Methylsulfonylmethane-Based Deep Eutectic Solvent as a New Type

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Methylsulfonylmethane-Based Deep Eutectic Solvent as a New Type of Green Electrolyte for a High-Energy-Density Aqueous Lithium-Ion Battery Ping Jiang,†,‡,⊥ Liang Chen,*,† Hezhu Shao,† Shaohua Huang,§ Qiushi Wang,† Yuebin Su,† Xiaoshuang Yan,‡ Xinmiao Liang,‡ Jiujun Zhang,∥ Jiwen Feng,*,‡ and Zhaoping Liu*,† †

Advanced Li-Ion Battery Engineering Laboratory and Key Laboratory of Graphene Technologies and Applications of Zhejiang Province, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Science, Ningbo 315201, P. R. China ‡ Wuhan Institute of Physics and Mathematics, Chinese Academy of Science, Wuhan 430071, P. R. China § Institute of Drug Discovery Technology, Ningbo University, Ningbo 315211, P. R. China ∥ Institute for Sustainable Energy/College of Sciences, Shanghai University, Shanghai 200444, P. R. China ⊥ University of Chinese Academy of Sciences, Beijing 100000, P. R. China S Supporting Information *

ABSTRACT: Currently used aqueous electrolytes for lithium-ion batteries suffer from narrow potential windows of 0.3). This DES electrolyte performs with a satisfying electrochemical stability window (∼3.5 V), which enables the LiMn2O4/Li4Ti5O12 aqueous lithium-ion battery with both high energy density (>160 W h kg−1) and high capacity retention (72.2% after 1000 cycles). Overall, our study of this new deep DES electrolyte can enrich room-temperature “water-in-salt” aqueous electrolyte and offers new insights into exploring green aqueous electrolyte systems for lithium-ion batteries.

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Regarding the application feasibility of such aqueous electrolytebased lithium batteries, effectively restraining water decomposition side reactions for a wide electrochemical stability widow (>3.0 V) should be the major research effort. In previous work, superconcentrated aqueous electrolytes with the molarity of charge-transfer cations above 20 mol kg−1, namely, “water-insalt” electrolytes such as highly concentrated LiN(SO2CF3)2 (LiTFSI), LiTFSI/Li(SO2C2F5)2, LiTFSI/Zn[N(SO2CF3)2]2, LiN(SO2CF3)2, and LiN(SO2CF3)2/CF3SO3Li solutions,5−7,9 etc., have been explored. Generally, the strong Lewis-acidic cations (Li+, Na+) or strong Lewis-basic (F−, Cl−) anions can form ionic crystals at room temperature, but in these electrolytes, Li+ can coordinate with the oxygen donor in water or the anion, forming a metal−coordination complexes cation and making these salts soluble. In such electrolytes, nearly

