High Active Magnesium Trifluoromethanesulfonate-Based Electrolytes

Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE

Energy, Environmental, and Catalysis Applications

High Active Magnesium TrifluoromethanesulfonateBased Electrolytes for Magnesium-Sulfur Batteries Yuanying Yang, Weiqin Wang, Yanna Nuli, Jun Yang, and Jiulin Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20180 • Publication Date (Web): 13 Feb 2019 Downloaded from http://pubs.acs.org on February 13, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

High Active Magnesium TrifluoromethanesulfonateBased Electrolytes for Magnesium-Sulfur Batteries Yuanying Yang, Weiqin Wang, Yanna NuLi,* Jun Yang and Jiulin Wang Shanghai Electrochemical Energy Devices Research Center, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China. E-mail: [email protected] KEYWORDS: Electrolytes, Trifluoromethanesulfonate, Anthracene, Magnesium-Sulfur batteries, Rechargeable Magnesium Batteries ABSTRACT. The shortage of high-performance and easily prepared electrolyte has hindered the progress of rechargeable magnesium-sulfur (Mg-S) batteries. In this paper, we develop a new electrolyte based on Mg(CF3SO3)2-AlCl3 dissolved in tetrahydrofuran and tetraglyme mixed solvents. Mg(SO3CF3)2 as a Mg2+ source is non-nucleophilic, easily handling and much cheaper than Mg(TFSI)2 (TFSI= bis(trifluoromethanesulfonyl)imide). After modification with anthracene (π stabilizing agent) as a coordinating ligand to stabilize the Mg2+ ions and MgCl2 to improve the interface properties by accelerating the reaction of Mg(CF3SO3)2 with AlCl3, the electrolyte exhibits a low over-potential for overall Mg deposition and dissolution, moderate anodic stability (3.25 V on Pt, 2.5 V on SS, 2.0 V on Cu and 1.85 V on Al, respectively) and a suitable ionic conductivity (1.88 mS cm-1). More importantly, this electrolyte modulated by Li-salt additives exhibits good compatibility with S cathode and can be applicable for Mg-S batteries. The

ACS Paragon Plus Environment

1

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 38

rational formulation of the new electrolyte could provide a new avenue for simply prepared Mg electrolytes of Mg-S and rechargeable magnesium batteries.

1. INTRODUCTION

Due to the higher safety than Li-metal batteries, rechargeable multivalent batteries (for example Al, Ca and Mg) have caused a growing number of concerns.1 In consideration of the benefits of Mg metal, that is reasonable reduction potential (-2.4 V vs. SHE), low cost, easy to be produced as a metal, high reversibility without dendrite formation and high volumetric energy density (3833 mAh cm-3), rechargeable magnesium batteries combining conversion or intercalation cathodes with an Mg metal anode in reversible Mg deposition-dissolution electrolytes have gained recognition as one of prospective energy storage systems.2,3 However, the large charge density and high polarity of Mg2+ ions often reduce the migration kinetics, limiting the choice of cathode materials.4,5 Furthermore, the development of electrolytes with reversible Mg deposition-dissolution, adequate Mg2+ concentration and high electrochemical stability is considered as a major issue for the commercialization of rechargeable magnesium batteries. The significant progress on Mg electrolytes is Mg(AlCl2BuEt)2/tetrahydrofuran (THF) (Bu:butyl, Et:ethyl) and (PhMgCl)2-AlCl3/THF electrolytes, which have up to 100% stable Mg deposition-dissolution Coulombic efficiency and high anodic stabilities on inert Pt (2.5 V and 3.3 V vs. Mg/Mg2+, respectively).6,7 However, these Lewis acid-base electrolytes show limited anodic stabilities on non-noble metals and are expensive, restricting the development and further commercialization of rechargeable Mg batteries. To further improve the electrolyte’s anodic stabilities, a few studies have been focused on


2

Page 3 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


new containing Mg2+ compounds with large volume and weakly coordinating anions

8-13

or Mg-HMDS (HDMS=hexamethyldisilazane) compounds, such as (HMDS)2MgAlCl3,14-19 (HMDS)2Mg-MgCl2,20 (HMDS)2Mg-AlCl3-MgCl2,21 soluble in ethereal solvents. Nevertheless, most of these electrolytes are synthesized through complicated procedures and/or from expensive precursors, limiting extensive research and application. It has been well known that most of simple salts for Li battery electrolytes, such as ClO4or PF6-, cannot be used in Mg battery electrolytes because the species forming on Mg surface passivates Mg electrode and blocks the transportation of both ions and electrons.22 Recently, several Mg electrolytes using simple magnesium salts, such as the commercially available MgCl2,23-31 Mg(BF4)2,32-35 Mg(TFSI)2

