Methylsulfonylmethane-Based Deep Eutectic Solvent as a New Type

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Methylsulfonylmethane-based Deep Eutectic Solvent as a New Type Green Electrolyte for 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 ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 23 May 2019 Downloaded from http://pubs.acs.org on May 24, 2019

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ACS Energy Letters

Methylsulfonylmethane-based

Deep

Eutectic

Solvent as a New Type Green Electrolyte for HighEnergy-Density Aqueous Lithium-Ion Battery

Ping Jianga,b,e, Liang Chena*, Hezhu Shaoa, Shaohua Huangc, Qiushi Wanga, Yuebin Sua, Xiaoshuang Yanb, Xinmiao Liangb, Jiujun Zhangd, Jiwen Fengb*, Zhaoping Liua*

aAdvanced

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 bWuhan

Institute of Physics and Mathematics, Chinese Academy of Science, Wuhan

430071, P.R. China cInstitute dInstitute

of Drug Discovery Technology, Ningbo University, Ningbo 315211, P.R. China for Sustainable Energy/College of Sciences, Shanghai University, Shanghai

200444, P.R. China eUniversity

of Chinese Academy of Sciences, Beijing 100000, P.R. China

E-mail: [email protected], [email protected], [email protected] AUTHOR INFORMATION Corresponding Author *Zhaoping Liu: [email protected]

*Jiwen Feng: [email protected]

*Liang Chen:

[email protected]

ACS Paragon Plus Environment

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ABSTRACT: Currently used aqueous electrolytes for lithium-ion batteries suffer from narrow potential windows of 0.3). This DES electrolyte performs a satisfying electrochemical stability window (~3.5 V), which enabling LiMn2O4/Li4Ti5O12 aqueous lithium-ion battery with both high energy density (> 160 Wh 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 lithiumion batteries.

TOC GRAPHICS


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Lithium-ion batteries as the most common rechargeable battery systems for vehicle electrification and grid energy storage perform high energy density, cycle stability, and coulombic efficiency1-3, but suffering from the toxic organic electrolytes and potential safety issue. Therefore, the nonflammable and low-toxicity aqueous electrolytes which can enable lithium battery with high electrochemical performance is highly desirable to substitute nonaqueous electrolyte4-9. However, the narrow electrochemical windows of aqueous electrolytes (< 1.5 V) have limited their application. If the aqueous electrolytes want to replace the non-aqueous electrolytes as the advanced lithium-based energy storage device, they should possess the following properties : (i) the enhanced stability for overcoming the narrow electrochemical stability window4-7, 9, (ii) high thermal stability for maintaining a stable liquid state over a large range of temperature4, 9, (iii) high conductivity for rapid ion transport to the electrode surfaces during charge-discharge4,

9-10,

(iv) high salt concentration for increasing the ion insertion

potential4-7, 10, and (v) low cost. Regarding 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, super concentrated aqueous electrolytes with the molarity of charge-transfer cations above 20 mol Kg-1, namely “water-in-salt” 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 solutions5-7, 9, etc., have been explored. Generally, the strong Lewisacidic 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 oxygen donor in water or anion, forming metal-coordination complexes cation, and making these salts soluble. In such electrolytes, nearly all water molecules form primary solvation sheath surrounding cations


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(coordinated-water), and there are almost no free water molecules (uncoordinated-water). As a result, the water activity is largely suppressed, leading to remarkably increased over-potentials of hydrogen/oxygen evolution, which is beneficial to the widening of potential widows of the electrolyte. Meantime, the potential equilibrium of the cation-intercalation reaction is also thermodynamically upshifted in “water-in-salt” electrolytes when compared to that of dilute ones, which is also a beneficial factor. However, these “water-in-salt” electrolytes are limited by the high price and toxicity in terms of practical applications. Therefore, new safe, green and lowcost electrolytes for aqueous electrolyte lithium batteries are demanded to be further explored and investigated. In literatures, some effort has been devoted to non-aqueous deep eutectic solvents (DES) based on some organic solvates and lithium salts for aqueous electrolyte for lithium-ion batteries.8-9,