ithium-ion batteries as the most common rechargeable battery systems for vehicle electrification and grid energy storage perform with high energy density, cycle stability, and Coulombic efficiency1−3 but suffer from the toxic organic electrolytes and potential safety issues. Therefore, the nonflammable and low-toxicity aqueous electrolytes which can enable the lithium battery with a high electrochemical performance is highly desirable to substitute nonaqueous electrolyte.4−9 However, the narrow electrochemical windows of aqueous electrolytes ( 0.3. Moreover, when the molar ratios of LiClO4/MSM are not in the range from 1.8 to 3, the composition of MSM− LiClO4−H2O mixtures can hardly reach a stable liquid phase at room temperature when the water content is low. As a result, ternary MSM−LiClO4−H2O mixtures can only form a liquid DES consisting of an appropriate proportion of MSM, LiClO4, and H2O. The physicochemical properties of MSM−LiClO4− H2O with a molar ratio of 1.8:1:z (0.5 ≤ z ≤ 16) are presented in Figure 1c and Table S1 in the Supporting Information. Obviously, raising the water ratio z in MSM−LiClO4−H2O (1.8:1:z) can improve the ionic conductivity and Li+ diffusion coefficient, while the viscosity of this DES system is decreased. The ionic conductivity rises with increasing the water molar ratio from 0.3 to 16, and the same trend can be observed of the viscosity property and Li+ diffusion coefficient. When the molality of water in MSM:LiClO4:H2O = 1.8:1:0.5 (DES-0.5) is as high as 111.1 mol kg−1, the viscosity, conductivity, and Li+ diffusion coefficient are 213 mPa s at 22 °C, 2.27 mS cm−1, and 0.9 × 10−11 m2 S−1, respectively. The Li+ diffusion coefficient is lower than that of Li-salt-based nonaqueous solutions because the Li+ is relatively immobilized in DES, which results from the strong coordination with MSM when DES has a low water content (X < 0.5). However, the Li+ is still closer to the diffusion state when the water is unevenly distributed in the section around Li+. In this work, a DES solution (named as DES-1 with a conductivity of 3.71 mS cm−1, viscosity of 148 mPa s, and Li+ diffusion coefficient of 2.27 × 10−11 m2 S−1) is used as an electrolyte in the following study. Liquid Structure. Many studies have discussed the structure of Li+-based “water-in-salt” electrolyte or solvate ionic liquids,21−29 since the property of the electrolyte depends highly on their composition, structure, and dynamics. Thus, the structures of the new room-temperature DES solution and the interaction among MSM, LiClO4, and H2O were further studied for a fundamental understanding. The structural characteristics of MSM−LiClO4−H2O with different molar ratios were first explored by vibration spectra (Raman and FTIR), 17O NMR, and DFT calculation. Figure 2a,b and Figures S1−S4 in the Supporting Information display the OH, SO, and ClO stretching vibration modes of various DES solutions with different water component proportions, respectively. It can be seen that, for 1 M LiClO4 and the saturated MSM and DES-38 solutions, the OH stretching vibration gives rise to a broad Raman bands in the range from 2900 to 3700 cm−1. These bands could be attributed to hydrogen-bonded water molecules, which is similar to pure water with a strong hydrogen bonding.30,31 The narrow peak on 1422

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Figure 3. (a) Structure of MSM:LiClO4 = 2:1 DES solution calculated from DFT. (b) Liquid structure of MSM:LiClO4:H2O = 2:1:1 DES electrolyte calculated from DFT.

and Figure S5. Figure 2d summarizes the change of 17O chemical shifts (H2O, MSM, ClO4−) of various DES solutions. It can be seen that the 17O signal of DES is rather sensitive to the chemical surrounding of the O atom due to the concentration change and the direct interaction among Li+, oxygen in H2O, MSM, and ClO4−. When water content is low (z = 1), the 17O resonance of H2O appears at −8.2 ppm, about a −8.2 ppm upfield shift relative to one of pure water. This is a clear indication of direct Li+−H2O interaction, since Li+−H2O significantly shifts the 17O peak upfield whereas the ClO4−−H2O interaction is expected to slightly move the 17O peak of H2O upfield.36 With the water content increasing, the 17O chemical shift of H2O increases from −8.2 to −0.05 ppm. The same trend of MSM 17O signals can be found when the water molar ratio x is less than 16. Similarly, this increase in 17O chemical shift of MSM molecules can be attributed to a decrease in MSM molecules coordinated with Li+ ions. As water content is further increased, the 17O chemical shift of MSM decreases to 158 ppm, which is close to that of the saturated MSM (157 ppm), indicating that the MSM−H2O interaction prevails at a high water content. Meanwhile, ClO4− signals increased from 292.8 to 293.8 ppm when the molar ratio of water x rises to 6 and then decreased to 291 ppm in DES-38. As a result, the coordination of three oxygen lone pair electrons in H2O, MSM, and ClO4− with Li+ is mostly affected by water content. The broad 17O peaks of MSM and H2O and the smooth 17 O peak of ClO4− are induced by the high-viscosity liquid (>148 mPa S) when the molar ratio of water is less than 2. Such an intimate interaction between the Li+ cation and ClO4− anion, which has been revealed, respectively, by Raman, FTIR, and NMR spectra, is like ionic liquid, where each Li+ is surrounded by at least one ClO4− anion on average. Considering that Li+ is in a relatively complex environment, which could be coordinated with H2O, MSM, and ClO4−, the 1H NMR experiment of the MSM−LiClO4−H2O DES solutions was further performed to observe the stoichiometry between LiClO4 and H2O or between MSM and H2O, and the results obtained are shown in Figure 2e and Figure S6 and Table S2, respectively. The 1H NMR results show a nearly swelling curve (Figure 2f,g), probably suggesting that the H2O−LiClO4 can form a 1:1 complexation in solutions, and H2O−MSM forms a 1:2 complex structure by hydrogen bonding.37 Usually, Li+ coordinated with the oxygen in water can contribute to the deshielding of the proton. The hydrogen nucleus becomes more deshielded with successively increasing the initial water content