36-45

have been extensively

researched. However, such salts are slightly soluble in ethers, which are inert to metallic magnesium. Among those ionic salts, Mg(TFSI)2 is particularly attractive owing to high anodic stability, ionic conductivity and solubility in ethers. Unfortunately, Mg(TFSI)2 with high purity and ultra-low moisture is very expensive. Especially, due to purity level and inevitable presence of trace amount of moisture, large over-potential and inferior Coulombic efficiency are observed in Mg deposition-dissolution processes owing to a low-conductivity layer formed on the Mg electrode. The electrolytes need to be chemically and electrochemically conditioned to initiate the initial dissolution and facilitate the reversible deposition.42, 43 To address these issues, new Mg electrolyte formulation should be identified with a simple approach to provide reversible Mg deposition-dissolution. Herein, we develop a new

class

of

electrolytes

containing

a

“simple”

magnesium

salt,

trifluoromethanesulfonate (Mg(CF3SO3)2), which is cheap and commercially available


3


Page 4 of 38

with high purity and ultra-low moisture, and firstly represent the Mg-S battery with the electrolyte containing Mg(CF3SO3)2 without the conditional processes appearing in Mg(TFSI)2 based electrolytes. 2. MATERIALS AND METHODS 2. 1 Preparation of sulfur@microporous carbon (S@MC) composite A simple melt-diffusion method was used to prepare S@MC composite with 55 wt% sulfur content. The mixture of microporous carbon (ACS Material LLC) and sublimed sulfur (99.99%, Sigma-Aldrich), which was obtained by ball-milling at 350 rpm for 1 h with a weight ratio of 1:4, was heated at 155 °C for 20 h, then 300 °C for 2 h under Ar atmosphere. 2.2 Preparation of electrolytes The predetermined amounts of Mg(CF3SO3)2 (98%, Alfa Aesar), AlCl3 (99.99%, SigmaAldrich), MgCl2 (99.9%, Sigma-Aldrich) and anthracene (99%, Sigma-Aldrich) were dissolved in THF (Aladdin, dried by molecular sieve) and tetraglyme (TG) (Aladdin, dried by molecular sieve) under stirring over 6 h at room temperature in a glove-box filled with argon gas (Mbraun). The obtained solutions without further purification were used as electrolytes. LiCF3SO3 (98%, Alfa Aesar) and LiCl (99.99%, Sigma-Aldrich) were used as the additives of the electrolytes to enhance the compatibility of S cathode with the electrolytes. 2.3 Characterization DDB-303A conductivity meter (INESA INSTRUMENT) was used to measure the conductivity and spectrum 100 FT-IR spectrometer (Perkin Elmer, Inc., USA) was run to conduct Fourier transform infrared spectroscopy (FTIR) analysis of the electrolytes. X-ray diffraction (XRD) was carried out on a Rigaku diffractometer (D/MAX-2200/PC, Cu Kα


4

Page 5 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


radiation), and scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analysis were conducted on a JEOL field-emission microscope (JSM-7401F) to analyze to components and morphology of the deposits. The samples deposited at 0.1 mA for 24 h (7.64 C cm-2) and 100 h (31.8 C cm-2) and 1 mA for 24 h (76.4 C cm-2) on SS, Cu or Ag substrate (Ф12 mm) were washed with drying THF solvent in the glove box to remove soluble residue before the analysis of the deposits. 2.4 Electrochemical Measurements. Cyclic voltammograms (CVs) of the electrolytes were carried out in three-electrode cells on a CHI604A Electrochemical Workstation. Magnesium ribbons (≥99.5%, Aldrich) were used as counter and reference electrode, and stainless steel (SS), aluminum (Al), copper (Cu) or platinum (Pt) disks (3.14×10-2 cm2) as working electrode. Magnesium deposition-dissolution processes were tested on a battery measurement system (land, China) via CR2016 coin cells assembled in the glove box. SS foil (Ф12 mm) or Mg ribbon was used as working electrode, an Entek PE as separator, and Mg ribbon as counter electrode. Magnesium deposited onto SS or Mg for a fixed period was dissolved to the voltage limit of 0.8 V vs. Mg at 0.088 mA cm-2. The discharge and charge process was referred to magnesium deposition and dissolution, respectively. The deposition-dissolution Coulombic efficiency was calculated by dividing the times of charge by discharge. In addition, the cycling Coulombic efficiency of Mg was also measured in a SS|Mg cell using the method suggested by Aurbach et al.42 0.5 Coulomb cm-2 reservoir charge (Qin) was electrochemically deposited at a current density of 1 mA cm-2, subsequently, the deposited magnesium was striped and re-deposited at 1 mA cm-2 with 0.1 Coulomb cm-2 charge (QC) (i.e. 20% DOD) for 100 (n) cycles. Finally, the surplus Mg was dissolved up to 0.8 V at 1 mA cm-2.