11-13

As recognized, a non-aqueous DES always exhibits low vapor pressure14,

relatively wide-liquid range, and non-flammability. MSM as the solvate is valuable for forming eutectic mixture with lithium salts, such as LiN(CF3SO2)2-MSM, LiBF4-MSM, LiClO4-MSM and LiNO3-MSM system15-16. However, all of these non-aqueous DES can only be used at high temperatures that set an intrinsic limit on their practical applications of energy storage devices at low temperatures. For instance, MSM-LiClO4 with a molar ratio of 1.8:1 has the melting point of 49 oC that still do not meet the criteria of room-temperature application of battery. In this work, we report a new aqueous deep eutectic solvent (DES) based on methylsulfonylmethane (MSM), LiClO4 and H2O as aqueous electrolyte for lithium-ion batteries. This new room temperature eutectic solvent can be obtained just by mixing a amount of MSM, LiClO4 and LiClO4·3H2O according to a certain percentage. The formed electrolyte shows a significantly improved conductivity and viscosity if the


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content of water in the DES is controlled. We have also found that the distinct differences of the formed Li-complexes between the conventional “water-in-salt” electrolyte and the new DES4,

6-7, 10, 17.

In a [Li-MSM1.8H2O0.3]+ cation complexes, the Li+ cation is

coordinated with oxygens in all three MSM, H2O and ClO4- species, forming a superconcentrated Li-salt electrolyte (185 mol Kg-1, molality in water). A super ionic conductivity as high as 1.41 mS cm-1 at room temperature is achieved. In addition, the electrochemical stability of a DES named as DES-1(MSM:LiClO4:H2O=1.8:1:1, 56 mol Kg-1, 3.87 mS cm-1) is used as the electrolyte to construct a LiMn2O4 /Li4Ti5O12 aqueous lithium-ion battery, which is electrochemically tested for 1000 charge/discharge cycles at a high current density of 4.5C, and nearly 100% (98.9%-99.5%) coulombic efficiency and 72.2% of retention capacity are obtained, respectively. To fundamentally understand the performance mechanism, the Li+ solvation/local structure and the macroscopic properties of the electrolyte solutions are studied by Raman spectra analysis /Micro-infrared spectra, 17O

nuclear magnetic resonance (NMR) chemical shift, and 1H NMR experiment, as well

as theoretical calculations (ab initio molecular orbital calculations). New deep eutectic solvent at room temperature Usually, deep eutectic solvents (DES), classified as types of ionic solvents with special properties, are systems formed from a eutectic mixture of anionic Lewis or Brønsted acid and cationic base species. DES can incorporate one or more compound as a third component in a mixture form, to give a eutectic with a melting point much lower than either of the individual components. The component of Lewis or Brønsted acid is in principle any cation such as ammonium, phosphonium, or metal ions, and the Lewis base is generally a halide anion, and the third component can coordinate with anion and/or cation to form complex(s).


