Magnesium Borohydride-Based Electrolytes Containing 1-butyl-1-methylpiperidinium bis(triﬂuoromethyl sulfonyl)imide Ionic Liquid for Rechargeable Magnesium Batteries

The electrolytes containing halide-free inorganic magnesium salt Mg(BH 4 ) 2 dissolved in ether solvents have shown reversible Mg deposition-dissolution performance. Herein, we improved the anodic stability of the electrolytes on non-inert stainless-steel electrode by mixing PP 14 TFSI ionic liquid with tetraglyme (TG) and dimethoxyethane (DME) ether solvents. The effect of mixing ratios and salt concentrations on the electrochemical behavior of the electrolyte were investigated. High anodic stability, good ionic conductivity, excellent cycling efﬁciency, feasibility of the preparation and good compatibility toward Mo 6 S 8 and TiO 2 insertion cathode make the electrolytes promising for the potential application in rechargeable magnesium batteries. of the any

The demand for inexpensive and safe rechargeable batteries for applications such as electric vehicles and large scale power storage systems is constantly increasing because of fossil energy shortage and environmental issues. 1,2 Rechargeable magnesium batteries with magnesium anode have recently gained recognition as a promising candidate for next-generation battery systems due to the high volumetric capacity (3,832 mAh cm −3 ), high specific capacity (2,230 mAh g −1 ), lower reduction potential (−2.356 V vs. standard hydrogen electrode) of Mg metal compared with other multivalent metals such as zinc (−0.763 V) and aluminum (−1.676 V), dendrite-free Mg deposition, and the natural abundance of Mg resources. [3][4][5] The development of magnesium electrolytes is considered as the most important challenge for the commercial application of rechargeable magnesium batteries because electrolyte properties govern battery performance and determine the class of cathodes to be utilized. 6 The significant progress is the reports of 0.25 mol L −1 Mg(AlCl 2 BuEt) 2 /THF (Bu=butyl, Et=ethyl) electrolyte 7,8 and 0.4 mol L −1 (PhMgCl) 2 -AlCl 3 /THF electrolyte, 9,10 which have high anodic stability (2.5 V and 3.3 V vs. Mg RE on inert Pt electrode, respectively) and reversibility of Mg deposition-dissolution. Recently, a family of novel boron based electrolytes with high ionic conductivity, excellent Mg deposition reversibility as well as high anodic potential were proposed. 11,12 On the other hand, phenolate-based 13 and alkoxide-based 14 electrolytes exhibit air insensitive character and excellent magnesium depositiondissolution performance. Meanwhile, inorganic magnesium salt solutions synthesized by the acid-base reaction of MgCl 2 and Lewis acidic compounds such as AlCl 3 show high coulombic efficiency, low overpotential for magnesium deposition-dissolution and good anodic stability. [15][16][17][18] However, these electrolytes may corrode non-inert current collectors at lower anodic potentials duo to the presence of halides in the cation and anion components of the electrolytes, although some of these electrolytes have shown impressive stability against electrochemical oxidation. 19 Hence, it is still necessary to find electrolytes with high stabilities on non-inert current collectors for a practical rechargeable Mg battery system. Nelson et al. showed that decreasing the chloride content in Mg electrolytes by switching the Lewis acid from AlCl 3 to Al(OPh) 3 greatly improves the anodic stability up to 5 V on both Pt and stainless steel electrodes. 20 Ha et al. proposed a new class of electrolytes based on magnesium (II) bis(trifluoromethane sulfonyl)imide (Mg[N-(SO 2 CF 3 ) 2 ] 2 , Mg(TFSI) 2 ) dissolved in glymebased solvents with unique characteristics such as highly reduced corrosive nature toward the current collector, low volatility, high solvating power, and the ability to form an appropriate solvation sheath structure for Mg deposition-dissolution. 21 Recently, a whole new promising family of halide-free salts containing the B-H motif z E-mail: nlyn@sjtu.edu.cn expanded the portfolio of Mg-compatible electrolytes. 22 Carboranebased electrolytes display non-corrosive nature and permit standardized methods of high-voltage cathode testing in a typical coin cell. 23,24 Magnesium borohydride Mg(BH 4 ) 2 , with LiBH 4 shows unprecedented reversible Mg deposition-dissolution in tetrahydrofuran (THF), dimethoxyethane (DME) and DGM (diglyme) solvents. 25,26 Quinquedentate ligand tetraglyme (TG) with higher safety (the boiling/flash points of TG, DGM, DME and THF are 275 • C/141 • C, 162 • C/57 • C, 85 • C/−2 • C and 66 • C/−14 • C, respectively) was further developed as the solvent of Mg(BH 4 ) 2 . 27 More importantly, a high concentration electrolyte based on Mg(BH 4 ) 2 in TG with a higher anodic stability on non-inert metal electrode was pursued for practical application. In general, however, as ether solvents have high vapor pressure and strong flammability, the proposed battery systems still have practical problems of safety and reliability.