and decreasing the initial concentration of lithium. After further increasing the water content, the 1H nucleus becomes less deshielded but still has a lower chemical shift than free-water (4.8 ppm) (Figure 2e). The asymmetry of the curve is possibly caused by the Li+ cation coordinating to the SO bond in MSM as well as ClO4−. Thus, adding a tiny amount of water could lead to a Li-coordinated complex with hydrogen bonding in the MSM−LiClO4 DES solution, which could decrease the melting point to −48 °C (Figure S7). From all the information above on the spectra, Li+ remains well-hydrated in its primary solvation sheath by coordinating with the oxygen atom in H2O, MSM, and ClO4− in DES-1 solutions. In addition, one Li+ coordinates with one H2O, and one H2O hydrogen bonds with two MSMs. The relationship between solution structure and electrochemical properties is further studied from DFT calculations as described in the following section. DFT calculations are used to gain theoretical insights into the possible structure of MSM−LiClO4−H2O DES-1 electrolyte. The results further indicate that, on average, two MSM solvates (coordinated-MSM and free-MSM), one water molecule, and one ClO4− anion are in each Li+ primary solvation sheath. The possible geometrical structures of MSM−LiClO4 (1:1) and MSM−LiClO4−H2O (1.8:1:1) are simulated, as shown in Figure 2 and Figure S8. It can be seen that the oxygen atoms in ClO4−, MSM, and H2O have lone pairs that can interact with Li+. The Li−O bond length between Li and water (1.97 Å) is slightly shorter than that between Li+ and the coordinated-MSM (2.00 Å), indicating that the Li+ has a little stronger affinity to the O atom in water than to MSM. Such an effect can restrain the movement of water molecules, thus lowering the water activity. The distance of Li+ and the nearest O in ClO4− is 1.98 Å, which is slightly longer than the Li−O bond. Therefore, in DES-1 solution, Li+ and ClO4− or MSM can be separated freely, which is beneficial to Li+ transportation. Moreover, Figure S9 shows DFT calculations of H2O IR spectra; there are no broad bands observed for MSM−LiClO4−H2O DES-1, suggesting that nearly all water molecules are coordinated with Li+, and almost no free-water exists in such a Li+-coordination. Such an effect can restrain the movement of water molecules, thus lowering the water activity, leading to a highly limited hydrogen evolution reduction. In addition, the melting point of DES electrolyte is about 49 °C in the LiClO4−MSM system (MSM:LiClO4 = 1.8:1),15 in which one Li+ coordinates with two O atoms in MSM (Figure 1423

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Figure 4. (a, upper part) linear sweep voltammetry (LSV) of current collector Ti in DES-1 (MSM:LiClO4:H2O = 1.8:1:1) electrolyte and a typical aqueous solution of 1 mol L−1 LiClO4/H2O and (a, lower part) cyclic voltammograms (CVs) of LiMn2O4 (LMO) and Li4Ti5O12 (LTO) electrodes in DES-1 electrolyte. All CVs are obtained with the geometric electrode area of 1.32 cm2. For the LSVs in the upper part, the potential scan rate is 2 mV s−1, and that in the CVs in the lower part is 0.2 mV s−1. Note that the CVs at the right-hand side of the lower part are obtained on the LMO electrode in three different electrolytes; one is the DES-1 (red curve), one the LITFSI (green dashed line), and the other the 1 M LiClO4 (light red short line). The CV at the left-hand side of the lower part is obtained on the LTO electrode in DES-1 electrolyte. (b) Illustration of the expanded electrochemical stability window for water-in-salt electrolytes (DES-1 and LiTFSI (21 M)) together with the formal potentials of the LMO cathode and LTO anode, induced by a high salt concentration, as well as the comparison with those in 1 M LiClO4 solution and pure water. (c) Galvanostatic profiles of the LMO/DES-1/LTO battery along with the voltage profiles of their individual anode and cathode electrodes vs SHE at a rate of 1.5C. (d) Cycling performance of the cathode LMO at a rate of 4.5C, anode LTO at a rate of 20C, and LMO/DES-1/LTO battery at a rate of 4.5C, respectively.