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Regarding the base in DES solvent, the notably large dielectric constants (47.39) and dipole moments (4.44 D)18 of MSM make it a unique candidate solvent for polar compounds with low polarity. Moreover, owning to the abundant sulfony group, MSM is considered to be a good oxygen donor (lone electron pair donor) and can serve as a strong Lewis base to coordinate with cations, especially alkaline cations (electron-withdrawing group). Due to the strong interaction between MSM and cations (C+), they can form C+-MSM coordination complexes as strong Lewis acid for anions (A-), which then dissociate crystal ionic salt (C+A-) by “plasticizing” effect, leading to a C+-coordinate8. In this work, we have discovered that adding a tiny of water into the binary LiNO3-MSM or LiClO4-MSM mixtures can substantially decrease the melting points19, which is beneficial to their practical application as green electrolytes for aqueous lithium batteries, and the conductivity and viscosity of such electrolytes can be adjusted by the proportion of water in eutectic solvent. As shown in Figure 1a, when a water content19-20 as low as about 1.9% (mass ratio) is added into the 1.8MSM-LiClO4, the formed DES shows an extremely high miscibility with water at room temperature, forming a stable, transparent and colorless homogeneous liquid. Figure 1b summarizes the states of MSM-LiClO4-H2O mixtures (solid, liquid and solid/liquid) with different compositions. When the molar ratios of MSM/LiClO4 (n) keep between 1.8 and 3, ternary MSM-LiClO4-H2O mixtures with a molar ratio of n:1:x can maintain the liquid state with x > 0.3. Moreover, when the molar ratios of LiClO4/MSM is not in the range of 1.8 and 3, the composition of MSM-LiClO4-H2O mixtures can hardly get 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 consisted of 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≤x≤16) is presented in Figure 1c and supporting information


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Table S1. Obviously, raising the water ratio x 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 raises 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 is 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 non-aqueous solutions because the Li+ is relatively immobilized in DES, which is resulted from the strongly coordinating with MSM when DES has a low water content (X148 mPa S) when the molar ratio of water is less than 2. Such an intimate interaction between Li+ cation and ClO4- anion, which has 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 Li+ is in a relatively complex environment, which could be coordinated with H2O, MSM and ClO4-, 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, Figure S6 and Table S2, respectively. The 1H NMR results show a nearly swelling curve (Figure 2f, 2g), probably suggesting that the H2O-


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LiClO4 can form a 1:1 complexation in solutions, and H2O-MSM does a 1:2 complex structure by hydrogen bonding37. Usually, Li+ coordinated with the oxygen in water can contribute to the deshielding of proton. The hydrogen nucleus becomes more deshielded with successively increasing the initial water content and decreasing the initial concentration of lithium. After further increasing water content, the 1H nucleus becomes less deshielded, but still has a lower chemical shift than that of free water (4.8 ppm) (Figure 2e). The asymmetry of the curve is possibly caused by Li+ cation coordinating to the S=O bond in MSM as well as ClO4-. Thus, adding tiny water could lead to a Li-coordinated complex with hydrogen bonding in the MSMLiClO4 DEC solution, which could decrease the melting point to -48 oC (Figure S7). From all the information of spectra above, Li+ remains well hydrated in its primary solvation sheath by coordinating with 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 MSM. 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 get theoretical insights into the possible structure of MSMLiClO4-H2O DES-1 electrolyte. The results further indicate that on average, two MSM solvates (coordinated MSM and free MSM), one water molecule, one ClO4- anion are in each Li+ primary solvation sheath. The possible geometrical structure of MSM-LiClO4 (1:1) and MSM-LiClO4H2O (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 O atom in water than to MSM. Such an effect can restrain the movement of water molecules, thus lower the water


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activity. The distance of Li+ and nearest O in ClO4- is 1.98 Å, which is slightly longer than Li-O bond. Therefore, in DES-1 solution, Li+ and ClO4- or MSM can be separated freely, that 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 are existed in such a Li+coordination. Such an effect can restrain the movement of water molecules, thus lower the water activity, leading to a highly limited hydrogen evolution reduction. In addition, the melting point of DES electrolyte is about 49 oC in LiClO4-MSM system (MSM:LiClO4=1.8:1)15, in which one Li+ coordinates with two O atom in MSM (Figure 3a), but in EDS-1 solution (Figure 3b), the hydrogen bond length between H2O and free MSM is about 1.80 Å, and 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 tiny water into LiClO4-MSM system can obtain a room temperature MSM-LiClO4-H2O DES electrolyte solution due to that the formed hydrogen bond between H2O and MSM (free MSM and coordinated MSM) can break the coordination of Li+ with MSM, forming Li-H2O bond, resulting in strong effect on the melt point through H2O-MSM hydrogen bonding. New deep eutectic solvent for Li-ion batteries 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


