electrolyte plays an important role to prevent passivation even for the Mg(TFSA)2/MgCl2 1:2 mixtures in the Mg(TFSA)2−MgCl2/G3 system as well. In the previous section, we have clarified that the solvation structure of Mg2+ ions is modified in such highly concentrated solutions. Thus the improved compatibility with Mg metal can be attributed to the specific coordination environment of TFSA anions in the highly concentrated solutions. As for the dilute Mg(TFSA)2−MgCl2/G3 solutions, TFSA anions would easily approach to react with the Mg metal because all TFSA anions probably exist as free or SSIPs. On the contrary, in the highly concentrated Mg(TFSA)2−MgCl2/ G3 solutions, it is considered that TFSA anions are confined in the bulk solution to form CIPs or AGGs with Mg2+ ions, which consequently would prevent the reaction on the Mg metal surface. Indeed, the presence of Cl− ions would play some roles on Mg-metal surface to help reversible Mg deposition/dissolution;10,11,24 in fact, poor reversibility was observed for the binary Mg(TFSA)2/G3 solutions (Figure S5). Actually, the generation of Mg−Cl containing active species is known to be important, especially for the deposition of Mg.5,13,25 However, to completely prevent the passivation of Mg-metal surface in TFSA-containing electrolytes, we have demonstrated that decreasing the reactivity of TFSA anions would be crucial. One drawback of the highly concentrated Mg(TFSA)2− MgCl2/G3 solutions is the low Coulombic efficiency of Mg deposition/dissolution, which is probably because TFSA anions are also reduced along with the deposition of Mg. In particular, it is considered that TFSA anions forming CIPs or AGGs with Mg2+ ions tend to be easily reduced in deposition process compared with those of SSIPs.27 Actually, the Coulombic efficiency becomes lower with the increase in the concentration in the range of 1 ≤ G3/Mg-salts ≤ 2. Thus
Figure 4. First discharge curves of MgCo2O4/Mg-metal cells with several electrolytes. The discharge was conducted at C/20 without any precharge process.
discharge curves of the full-cells using MgCo2O4 cathode and Mg-metal anode, with several electrolytes. It has been reported that MgCo2O4 can accommodate a large amount of Mg at elevated temperatures around 150 °C by utilizing the spinel-torocksalt transition (its theoretical energy density with Mgmetal anode exceeds 600 Wh kg−1).4 In the electrolyte of 0.5 M Mg(TFSA)2/G3, the cell voltage is rapidly decreased to 0 V, which indicates that the cathode does not work well at room temperature due to the sluggish Mg diffusion inside the oxide. On the contrary, in the electrolytes at 150 °C, the discharge capacity corresponding to Mg insertion into the MgCo2O4 cathode amounts to >150 mAh g−1, which is consistent with previous reports.4,28 The most important point for constructing high-potential MRBs is to enhance the performance on the Mg-metal anode. In fact, the cell voltage exhibited only ∼1 V during the discharge in the electrolytes of Mg(TFSA)2/G3 1:1 and (Mg10/Cs90)TFSA due to the large overpotential for Mg dissolution on the Mg-metal anode. However, the cell with the electrolyte of Mg(TFSA)2/MgCl2/G3 1:1:2 delivered the highest cell voltage of ∼2 V because both the MgCo2O4 cathode and the Mg-metal anode can work with low overpotential. From the obtained discharge curve, the electrode energy density amounts to 256 Wh kg−1, which is much higher than that of the prototyped Mg battery using Mo6S8 cathode and Mg-metal anode.29 The anodic limit of this electrolyte is not high enough to fully charge the MgCo2O4 cathode (Figure S7), so the improvement of the oxidative stability of the electrolyte would be essential to improve the cyclability in this MRB system. In conclusion, we have revealed that the passivation of Mg metal can be prevented by controlling the solvation structure of Mg2+ ions in the Mg(TFSA)2−MgCl2/G3 solutions. Although MgCl2 is hardly soluble in pure G3 solvent, it can be dissolved in high concentration by utilizing a Schlenk-like reaction with Mg(TFSA)2 and strong chelating ability of G3. On the basis of the results of Raman spectroscopy, we have clarified that the formation of Mg(G3)1 inevitably confines 4735
DOI: 10.1021/acs.jpclett.8b02209 J. Phys. Chem. Lett. 2018, 9, 4732−4737
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The Journal of Physical Chemistry Letters TFSA anions in highly concentrated Mg(TFSA)2−MgCl2/G3 solutions. Actually, by the electrochemical measurements at elevated temperatures around 150 °C, we have shown that the overpotential for Mg dissolution is markedly suppressed in such highly concentrated Mg(TFSA)2−MgCl2/G3 solutions in the range of 1 ≤ G3/Mg-salts ≤ 2. Consequently, we can conclude that the capability of preventing the passivation is attributed to the substantial decrease in free TFSA anions. By utilizing the improved electrochemical property and the enhanced thermal stability of the highly concentrated Mg(TFSA)2−MgCl2/G3 solution, we have demonstrated the fullcell using MgCo2O4 cathode, and Mg-metal anode can deliver a cell voltage of ∼2 V during a discharge process at 150 °C.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.8b02209. Experimental details, additional figures for Raman spectra, cyclic voltammograms, and discharge/charge performance. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Tetsu Ichitsubo: 0000-0002-1127-3034 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST) and Grantin-Aid for JSPS Research Fellow (No. 17J08120) from the Japan Society for the Promotion of Science (JSPS).
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REFERENCES
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