3a), but in EDS-1 solution (Figure 3b), the hydrogen bond length between H2O and free-MSM is about 1.80 Å; free-MSM does not coordinate with Li+ but participates in the Li+ solvation sheath. These data indicate that MSM can dissociate crystal ionic salt LiClO4 to form DES electrolyte solution. Therefore, adding a tiny amount of water into the LiClO4−MSM system can lead to a room-temperature MSM−LiClO4−H2O DES electrolyte solution due to the fact that the formed hydrogen bond between H2O and MSM (free-MSM and coordinatedMSM) can break the coordination of Li+ with MSM, forming the Li−H2O bond, resulting in a strong effect on the melting point through H2O−MSM hydrogen bonding. New Deep Eutectic Solvent for Li-Ion Batteries. The linear sweep voltammetry (LSV) technique was employed to evaluate the electrochemical stability window of DES-1. Figure 4 and Figure S10 display the LSVs of bare carbon paper and Ti electrodes (current collectors) at a scan rate of 2 mV s−1 in DES-1 solution together with aqueous LiClO4 (1 M) as a reference. As shown, DES-1 shows the remarkable wide electrochemical stability windows with two electrodes (≥3.5 V), which are 2 times higher than those of aqueous LiClO4. Moreover, the electrochemical stability of the DES-1 is also found to be strongly dependent on current collectors, which follow the order in DES-1 of carbon paper < steel < Ti (see Figure S10). The onset reduction potential of DES-1 with Ti is lower than the one with the carbon paper electrode. The same trend can be observed for the

electrochemical oxidation. Thus, Ti is preferable for the current collector. Spinel Li4Ti5O12 and LiMn2O4 with reaction potentials of approximately −1.45 and 1.2 V vs SHE, respectively, were selected as active electrode materials to verify the expanded electrochemical stability window. Notably, a reversible redox pair corresponding to the Li+-(de)intercalation reaction of Li4Ti5O12 appears at −1.45 V versus SHE (see Figure 4a). However, in saturated lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) and 1 M LiClO4 aqueous solutions, no redox pairs could be observed (not shown here), indicating that hydrogen evolution could still overwhelm the process during the Li+-(de)intercalation reaction. Two reasons account for this: one is that the increase of Li+-activity in DES-1 compared with the 1 M LiClO4 and saturated LiTFSI could lead to the upshift of the potential for the Li+-(de)intercalation reaction in Li4Ti5O12 (Nernst shift), and the other is that the suppression of water activity of DES-1 could result in a large overpotential for hydrogen evolution. For comparison, the E1/2 (formal potential) of LiMn2O4 anodic and Li4Ti5O12 cathodic potentials in DES-1, LiTFSI, and 1 M LiClO4 and the electrochemical windows are shown in Figure 4b. It can be seen, for the spinel LiMn2O4, that the remarkable upshifts of the redox potentials can be observed in DES-1 and saturated LiTFSI compared to 1 M LiClO4. At a scan rate of 0.2 mV s−1, the values of E1/2 follow the order LiTFSI (1.40 V) > DES-1 (1.39 V) > 1 M LiClO4 (1.22 V). Obviously, the kinetic rates of the electrochemical Li+1424

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calculations. The obtained results show that the possible structure of the DES-1 (MSM:LiClO4:H2O = 1.8:1:1) solution has one Li+ coordinated to oxygen atoms from one H2O, one ClO4−, and two MSMs and that one H2O hydrogen is bonded with two MSMs. This hydrogen bonding can significantly decrease the melting point to −48 °C, resulting in a roomtemperature solution. Particularly, when the water content molar ratio is 1, the formed DES-1 shows a satisfying electrochemical stability window (about 3.3 V). This environmentally friendly and nonflammable electrolyte enables the aqueous lithium-ion battery with high energy density (>160 W h kg−1) and high working voltage of 2.4 V. A lithium-ion battery consists of positive LiMn2O4 (LMO) and negative Li4Ti5O12 (LTO) electrodes and DES-1 aqueous electrolyte which can have a capacity retention of 72.2% after 1000 cycles. Overall, our study of this new deep eutectic solvent can enrich the roomtemperature “water-in-salt” aqueous electrolyte and offers new insights into exploring a green aqueous electrolyte system for lithium-ion batteries, which greatly broadens our horizons for electrolyte and battery research.