Herein, 1-butyl-1-methylpiperidinium bis(trifluoromethyl sulfonyl)imide (PP 14 TFSI) ionic liquid, which has excellent properties such as liquidity over a wide temperature range, wide electrochemical window, good thermal and chemical stability, non-flammability, non-volatility, and high ionic conductivity, 28 was used as a mixed solvent to suppress the vapor pressure, increase the ionic conductivity and improve the electrochemical performance of the Mg(BH 4 ) 2 /TG electrolyte. Ionic liquids (ILs), also known as room temperature molten salts, consist of organic cations with large-size anions, and have attracted much attention as a novel ion conductive media for lithium-ion batteries, 29 electrochemical capacitors, 30 and dye-sensitized solar cells 31 duo to their high ionic conductivity, nonflammability and wide-electrochemical window. ILs have been applied as "ionic solvents" of the electrolytes containing organo-magnesium complexes, and the resulting Mg-complex/IL systems can be worked as efficient electrolytes for reversible Mg deposition-dissolution. [32][33][34][35][36] As a result, higher efficiencies of Mg deposition-dissolution were achieved in those systems than that without IL. Recently, Bertasi et al. reported Mg electrolytes based on 1-ethyl-3-methylimidazolium chloride (EMImCl) with AlCl 3 and amorphous δ-MgCl 2 exhibited highly reversible behavior. 37 However, Vardar et al. suggested that no reversible Mg plating was observed in the electrolytes consisting of Mg salts Mg(BH 4 ) 2 , Mg(TfO) 2 (TfO − : trifluoromethanesulfonate), and Mg(TFSI) 2 , IL solvents BMIM-TFSI (l-n-butyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide), PP 13 -TFSI (N-methyl-N-propylpiperidinium bis(trifluoromethyl sulfonyl)imide), and DEME-BF 4 (N,N-diethyl-N-methyl(2-methoxyethyl)-ammonium tetrafluoroborate), and DME and ACN (acetonitrile) organic cosolvents. 38 The failure owes to the strong coulombic attraction between the anions and the extremely charge-dense Mg 2+ cation, which prevents dissociation in solution to produce solvated mobile Mg 2+ and the strong association could not Recently, pure alkoxy-ammonium based ILs 39 and PEGylated-ILs (in which polyether chains are pendent from the organic pyrrolidinium cation of ILs) 40,41 with TFSI − anion were designed and prepared to meet the coordination conditions required for Mg deposition/dissolution from the Mg(BH 4 ) 2 source. The presence of polyether chains in the ILs can displace TFSI − and BH 4 − anions to coordinate with Mg 2+ , thus facilitate reversible electrochemical deposition-dissolution of Mg. Herein, in order to simplify the preparation technology, develop IL and cosolvent system for promoting dissociation of Mg-containing ions, the electrochemical behavior of the electrolytes simply dissolving the Mg(BH 4 ) 2 -LiBH 4 complex in TG, DME and PP 14 TFSI mixed solvents with different ratios were investigated based on previous Mg(BH 4 ) 2 -LiBH 4 /TG electrolytes. 27 In view of few reports about the compatibility of ILs-based magnesium electrolytes with cathodes, an appropriate electrolyte formulation for Mg batteries and hybrid Mg-Li batteries was further identified for practical application.
The synthetic work for the electrolytes were conducted in an argon-filled glove box (Mbraun, Unilab, Germany) containing less than 2 ppm water and O 2 by dissolving the predetermined amount of Mg(BH 4 ) 2 and LiBH 4 in solvents under stirring for at least 2 hours. Synthesis route of Mo 6 S 8 followed the literature. 42 Measurement procedures and apparatus.-The conductivity of the electrolytes was measured using a DDB-303A conductivity meter (INESA INSTRUMENT). IR analysis of the solutions was run using a Spectrum 100 FT-IR spectrometer (Perkin Elmer, Inc., USA). X-ray Photoelectron Spectroscopy (XPS) measurements were conducted on a Kratos spectrometer (AXIS Ultra DLD). X-ray diffraction (XRD) analysis of the magnesium deposits was conducted on a Rigaku diffractometer D/MAX-2200/PC equipped with Cu Kα radiation. The morphology of the deposits was observed using scanning electron microscopy (SEM) on a FEI SIRION200 field-emission microscope. Before the analysis of the deposits, the sample deposited for 8 hour at −0.1 mA was washed in the glove box with drying THF solvent to remove soluble residue and then transferred out of the box and kept without exposure to the atmosphere.