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ACS Energy Letters

electrochemical stability windows with two electrodes (≥ 3.5 V), which are two 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: carbon paper DES-1 (1.39 V) > 1 M LiClO4 (1.22 V). Obviously, the kinetic rates of electrochemical Li+-(de)intercalation reaction in DES-1 and saturated LiTFSI become much


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slower, as their ionic conductivity 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 galvanostatic technique. As shown in supplementary 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% (1 C) to 99.5% (20 C) 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 mAhg-1 at 1C, 2C, 4C and 5C, respectively, while the coulombic efficiency keeps nearly 100% at various rates (≥99.6%). These results suggest that oxygen evolution at LiMn2O4 electrode is almost eliminated despite the high voltage charge limits (1.3 V vs. 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%. Based on 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 supplementary Figure S13. As indicated, Li4Ti5O12 anode exhibits a voltage plateaus of -1.13 V vs. SHE, and LiMn2O4 cathode shows a voltage plateaus of 1.27 V vs. SHE. Meanwhile, the full cell can deliver a high voltage output of 2.4 V with high energy density (> 160 Wh Kg-1), despite the relatively low coulombic efficiency of 92.4%. Under high-rate cycling at 4.5 C for 1000 cycles, the capacity retention is 72.2% and the coulombic efficiency approaches nearly 100% (98.9%-99.5%) (see supplementary Figure S14). But under the rate of 2C show in supplementary Figure S15, the capacity decreases rapidly. That is maybe because the electrode/electrolyte interface is not indeed stabilized, considering there is no obviously SEI-


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ACS Energy Letters

related semicircle of LTO in EDS-1 (see supplementary Figure S16) when the polarization is not very high (about 80 Ω). All above electrochemical results have demonstrated that DES-1 can be successfully used as electrolytes for high-voltage aqueous Li-ion batteries at 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. Discharging process generates more free-MSM and free-water molecules on the cathode surface. This may be the reason of unstable electrode/electrolyte interface on 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 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 high rate when short time is spent in the limit of the electrolyte instability. In summary, a brand new type of lithium salt deep eutectic solvent (DES) consisted of MSM, LiClO4 and H2O is reported in this paper. The Li+ cation is coordinated with oxygens in MSM, H2O and ClO4- to form Li+-coordinated complexes, leading to a super-concentrated 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, and cyclic voltammetry, DFT calculations. The obtained results show that the possible structure of DES-1 (MSM:LiClO4:H2O=1.8:1:1) solution has one Li+ coordinated to oxygen atoms from one H2O, one ClO4- and two MSM, and that one H2O hydrogens are bonded with two MSM. This hydrogen bonding can significantly decrease the melting point to -48 oC, resulting in a room temperature 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-


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friendly and nonflammable electrolyte enables aqueous lithium-ion battery with high energy density (> 160 Wh Kg-1) and high working voltage of 2.4 V. A lithium-ion battery consisted of positive LiMn2O4 (LMO), 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 room temperature ‘water in salt’ aqueous electrolyte, and offer new insights into exploring green aqueous electrolyte system for lithium-ion batteries, which greatly broaden our horizons for electrolyte and battery research.

ASSOCIATED CONTENT Supporting Information accompanies this paper. Experimental Methods lists in Supporting Information. Figure S1-S16 are the electrochemical performance, apectral analysis, DSC experiment and DFT caculation supplement, Table S1-S2 lists the composition and properties of a series of DES. AUTHOR INFORMATION Corresponding Author *Zhaoping Liu: [email protected] *Jiwen Feng: [email protected] *Liang Chen: [email protected] Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT The authors acknowledge financial supports from Key Research Program of Chinese Academy of Sciences (Grant No. 2016YFB0100100), National Natural Science Foundation of China (Grant No. 11474314, 21603267), and Youth Innovation Promotion Association CAS (Grant No. 2017341).