5


Page 6 of 38

Qex is the last charge of dissolved Mg. The cycling Coulombic efficiency was calculated through the formula listed below: CE% =

𝑛𝑄𝑐 ― 𝑄𝑖𝑛 + 𝑄𝑒𝑥 𝑛𝑄𝑐

× 100

The S@MC electrode was obtained by casting and drying a mixture of S@MC, carbon powder (super-P, timcal) and polyvinylidene fluoride (PVDF) binder (N-methyl-2-pyrrolidinone as the solvent) at 8:1:1 weight-ratio onto copper current collector. CR2016 coin cells were assembled using S@MC cathode, Entek PE separator, Mg anode. 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.25 M MgCl2+0.025 M anthracene/THF+TG (1:1 volume ratio) solution was used as the electrolyte. 0.125 M LiCl (99.9%, Alfa Aesar) or LiCF3SO3 (J&K, 99.5%) was further added in the electrolyte to enhance the performance of the Mg-S batteries. Discharge-charge tests of the coin cells were conducted with the voltage limit of 0.5/1.7 V vs. Mg on a battery measurement system (land, China). 3. RESULTS AND DISCUSSION


6

Page 7 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. The first fifteen discharge-charge profiles via SS|Mg coin-cells with electrolytes of 0.125 M Mg(CF3SO3)2+0.25 M MgCl2/THF (a), 0.125 M Mg(CF3SO3)2+0.25 M AlCl3/THF (b), 0.25 M Mg(CF3SO3)2+0.25 M AlCl3/THF (c) and 0.50 M Mg(CF3SO3)2+0.25 M AlCl3/THF (d).

Mg(CF3SO3)2 is a readily tetrahydrofuran (THF)-soluble “simple” magnesium salt. As shown in Fig. S1a, 0.125 and 0.25 M Mg(CF3SO3)2/THF electrolytes are transparent. At a higher concentration (0.5 M), the solution becomes turbid, indicating that Mg(CF3SO3)2 in THF cleavages into ions in part. Dimethoxyethane (DME) and tetraglyme (TG) with long ether chains were also used as the solvents to enhance the solubility of Mg(CF3SO3)2. Although DME has a relatively high donor number (D.N.= 20.0 and 18.6 kcal mol-1 for THF and DME, respectively)


7


Page 8 of 38

but lower dielectric constant (7.6, 7.2 for THF and DME, respectively),46 Mg(CF3SO3)2 salt at 0.5 M concentration still does not dissolve in DME, as shown in Figure S1b. However, transparent solution can be obtained in TG with a lower donor number (16.6 kcal mol-1) but a higher dielectric constant (7.7). Compared to donor number, the dielectric constant of ethereal solvents obviously takes a more important effect to promote the dissolution of Mg(CF3SO3)2. 0.25 and 0.125 M Mg(CF3SO3)2/THF solutions were consequently examined for activity as the electrolytes for Mg deposition and dissolution. 0.25 M Mg(CF3SO3)2/TG electrolyte was also conducted for a comparison. Fig.S2a demonstrates the galvanostatic cycling curves of dischargecharge via stainless steel (SS)|Mg coin-cells with Mg(CF3SO3)2/THF electrolytes. In both electrolytes, the cathodic (discharge) processes generate a high over-potential after the voltages steeply decrease to the extremum, while the corresponding anodic (charge) processes do not appear but exhibiting instant voltage fluctuation. A higher concentration results in a larger overpotential and voltage skip. In the case of 0.25 M Mg(CF3SO3)2/TG (Fig. S2b), same cathodic over-potential appears at about -1.5 V vs. Mg and andic voltage skip at 1.5 V vs. Mg as that in 0.125 M Mg(CF3SO3)2/THF. Mg generally deposits on SS electrode during the cathodic scan, and Mg2+ is dissolved from Mg electrode. The high over-potentials indicate Mg depositiondissolution is difficult, which could originate from the formation of quasi-passivating layer containing insoluble species on SS electrode and the presence of inherent oxide layer on Mg electrode. The results in Fig.S2 mean that THF with Mg(CF3SO3)2 has less sufficient solvating power to get rid of the passivating layers on SS and Mg electrodes. The relatively high dielectric constant of TG cannot also initiate Mg deposition from Mg(CF3SO3)2 on SS electrode. As are the non-aqueous solutions based on conventional Mg salts,22 it is not easy to achieve Mg deposition when Mg(CF3SO3)2 is investigated solely. The Cl-free solution tends to passivate the surface of


8

Page 9 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


magnesium, resulting in the incompatibility.47 Recent reports have demonstrated that MgTFSI2+MgCl2/DME solutions support highly reversible Mg deposition.42-44 Mg2+ ions in electrolytes can be stabilized by the chloride anions, which also accelerates Mg2+ ions close to the electrode and inhibits the passivation phenomena. It has so far been reported that DME is an effective solvent for Mg(TFSI)2+MgCl2 electrolyte due to the improved solubility of MgCl2 in the solution.42 However, adding MgCl2 in Mg(CF3SO3)2/DME electrolyte cannot reduce the overpotentials for both cathodic and anodic processes of most cycles (Fig. S3a) and few cycles exhibit low over-potentials (Fig. S3b). For comparison, the same experiment is also performed on Mg(CF3SO3)2+MgCl2/THF electrolyte. The curve in Fig. 1a exhibits an initial large overpotential, suggesting the initiation of Mg deposition is difficult, and the voltage is dramatically reduced to -0.8 V in the further cathodic process, proceeding about 45 cycles. This phenomenon is similar to previous reports for the electrolytes containing Mg(TFSI)2 or Mg(TFSI) 2+MgCl2, which is called as the pretreatment cycles.39,