Cyclic voltammograms (CVs) were conducted in three-electrode cells inside an argon-filled glove box using an electrochemical instrument of CHI604A Electrochemical Workstation (Shanghai, China). The working electrode was a stainless steel (type 316), platinum or aluminum disk (geometric area = 3.14 × 10 −2 cm 2 ), which was polished with a corundum suspension and rinsed with dry acetone before use, and magnesium ribbon (99.5%, 0.15 mm thickness, Sigma-Aldrich), which was polished by metallographic abrasive paper (800Cw) and then cleaned up using tissues, as counter and reference electrodes. Mg ribbon can be used directly as a pseudo reference electrode, 43 simplifying the experiments. Electrochemical magnesium depositiondissolution cycles were examined with CR2016 experimental coin cells on a land battery measurement system (Wuhan, China). Stainless steel foil ( 12 mm, type 316, 25 μm thickness, Shenda Metals Co., Ltd.) was served as the working electrode (substrate). Mg ribbon as the counter electrode. An Entek PE separator (ET 20-60, 37% porosity, 20 μm thickness) and a fiber membrane as the separator.
The cells were assembled in the glove box. Magnesium was deposited onto the stainless steel substrate for fixed periods of 30 min followed by stripping to a fixed potential limit of 0.8 V vs. Mg at a constant current density of 0.1 mA. There was a 30 second rest between deposition and dissolution. The magnesium deposition and dissolution on the substrate were referred to as the discharge and charge process, respectively. The time of charge divided by the time of discharge was defined as the deposition-dissolution efficiency.
Cathode electrode slurry was prepared by mixing 80 wt% active material, 10 wt% super-P carbon powder (Timcal) and 10 wt% poly(vinylidene fluoride) (PVDF) dissolved in N-methyl-2pyrrolidinone. The electrodes were formed by coating the slurry onto stainless steel foil current collectors, drying at 80 • C for 1 hour, pressing at 2 MPa, and drying again at 80 • C for at least 12 hours under vacuum. The electrode, with a diameter of 12 mm, contains 0.5∼0.7 mg active material, and the typical thickness of the active layer is 100 μm. Electrochemical behavior was examined via CR2016 coin cells with a magnesium counter electrode, an Entek PE membrane separator. The cells were assembled in an argon-filled glove box. Galvanostatic discharge-charge measurements were conducted at ambient temperature on a Land battery measurement system (Wuhan, China).  38 ), which acts as ligands to form solvated Mg complexes, plays a significant role in reversible magnesium deposition. 25 The solvation of Mg 2+ ions via ion-dipole interaction can be improved by the use of TG with high electron donicity (different from DME with low electron donicity reported in literature 38 ), promoting the formation of ionic Mg-containing complexes that dissociate readily with increased ionic conductivity than Mg(BH 4 ) 2 /PP 14 TFSI (1.33, 0.65 mS cm −1 , respectively).

Results and Discussion
To clarify the influence of TG on coordination with Mg 2+ ions, Fourier transform infrared (FTIR) analyses were performed. Fig. 1b shows a disparity between TG, PP 14  ions, appear. The ion-dipole interaction between the TG moiety and the Mg 2+ ions affects the stretching vibration mode. Fig. 1c shows the CVs of Mg electrochemical depositiondissolution on Pt disk electrode from three solutions. There is still few magnesium deposition-dissolution from Mg(BH 4 ) 2 /PP 14 TFSI solution when the electrode is changed from stain steel to platinum, and the addition of TG improves the reversibility. Although the solution shows higher reversibility on Pt electrode duo to an apparent limitation of SS electrode to intermediate efficiencies of Mg depositiondissolution, 11 a higher anodic stability is obtained on SS electrode (shown in inset of Fig. 1c). A similar trend was also observed in the electrolytes without PP 14 TFSI. 27 The thermodynamic stability limit of the electrolyte should be independent of electrode material. 44 The difference probably reflects the varying kinetics duo to different properties of electrode/electrolyte interface.