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Figure 1. Preparation of a methylsulfonylmethane (MSM) based room temperature deep eutectic solvent (DES). a: Stoichiometric amounts of MSM, LiClO4 and LiClO4·3H2O is used to prepare a room temperature DES-0.3 (MSM:LiClO4:H2O=1.8:1:0.3) having the lowest water content. b: State of mMSM-nLiClO4-zH2O salt-water mixtures, where m, n, and x are the molar ratios for MSM, LiClO4 and H2O respectively. The stable liquids state is drawn by black squares that are experimentally obtained through the gradual addition of water to of m’MSM-n’LiClO4 salts, where m’ and n’ are the molar ratios of MSM and LiClO4 before water addition. The blue triangles represent the solid phase, and the red circles represent a partially miscible phase of salts and water containing the deposited crystalline salt or hydrate. c: the molality, conductivity and viscosity of salt/solvent ratios in mMSM-nLiClO4-zH2O system, and the viscosity and molar concentration are shown in Table S1 (Supporting Information)


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Figure 2. a: Raman spectra of DES electrolytes (MSM:LiClO4:H2O=1.8:1:z, z=1, 2, 6, 16, 38) using the saturated LiClO4-H2O and saturated MSM-H2O as the references. The left Raman bands observed in the range of 3000-4000 cm-1 correspond to the O-H stretching modes of water molecules, the right Raman bands in the range of 1050-1250 cm-1 correspond to the S=O stretching modes of MSM molecules. b: FTIR microscope spectra of DES electrolytes using 1M LiClO4-H2O and the saturated MSM-H2O as the references. The left vibration in the range of 3000-4000 cm-1 correspond to the O-H stretching modes of water molecules, and the right


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vibration in the range of 950-1200 cm-1 correspond to the S=O stretching modes of MSM molecules and Cl=O stretching modes of ClO4- ion. c: Chemical shifts for 17O nuclei of various DES electrolytes using the saturated LiClO4-H2O and the saturated MSM-H2O as the references. The left chemical shifts in the range of 302-287 ppm correspond to the 17O nuclei of ClO4-, the middle chemical shifts in the range of 160-135 ppm correspond to the 17O nuclei of MSM, and the right chemical shifts in the range of 302-287 ppm correspond to the

17O

nuclei of H2O, in

which D2O in capillary as external standard and calibrated the chemical shift at 0 ppm. d: Summary of the chemical shifts of ClO4-, MSM, and H2O of MSM:LiClO4:H2O=1.8:1:z (z=1, 2, 6, 16, 38), the saturated MSM-H2O and LiClO4-H2O. e: 1H NMR chemical shift of DES electrolytes of MSM:LiClO4:H2O=1.8:1:z (z=0.5, 0.7, 0.9, 1, 1.2, 1.6, 2, 2.5, 3, 3.5, 4, 5, 6, 7). f, g: Illustration of the Job Plot for Determination of Stoichiometry, Li+ and MSM is used as the host and H2O as the guest, respectively.


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


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Figure 4. a, Upper part of linear sweep voltammetry (LSVs) 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 the 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 upper part LSVs, the potential scan rate is 2 mV s-1, and the lower part CVs is 0.2 mV s1.

Note that the CVs at the right hand of the lower part figure are obtained on LMO electrode in

three different electrolytes, one is the DES-1 (red curve), one is LITFSI (green dashed line), and the other is 1 M LiClO4 (light red short line) and the CV at the left hand of the lower part figure is obtained on LTO electrode in DES-1 electrolyte. b: Illustration of the expanded electrochemical stability window for water in salt electrolytes (DES-1 and LiTFSI (21M))


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together with the formal potentials of LMO cathode and LTO anode, induced by high salt concentration, as well as the comparison with those in 1M LiClO4 solution and pure water. c: Galvanostatic profiles of 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 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.


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