42-44

Thereafter, the over-potential significantly

decreases to approximately 0.25 V during both reduction and oxidation scans (inset of Fig. 1a). Compared to Mg(CF3SO3)2+MgCl2/DME solution (Fig. S3), the improved Mg depositiondissolution performance in Mg(CF3SO3)2+MgCl2/THF solution (inset of Fig. 1) should be related to the enhanced solubility in the solution (Fig. S4). This indicates that the formed passivating layer on SS electrode and the inherent oxide layer on Mg electrode have been removed upon cycling. Moreover, there is active species related with reversible Mg deposition in the reaction between Mg(CF3SO3)2 and MgCl2. The changes in the voltage curves are mainly resulted from the transformation of components and morphologies on the surface of SS and Mg electrodes. However, it is still not enough to achieve satisfying Mg deposition-dissolution performance.


9


Page 10 of 38

Inspired by the solutions obtained by the reacting magnesium compounds with Lewis acids through transmetalation reactions,23-30 a typical Lewis acid containing Cl-, AlCl3, was added and reacted with Mg(CF3SO3)2 in order to obtain effective active species containing in the solutions. Fig.1b, 1c and 1d show the Mg deposition-dissolution curves via SS|Mg coin-cells using Mg(CF3SO3)2+0.25 M AlCl3/THF electrolytes at Mg:Al molar ratios of 2:1, 1:1 and 1:2, respectively. It is observed that all electrolytes provide Mg deposition-dissolution behavior and the performance of the electrolytes is related with the Mg:Al molar ratios. The strong Lewis acid, AlCl3, presumably overcomes the Coulomb force in Mg(CF3SO3)2 and facilitates the dissociation of CF3SO3- from Mg2+ due to the strong electron-withdrawing.23, 47 Changing the molar ratio of Mg(CF3SO3)2 to AlCl3 probably produces diverse electrochemically active species, however, all of them enable a complete remove of passivating layer from the SS electrode and inherent oxide layer from the Mg electrode. For the electrolyte with Mg:Al molar ratio of 1:2 shown in Fig. 1b, large over-potential is observed at the initial stages of the Mg dissolution-dissolution processes. After three cycles, however, the over-potential suddenly decreases to lower than 0.2 V in both reduction and oxidation processes, indicating low resistance for Mg deposition at the SS electrode and Mg dissolution from the Mg electrode, and keep stable well during subsequent cycles (inset of Fig. 1b). This suggests that the passivating layer from the SS electrode and the inherent oxide layer from the Mg electrode have been at least removed along cycling, and effective Mg deposition-dissolution processes are initiated. Note that the electrolytes with Mg:Al molar ratios of 1:1 and 2:1 (Fig. 1c and 1d) even exhibit a dramatically reduced voltage for Mg deposition-dissolution at the 1st cycle. However, higher voltage plateaus at about 0.79 V appear in some anodic processes upon cycling (inset of Fig. 1c and 1d), which are probably resulted from some side reactions of the counter anions generated from the combinations of


10

Page 11 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Mg(CF3SO3)2 and AlCl3. It is obvious that Mg:Al molar ratio in the electrolytes is related to the over-potentials of initial Mg deposition. The amount of AlCl3 in the electrolyte is higher, and the over-potential for the initial deposition is larger, as demonstrated by the result of 1:2 molar ratio shown in Fig. 1b. Electrochemical active chloride containing species can avoid the passivation of electrode but are strongly adsorbed on the surface of the electrode compared to other solution species,42 increasing the surface resistance and leading to larger over-potentials for initial Mg deposition.

Figure 2. The first fifteen discharge-charge curves via SS|Mg coin-cells with the electrolytes of 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.025 M anthracene (a), 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.25 M MgCl2/THF (b), 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.25 M MgCl2+0.025 M


11


Page 12 of 38

anthracene/THF (c) and 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.25 M MgCl2+0.025 M anthracene/THF+TG (1:1 volume ratio) (d). Insets are the corresponding Coulombic efficiency upon cycling.