(de)intercalation reaction in DES-1 and saturated LiTFSI become much slower, as their ionic conductivities are two magnitudes lower than that of 1 M LiClO4. Moreover, the rate capability and cycling stability of Li4Ti5O12 and LiMn2O4 in DES-1 were investigated by a galvanostatic technique. As shown in Figure S11, the high-rate capacity retention of Li4Ti5O12 is excellent in DES-1, such as for its 90% capacity retention at 10C and 81% at 20C. Its Coulombic efficiency increases from 89.4% (1C) to 99.5% (20C) with increasing the charging rates, and about 85.2% retention of the initial discharge capacity after 1000 cycles at 20C (Figure 4d, Figure S12) can be obtained. As for LiMn2O4, its capacities are 106.2, 91.1, 52.8, and 38.5 mA h g−1 at 1C, 2C, 4C, and 5C, respectively, while the Coulombic efficiency remains nearly 100% at various rates (≥99.6%). These results suggest that oxygen evolution at the LiMn2O4 electrode is almost eliminated despite the high voltage charge limits (1.3 V versus SHE) for DES-1. Additionally, the cycling life of LiMn2O4 in DES-1 is also very impressive. For example, after 1000 cycles at a rate of 4.5C, the capacity retention of LiMn2O4 can remain 91%. On the basis of the excellent electrochemical performances of Li4Ti5O12 and LiMn2O4, a full cell Li4Ti5O12/DES-1/LiMn2O4 is assembled for performance validation. Its galvanostatic profiles at a rate of 1.5C along with the individual Li4Ti5O12 and LiMn2O4 electrode potentials are shown in Figure 4c and Figure S13. As indicated, the Li4Ti5O12 anode exhibits a voltage plateau of −1.13 V versus SHE, and the LiMn2O4 cathode shows a voltage plateau of 1.27 V versus SHE. Meanwhile, the full cell can deliver a high voltage output of 2.4 V with high energy density (>160 W h kg−1), despite the relatively low Coulombic efficiency of 92.4%. Under high-rate cycling at 4.5C for 1000 cycles, the capacity retention is 72.2%, and the Coulombic efficiency approaches nearly 100% (98.9−99.5%) (see Figure S14). However, under the rate of 2C shown in Figure S15, the capacity decreases rapidly. That is maybe because the electrode/ electrolyte interface is indeed not stabilized, considering that there is no obviously solid electrolyte interface (SEI)-related semicircle of LTO in EDS-1 (see Figure S16) when the polarization is not very high (about 80 Ω). All of the above electrochemical results have demonstrated that DES-1 can be successfully used as electrolytes for high-voltage aqueous Li-ion batteries at a high rate beyond 2.4 V. Regarding the charging/ discharging processes of lithium-ion batteries, the Li+-coordinating structure in the electrolyte can change by solvation/ desolvation of the Li+ cation at the electrode/electrolyte interfaces. The discharging process generates more free-MSM and free-water molecules on the cathode surface. This may be the reason for the unstable electrode/electrolyte interface on the LTO electrode, which causes the degradation of the battery at low charge/discharge rates. However, it is reasonable to consider that the free-MSM and free-water molecules have a hydrogen bonding effect when the DES solution has a relatively low water content, which can partly limit the water decomposing on the anode electrode/electrolyte surface at a high rate when a short time is spent in the limit of the electrolyte instability. In summary, a brand new type of lithium salt deep eutectic solvent (DES) consisting of MSM, LiClO4, and H2O is reported in this Letter. The Li+ cation is coordinated with oxygens in MSM, H2O, and ClO4− to form Li+-coordinated complexes, leading to a superconcentrated Li salt solvate. To analyze the structure of DES electrolytes and their relationship with the battery performance, several methods are employed, including Raman, FTIR, micro-FTIR, NMR, cyclic voltammetry, and DFT

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.9b00968. Experimental methods and additional data and figures including electrochemical performance, spectral analysis, DSC experiment, and DFT calculation supplement, and the composition and properties of a series of DES (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hezhu Shao: 0000-0002-8945-6973 Jiujun Zhang: 0000-0002-6858-4060 Jiwen Feng: 0000-0002-9691-8574 Zhaoping Liu: 0000-0002-3943-8953 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from Key Research Program of Chinese Academy of Sciences (Grant 2016YFB0100100), National Natural Science Foundation of China (Grant 11474314, 21603267), and Youth Innovation Promotion Association CAS (Grant 2017341).

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REFERENCES

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ACS Energy Letters

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DOI: 10.1021/acsenergylett.9b00968 ACS Energy Lett. 2019, 4, 1419−1426