LiBH 4 was further added in the solution since it has been shown to have a remarkable effect to Mg(BH 4 ) 2 /THF, Mg(BH 4 ) 2 /DGM and Mg(BH 4 ) 2 /TG solutions due to the improved ionic conductivity as a result of increasing Mg(BH 4 ) 2 dissociation in the solution. [25][26][27] Herein, the deposition-dissolution currents decrease after adding LiBH 4 , which is resulted from the larger turbidity of the solution as shown in Fig. 1d, thus lower ionic conductivity (1.33 and 0.38 mS cm −1 , respectively). The anodic stability improves and reaches nearly 3.0 V (vs. Mg/Mg 2+ ) on SS electrode (Fig. 1a), probably related to the change of electronic structure of the Mg complexes in the solutions.
In order to keep the anodic satiability and improve the Mg deposition-dissolution currents, the effect of salt concentrations on the electrochemical performance was further investigated.  Table I). The higher concentration obviously increases the viscosity of the solutions and decreases the ionic conductivity. 0.25 mol L −1 Mg(BH 4 ) 2 +0.25 mol L −1 LiBH 4 /PP 14 TFSI+TG (2:1) solution has a high currents for Mg deposition and dissolution. Further decreasing the concentration may lead to the limited rate performance of Mg batteries for practical application. With increasing LiBH 4 concentration, the peak potentials of Mg dissolution shift toward 0 V, indicating enhanced reaction kinetics. This means that the difference in the elec-trolyte performance is not just due to their conductivity. Both solvents and BH 4 − can coordinate with Mg 2+ as ligands which affect the structure of Mg complexes, thus the performance of the electrolyte. Acting as the second coordination ligand, the addition of LiBH 4 and its increased concentration can speed up the stripping process at electrode surface from the kinetics viewpoint. 26 Fig. 2d, the negligible current density in chronoamperometirc experiments confirms 3.0 V anodic stability of the solution. As shown in Fig. 2c, the anodic stability of the solutions decreases with increasing the amount of the TG. The voltammetric response in the composition of 2:1 volume ratio of PP 14 TFSI:TG leads to the lower peak currents, however, a higher anodic stability. The former is related to the lower electric conductivity (1.44, 1.96 and 2.18 mS · cm −1 , respectively), and the latter is probably because the solution composition has a preferable ionic structure.
In our previous report, the heating-treatment process for the preparation of 0.5 mol L −1 Mg(BH 4 ) 2 +1.5 mol L −1 LiBH 4 /TG is a key issue to improve the electrochemical window duo to a lower electron cloud density of the radical group after heating treatment. 27 The influence of heating treatment for 0.25 mol L −1 Mg(BH 4 ) 2 +0.25 mol L −1 LiBH 4 /PP 14 +TG (2:1 volume ratio) on the Mg deposition-dissolution performance is shown in Fig. 3a. The electrochemical window changes little, maintains at approximately 3.0 V (vs. Mg/Mg 2+ ). The currents for Mg deposition-dissolution increases and the overpotential decreases a little with the increase of heating temperatures. Considering the simplification of operation, unheated process, that is, stirring at room temperature was chosen for further study. Constant-current electrodeposition was conducted from 0.25 mol L −1 Mg(BH 4 ) 2 +0.25 mol L −1 LiBH 4 /PP 14 TFSI+TG (2:1 volume ratio). A grayish uniform deposit was formed on the substrate. XPS measurement was carried out to analyze the component of the deposit. The XPS survey spectrum shown in Fig. 3b reveals the presence of F, O, C and Mg elements on the surface. The F and C elements come from the electrolyte components in the deposit which cannot be removed thoroughly. The highresolution Mg 2p and Mg 2s core level spectra presented in Fig. 3c show signals at 50.6 eV and 89.6 eV, corresponding to the Mg in oxide state. It has been reported that metallic Mg is highly reactive to O 2 even under a very high vacuum (∼10 −8 Torr) and therefore thin films prepared under different background pressures must exhibit the surface characteristics of the oxides in their respective XPS results. 