Considering the better Mg deposition-dissolution cycling behavior of Mg(CF3SO3)2+ AlCl3/THF electrolyte with Mg:Al molar ratio of 1:2, further strategies of adding anthracene or MgCl2 were focused on reducing the large over-potentials during the initial cycles. It has been proven that the addition of anthracene with a π–rich molecule can decrease the large overpotentials by stabilizing the Mg2+ reduced form related to reversible magnesium depositiondissolution in Mg(TFSI)2 based electrolyte.45 Fig. 2a exhibits the Mg deposition-dissolution cycling curves via SS|Mg coin-cells with the electrolyte of 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.025 M anthracene/THF. The addition of anthracene significantly reduces the overpotentials during initial Mg deposition-dissolution processes (Fig. S5). Compared to that without anthracene, deposition voltage reduces from 0.7 to 0.4 V, while dissolution voltage decreases from 0.53 to 0.45 V. The result seems to indicate that anthracene is referred to form intermediate compound with Mg2+, which enhances the kinetic of Mg deposition and dissolution due to the electron carrier properties of π-rich anthracene molecules.45 The Mg deposition-dissolution Coulombic efficiency (inset of Fig. 2a) is rather low (14.7, 29.7 and 39.8%) during initial three cycles and lower than that (42.2, 40.4 and 67.0%) without anthracene, may be resulted from the inhomogeneous deposition and dissolution on the electrodes due to the poor solubility of anthracene in ethers (close to 0.07 M in diglyme).45 Fig. 2b exhibits the Mg depositiondissolution cycling curves of via SS|Mg coin-cell with the electrolyte of 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.25 M MgCl2/THF. Adding MgCl2 relatively decreases the initial


12

Page 13 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


over-potential of Mg deposition-dissolution (Fig. S5). Note that the initial Coulombic efficiency (inset of Fig. 2b) obviously increases (74.8, 84.1 and 83.1% for the first three cycles), higher than that without MgCl2, suggesting that the reversibility of the Mg deposition-dissolution process are enhanced duo to the improved interface properties. However, there is over 100% Coulombic efficiency during long-term cycles (Fig. S6), which results from the side reaction of the counter anions generated from the combinations of Mg(CF3SO3)2, AlCl3 and MgCl2. According to the literature,26,

27

the possible reaction pathway of Mg(CF3SO3) with AlCl3 is

represented in Scheme 1: 2Mg(CF3SO3)2 + AlCl3

𝑇𝐻𝐹

[Mg2(𝜇-Cl)3(THF)6]+ [Al(CF3SO3)4]-

Scheme 1. Possible reaction pathway of Mg(CF3SO3) with AlCl3. Since MgCl2 has a strong ability to provide Cl- ions, the reaction in the electrolyte is more complete after the addition of MgCl2. The possible reaction pathways of Mg(CF3SO3) with MgCl2 and AlCl3 are represented in Scheme 2: Mg(CF3SO3)2+MgCl2 → 2(CF3SO3)MgCl (CF3SO3)MgCl +MgCl2 +AlCl3

𝑇𝐻𝐹

[Mg2(𝜇-Cl)3(THF)6]+ [(CF3SO3)AlCl3] –

The total reaction formula is: Mg(CF3SO3)2+3MgCl2+2AlCl3

𝑇𝐻𝐹

2[Mg2(𝜇-Cl)3(THF)6]+ [(CF3SO3)AlCl3] –

Scheme 2. Possible reaction pathways of Mg(CF3SO3) with MgCl2 and AlCl3.

To enhance the electrolyte performance, different solvents were also tried to be used for Mg(CF3SO3)2+AlCl3+MgCl2 system. Mixed binary solvents with one component for decreasing viscosity and the other for solvating power are usually used to optimize the electrolytes. 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.25 M MgCl2 in THF+TG mixed solvents exhibits high clarity


13


Page 14 of 38

(Fig. S7). TG is responsible for dissociating the salts and THF for decreasing the solution viscosity. However, high over-potentials and unstable voltage curves are observed upon cycling, which is similar to that only using TG as the solvent (Fig. S8). This result indicates only changing solvent is not a valid way to enhance the performance of the electrolytes containing Mg(CF3SO3)2+AlCl3+MgCl2. To further reduce the over-potentials and enhance the Coulombic efficiency

of

Mg

deposition-dissolution

process,

anthracene

are

added

in

Mg(CF3SO3)2+AlCl3+MgCl2 based electrolytes. Figure 2c shows Mg deposition-dissolution cycling profiles via SS|Mg coin-cell with the electrolyte of 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.25 M MgCl2+0.025 M anthracene/THF. The solution exhibits a more pronounced Coulombic efficiency, however, high voltage plateaus at about 0.78 V still appear during the anodic processes. When using THF+TG mixture (1:1 volume ratio) as the solvent, the electrolyte exhibits unique properties with low over-potential of 0.1 V for Mg deposition-dissolution (Figure 2d). Coulombic efficiency increases from 65.3 % to 89.6% after the first cycle and is stable at over 95% after the 3rd cycle, and the performance keep stable during 200 cycles (inset of Figure 2d). Note that no high polarization during the initial cycles is noticed and no high oxidation process appears during the whole cycling process. The cycling Coulombic efficiency of Mg was also measured in a SS|Mg cell by depositing 0.5 Coulomb cm-2 charge, followed by 20% DOD cycling. The Coulombic efficiency calculated from the result shown in Fig. S9 is 98.5% for the electrolyte after 100 cycles. The voltage profiles show a slightly incremental over-potentials for deposition-dissolution along cycling, reflecting the increase of impendence. This phenomenon can be attributed to the changes in the species on the electrodes, which do not passivate the surface and allow reversible Mg deposition-dissolution processes.42 The cycling curves of Mg deposition-dissolution via Mg|Mg symmetric coin-cell with the electrolyte also exhibit good


14

Page 15 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


reversibility and low polarization (Fig. S10). Thus, supported by the electrochemical performance, anthracene assisted by MgCl2 in Mg(CF3SO3)2+AlCl3/THF+TG solution should form a highly electrochemical active components related with chloride ligands and magnesiumanthracene complex solvated by THF and TG molecules. As shown in Fig. S11, Mg(CF3SO3)2+AlCl3+MgCl2+anthraene has a relatively poor solubility in THF, while can be easily dissolved in THF+TG mixed solvent. Overall, these results imply that 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.25 M MgCl2+0.025 M anthracene/THF+TG (1:1) electrolyte has beneficial properties of a low over-potential (0.1 V) and relatively high and stable Coulombic efficiency for Mg deposition-dissolution. A further increase of anthracene concentration to 0.05 M maintains the low over-potentials for Mg deposition-dissolution upon cycling (Fig. S12a), however, decreases the Coulombic efficiency (Fig. S12b) during first three cycles resulted from the poor solubility of anthracene in ethers. Further electrochemical experiments in this paper are emphasized

on

0.125

M

Mg(CF3SO3)2+0.25

M AlCl3+0.25

M MgCl2+0.025

M

anthracene/THF+TG (1:1) electrolyte.


15


Page 16 of 38

Figure 3. FTIR spectra of THF, TG, 0.25 M Mg(CF3SO3)2/THF and 0.25 M Mg(CF3SO3)2/TG (a), 0.25 M Mg(CF3SO3)2/THF, 0.125 M Mg(CF3SO3)2+0.25 M AlCl3/THF, 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.025 M anthracene/THF, 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.25 M MgCl2/THF, 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.25 M MgCl2+0.025 M anthracene/THF (b), 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.25 M MgCl2 in THF and THF+TG (1:1 volume ratio) (c), 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.25 M MgCl2+0.025 M anthracene in THF and THF+TG (1:1 volume ratio) (d).

To clarify the effects of THF and TG solvents on Mg(CF3SO3)2 and confirm the role of each component in the electrolyte, Fourier transform infrared (FTIR) measurements were


16

Page 17 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


conducted on different solutions. Fig. 3a exhibits a difference between Mg(CF3SO3)2 and solutions formed by dissolving Mg(CF3SO3)2 in THF and TG in C−O−C and C−C coupled with C−O stretching frequency regions. The spectra of THF and TG solvents are also presented for a clearer comparison. After adding THF to Mg(CF3SO3)2, the peaks of 715 and 1043 cm-1 corresponding to Mg(CF3SO3)2 disappear, and that at 1628 cm-1 shifts to 1648 cm-1 and becomes narrower. In addition, the characteristic wide peak of Mg(CF3SO3)2 at 1195 cm-1 shifts to 1173 cm-1 and becomes narrower, a strong new peak appears at 1259 cm-1, and weak new peaks at 855, 915 cm-1, indicating coordination between Mg2+ ions in Mg(CF3SO3)2 and C−O groups of THF. This result indicates that Mg2+ ions are solvated by THF owning a high donor number of 20.0 kcal mol-1. A new peak at 1080 cm-1 suggests ion-dipole interaction between Mg2+ ions and THF moiety and the solvation of THF to Mg2+ ions. In TG, the peak at 1080 cm-1 related to Mg2+ solvation disappears, while that at 1259 cm-1 maintains and broadens due to the influence of TG, indicating the coordination between Mg2+ ions in Mg(CF3SO3)2 and C−O groups in TG. New peak relevant to free CF3SO3- ions appears at 1290 cm-1, which arises from the sufficient decrease in ion interaction of Mg(CF3SO3)2 and prevention of the ion pairing when adding TG with a high dielectric constant (7.7). Fig. 3b exhibits the difference of Mg(CF3SO3)2, Mg(CF3SO3)2+AlCl3,

Mg(CF3SO3)2+AlCl3+anthracene,

Mg(CF3SO3)2+AlCl3+MgCl2,

Mg(CF3SO3)2+AlCl3+MgCl2+anthracene in THF. After adding AlCl3 into Mg(CF3SO3)2/THF solution, the peaks at 765 and 1664 cm-1 related to Mg(CF3SO3)2 disappear, 1173 cm-1 related to the coordination between Mg2+ ions and THF decreases, and new peaks arise at 615 and 842 cm1,

indicating reaction of AlCl3 with Mg(CF3SO3)2. After adding anthracene into

Mg(CF3SO3)2+AlCl3/THF solution, the peak at 1080 cm-1 drifts to 1069 cm-1 and becomes stronger due to the effect of anthracene, and that at 1200 cm-1 increases, indicating that Mg2+ ions