45 Ley et al. 46 have studied the XPS of pure Mg films in the pressure range 3 × 10 −10 -6 × 10 −11 Torr and the positions of the peaks in the core electron spectra were 49.4 eV and 88.55 eV for the Mg 2p and Mg 2s levels, respectively. Herein, the peak shifts for Mg on oxidation are 1.2 eV and 1.05 eV for the Mg 2p and Mg 2s levels, respectively. The O 1s spectrum shows a peak at 531.6 eV (inset of Fig. 3c), further ensuring the existence of MgO. 47 XRD measurement was further conducted to ensure the component of the deposit and the pattern is shown in Fig. 3d Surface SEM image shows the deposit layer is compact and uniform, as shown in the inset of Fig. 3d. Because 0.25 mol L −1 Mg(BH 4 ) 2 +0.25 mol L −1 LiBH 4 in PP 14 TFSI+TG (2:1) has a low ionic conductivity of 1.44 mS cm −1 , high potential barriers and unstable potential profiles for the Mg deposition-dissolution processes were observed during cycling (Fig. 4a). This result is likely because of the presence of undissociated salt retarding the movement of free ions in the electrolyte. In order to improve the ionic conductivity of the solution, dimethoxyethane (DME) as a mixed solvent was added. As expected, the conductivities of solutions with PP 14 TFSI+TG+DME mixed solvent are higher than those of PP 14 TFSI+TG mixed solvent (Table I). From a visual observation, the turbidity of the solution obviously decreases after the addition of DME. Therefore, the mixture of PP 14 TFSI, TG and DME with different volume ratios was used as solvents of Mg(BH 4 ) 2 +LiBH 4 . Fig. 4b demonstrates the galvanostatic cycling curves of Mg deposition-dissolution on SS substrate in 0.25 mol L −1  Mg(BH 4 ) 2 + 0.25 mol L −1 LiBH 4 /PP 14 TFSI+TG+DME solutions with different volume ratios of PP 14 TFSI:TG:DME. Note that the electrolytes with mixed solvents show a greatly reduced potential barrier for Mg deposition during the first cathodic scan, decreasing obviously compared with the solution without DME (Fig. 4a). Moreover, overpotentials appear and maintain at −0.25 V and 0.25 V during the subsequent cathodic and anodic scans, indicating low ohmic resistance for Mg deposition-dissolution on the SS electrode. Compared with the electrolyte with 2:0.5:0.5 PP 14 TFSI+TG+DME mixed solvent, 0.25 mol L −1 Mg(BH 4 ) 2 +0.25 mol L −1 LiBH 4 /PP 14 TFSI+TG+DME (2:1:1) shows lower Mg deposition-dissolution overpotentials, which is related to a higher ionic conductivity (Table I). Fig. 4c shows the Mg deposition-dissolution cycling efficiencies, which are calculated according to the ratios of the charge amounts of magnesium dissolution to those of magnesium deposition. 0.25 mol L −1 Mg(BH 4 ) 2 + 0.25 mol L −1 LiBH 4 /PP 14 TFSI+TG+DME (2:1:1) shows better cycling efficiencies. The coulombic efficiencies increase gradually from initial 79.9% with the progressive cycle numbers and reach to 91.5% after 3 cycles and are stable at above 92% after 20 cycles. The electrolyte with Mg-complex/IL systems exhibits higher anodic stability [33][34][35][36] and more stable Mg depositiondissolution efficiencies 36 than Grignard reagent/IL systems. However, higher efficiencies are necessary for the purpose of practical applications. Thus, we believe that further optimization of the ionic structure of Mg(BH 4 ) 2 +LiBH 4 /PP 14 TFSI+TG+DME system will realize a promising electrolyte system for rechargeable Mg battery. Fig. 4d compares the CVs of Mg electrochemical depositiondissolution on SS electrode from 0.25 mol L −1 Mg(BH 4 ) 2 + 0.25 mol L −1 LiBH 4 /PP 14 TFSI+TG+DME solutions with different volume ratios of PP 14 TFSI:TG:DME. The anodic stability of the solution with 2:1:1 ratio on SS electrode can maintain at approximately 3.0 V with higher current densities. The improved Mg deposition-dissolution performance seems to be partly due to the improved ionic conductivity (Table I). However, the electrolyte with PP 14 TFSI+DME (2:1) solvent does not exhibit the highest current density even if having the highest ionic conductivity among four solutions (Fig. 4d). It means that the CV properties of Mg deposition-dissolution of electrolytes are also influenced by other factors, for example, electrolyte viscidity, the so-lution structure of Mg ions in solvents, action intensity of solvents to salts and interfacial properties of electrolytes with electrode. The inset of Fig. 4d shows CVs on Al disk electrode at 50 mV s −1 from 0.25 mol L −1 Mg(BH 4 ) 2 +0.25 mol L −1 LiBH 4 /PP 14 TFSI+TG+DME (2:1:1). The electrolyte exhibits much lower Mg deposition-dissolution current and anodic stability on Al than those on SS, indicating slow deposition-dissolution speed on Al and the existence of Al corrosion in the electrolyte.