17


Page 18 of 38

are stabilized by the coordinating ligands of anthracene and THF. After adding MgCl2 into Mg(CF3SO3)2+AlCl3/THF solution, the peak at 1069 cm-1 also increases. This indicates that adding MgCl2 enhances Mg2+ solvation by THF moiety. Moreover, the peak at 615 cm-1 maintains and new one appears at 725 cm-1, suggesting a possible role of Cl- ions in stabilizing Mg2+ cations and accelerating the Lewis acid-base reaction. When anthracene and MgCl2 are simultaneously added into Mg(CF3SO3)2+AlCl3/THF solution, each of them takes respective effect. MgCl2 accelerate the reaction of Mg(CF3SO3)2 with AlCl3, and anthracene (π stabilizing agent) as a coordinating ligand stabilizes the Mg2+ ions. For Mg(CF3SO3)2+AlCl3+MgCl2 and Mg(CF3SO3)2+AlCl3+MgCl2+anthracene systems, the phenomenon for the decrease of ion interaction and prevention of the ion pairing after the addition of TG has also been observed, as shown

in

Fig.

3c

and

3d.

Comparing

the

different

spectra

between

Mg(CF3SO3)2+AlCl3+MgCl2/THF and Mg(CF3SO3)2+AlCl3+MgCl2/THF+TG, we found the peak at 1055 cm-1 related to the characteristics of MgCl2 in THF disappears in mixed solvent (Fig. 3c), explaining that MgCl2 has better solubility in TG. New peak relevant to free CF3SO3ions appears at 1290 cm-1, which arises from the decrease of ion interaction and prevention of ion pairing after the addition of TG. The same situation also occurs in the Mg(CF3SO3)2+AlCl3+MgCl2+anthracene solutions (Fig. 3d). In THF, peak at 1055 cm-1 connected with MgCl2 and that at 884 cm-1 with anthracene disappear, the peak at 1290 cm-1 relevant to free CF3SO3- ions appears in mixed solvent. This could increase the ionic conductivity of the solution (Table S1). Due to the formation of specific complexes, the synergy effect of anthracene and MgCl2 effectively promote the electrochemical Mg depositiondissolution process.


18

Page 19 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. CVs of Mg electrochemical deposition-dissolution at 50 mV s-1 in the electrolyte of 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.25 M MgCl2+0.025 M anthracene/THF+TG (1:1 volume ratio) on Pt disk electrode (a), and SS, Cu, Al disk electrodes (b). Insets are the enlarged portions showing the anodic stability.

Fig. 4a shows cyclic voltammograms (CVs) in 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.25 M MgCl2+0.025 M anthracene/THF+TG (1:1 volume ratio) on platinum (Pt) disk electrode. The cathodic current related to Mg deposition increases from around -0.27 V, and the anodic peak to electrochemical dissolution of deposited Mg appears at approximately 0.87 V. As shown in inset of Fig. 4a, the obvious increase of the anodic current arises at 3.25 V on Pt electrode. The anodic stability is higher than 3.0 V, which is comparable to PhMgCl-AlCl3/THF electrolyte

7

and

Mg(TFSI)2-MgCl2/DME electrolyte 43. Moreover, this electrolyte has a high current density of ca. 26 mA cm-2. The comparison indicates that the anions have a large effect on the electrolyte performance. The oxidatively stable anions are believed to contribute to the good anodic stability of the electrolytes. In order to optimize Mg deposition-dissolution and enhance the anodic


19


Page 20 of 38

stability through optimizing the electrochemically active anions, further studies on the structure understanding of the Mg(CF3SO3)2-AlCl3 based electrolytes are on the way. The CV performances of the electrolyte on SS, Al and Cu disk electrodes were further evaluated experimentally and the results are shown in Fig. 4b. Compared to Pt electrode, the electrolyte shows lower current densities and reversibility on non-inert electrodes, which is mainly related to an apparent limitation of the non-inert electrodes to Mg deposition-dissolution. Moreover, lower anodic stabilities are achieved on SS, Cu and Al electrodes (up to 2.5 V on SS, and 2.0 V on Cu and 1.85 V on Al, respectively), which is generally resulted from the corrosion of the non-inert metals by the chlorine ions containing in the solutions. Compared to PhMgCl-AlCl3 electrolyte (approximately 2.2 V on SS, 1.9 V for Cu and 1.3 V for Al, respectively), the anodic stability of the Mg(CF3SO3)2-AlCl3 based electrolyte on non-insert metals is significantly improved. Although there is still chlorine ions containing in the electrolyte, the corrosion to non-inert metals is obviously restrained due to no nucleophilic species. The high oxidation stability of the electrolyte on non-inert substrate permits the use of high potential cathodes. Notable improvement on the energy density for rechargeable magnesium batteries could be practically achieved.


20

Page 21 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. X-ray diffraction result (a) and SEM images (b, c and d) of electrodeposits from 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.25 M MgCl2+0.025 M anthracene/THF+TG (1:1 volume ratio) solution. The deposits for SEM measurements were obtained on SS substrate (Ф12 mm) at different charges: 7.64 C cm-2 (0.1 mA for 24 h) (b), 31.8 C cm-2 (0.1 mA for 100 h) (c), and 76.4 C cm-2 (1 mA for 24 h) (d).