Mg 2+ insertion material Mo 6 S 8 (128.8 mAh g −1 theoretical capacity) and Li + insertion material TiO 2 (168.0 mAh g −1 theoretical capacity) were chosen as cathode to test the compatibility with the electrolyte. The coin cell was constructed using 0.25 mol L −1 Mg(BH 4 ) 2 +0.25 mol · L −1 LiBH 4 /PP14+TG+DME (2:1:1) solution as the electrolyte, Mo 6 S 8 and TiO 2 as the cathode, and Mg as the anode. As shown in Fig. 5a, the cell with Mo 6 S 8 cathode delivers 74.3 mAh g −1 discharge capacity and 57.3 mAh g −1 charge capacity at 0.05 C rate. At the second and third cycles, the cell delivers 54.8 mAh g −1 and 54.4 mA g −1 discharge capacity, respectively. The plateau at approximately 1.35 V in the first discharge curve, which disappears in subsequent discharge curves, relates to ion intercalation in inner sites with more difficulty upon subsequent cycles and that at 1.1 V relates to that in outer ones. 3 The plateaus at approximately 1.3 V and 1.45 V (which is unconspicuous but always existent, as shown in inset of Fig. 5a) in charge curves are concerned with the de-intercalation from outer ones and inner sites, respectively. The discharge capacity increase slightly at the initial several cycles and is stabilized at approximately 55 mAh · g −1 for the remaining 55 cycles (Fig. 5b), indicating the electrolyte is compatible with Mg 2+ intercalation material. Fig.  5c shows the voltage profiles of the TiO 2 |Mg coin cell with 0.25 mol L −1 Mg(BH 4 ) 2 +0.25 mol L −1 LiBH 4 /PP 14 +TG+DME (2:1:1) electrolyte between 0.5 and 1.7 V at a rate of 0.2 C. During the first discharge, TiO 2 exhibits a well-defined plateau at approximately 0.72 V, corresponding to the Li + intercalation. The first charge plateau is observed at a slightly higher potential of 1.11 V. In the 5th and 10th cycles, the discharge platform moves up slightly to about 0.8 V, and the charge platform remains almost the same at 1.1 V, meaning a lower polarization. The first discharge capacity is 128.6 mAh g −1 , and the corresponding charge capacity is 124.1 mAh g −1 . There is a little increase in the 5th and 10th cycles, with the discharge capacities being 133.3 mAh g −1 and 133.2 mAh g −1 respectively. The activation process during the initial cycles is related to the slow infiltration of electrolyte in electrode. Fig. 5d displays the discharge and charge capacities with respect to cycle number at 0.2 C and 2 C over 200 cycles. The close match between the discharge and charge capacities manifests an excellent reversibility. The reversible specific capacity remains at the quite considerable value of ∼120 and 85 mAh g −1 for all of the cycles, demonstrating good cycling stability. It is worth noting that the electrochemical performance of two intercalation materials in the electrolyte with PP 14 TFSI ionic liquid is worse than those in the electrolyte without the ionic liquid, 27,48 which is probably resulted from slow penetration of the electrolyte between the cathode particles, poor contact property of electrode/electrolyte interface, and low ion diffusion rate in the electrolyte.

Conclusions
We proposed a ternary solvent system consisting tetraglyme (TG) and dimethoxyethane (DME) ether solvents and PP 14 TFSI ionic liquid for Mg(BH 4 ) 2 based Mg electrolytes and investigated the effects of the solvent ratios and salt concentrations on the electrochemical behavior. The electrolyte 0.25 mol L −1 Mg(BH 4 ) 2 +0.25 mol L −1 LiBH 4 /PP 14 TFSI+TG+DME (2:1:1) shows good ionic conductivity, high reversibility for Mg deposition-dissolution and 3.0 V (vs. Mg/Mg 2+ ) anodic stability on non-inert stainless steel. Furthermore, good compatibility with Mg 2+ insertion material Mo 6 S 8 and Li + insertion material TiO 2 indicates that the solution could be used as a potential electrolyte in rechargeable Mg battery system.