XRD measurement was carried out to confirm the component of the deposits. As shown in Fig. 5a, the diffraction peaks are in good accordance with the diffractions of metallic magnesium (JCPDS file 35-0821). SEM images in Fig. 5b (at 0.1 mA for 24 h, 7.64 C cm-2), 5c (at 0.1 mA for 100 h, 31.8 C cm-2) and 5d (1 mA for 24 h, 76.4 C cm-2) exhibit that the deposited


21


Page 22 of 38

charge has a critical influence on the morphology of the Mg deposits. With the increase of the involved charge, the deposition layer becomes compact and uniform (at same current density for a longer time or at a higher current density for a same time). The deposited Mg particles are circular with agglomerated morphology. Even at high deposition charges, the deposition layer is dendrite-free and smooth, which is very important for practical uses. SEM images in Fig. S13 demonstrate the morphological disparity of Mg deposits on the SS, Cu and Ag substrates. Mg deposits are particulate on SS and Cu substrates (Fig. S13a and S13b). However, the formation of Mg crystallites is more regular on Ag substrate (Fig. S13c), suggesting easier nucleation and growth processes for crystalline Mg deposits on Ag. As suggested above, AlCl3 is an effective factor to achieve reversible Mg deposition because the adsorption of chloride anions on electrode surfaces inhibits the formation of passivating films. Energy dispersive X-ray spectroscopy (EDS) analysis in Fig. S14 exhibits that the deposits contain mostly magnesium. In addition to Mg, the presence of small amount of C, F, S and Cl can be attributed to the by-products during reduction on the Mg deposits.


22

Page 23 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Discharge-charge curves and cycling performance of S@MC|Mg coin cell with 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.25 M MgCl2+0.025 M anthracene/THF+TG (1:1 volume ratio) electrolyte (a, b) and 0.25 M Mg(CF3SO3)2+0.5 M AlCl3+0.25 M MgCl2+0.025 M anthracene/THF+TG (1:1 volume ratio) electrolyte (c, d) between 0.5 and 1.7 V at 0.05 C.

S@MC|Mg coin cell was assembled using S@MC composite as the cathode, 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.25 M MgCl2+0.025 M anthracene/THF+TG (1:1 volume ratio) solution as the electrolyte, and Mg ribbon as the anode to detect the compatibility of S cathode with the electrolyte. During the first discharge-charge process at 0.05 C, as exhibited in Fig. 6a, a discharge voltage plateau appears at approximately 1.0 V with a slowly decaying tail from 0.9 V


23


Page 24 of 38

to 0.5 V, and a charge voltage plateau arises at around 1.5 V following a sloping voltage profile from 1.6 V to 1.7 V. These processes are related to the conversion from S to low-order polysulfides and further to MgS2 to MgS in discharging, and the transformation of MgS to loworder polysulfides and then to polysulfides or S in charging, respectively. The cell delivers 556.8 mAh g-1 discharge and 141.0 mAh g-1 charge capacities with 25.3% Coulombic efficiency. The low specific capacity indicates a substantial residual of unreacted sulfur, which is probably related to weaker Mg-ion dissociation capability in the electrolyte involved. Good Mg deposition-dissolution performance of the electrolyte suggests the chemical bonds to the Mg atom in the active species are relatively covalent in nature, however, the anions are not enough large to promote enough ionic dissociation in solution.49 Because Mg-ion dissociation can be activated by Li+ ions, the specific capacities would enhance in the first discharge process after adding Li salt in the electrolyte, as represented below. The capacity quickly decays to 129.5 mAh g-1 and 85.6 mAh g-1 in the 2nd and the 3th cycles with 71.1% and 85.7% Coulombic efficiency, respectively. The poor cycling retention suggests that the structure of S@MC material is not suitable for sulfur to be compatible with the electrolyte, which should be focused on in the next work. The discharge products with slow kinetics are also responsible for the sharp decline in capacity and greatly shorten the cycling life. It has been reported that Mg-S batteries suffer from the shuttle effect mainly arising from the reaction between intermediate polysulfides formed throughout discharge-charge process with the magnesium anode, which usually characterizes that the discharge capacity is less than the charge capacity (Coulombic efficiency higher than 100%).17 The dissolution of polysulfides brings about the unceasing loss of active material, which decreases the specific capacity and restrains the cycling stability of the Mg-S batteries. Herein, low Coulombic efficiency suggests that the serious polysulfide shuttling is not the main


24

Page 25 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


reason for the obvious decrease of the capacity. However, MgS/MgS2 could not be totally transformed to S, resulting in a quick decrease of the capacity. The capacity decays to about 40 mAh g-1 after 50 cycles, as shown in Fig. 6b. The inset of Fig. 6b exhibits the Coulomb efficiency gradually increases and becomes stable, embodying that the charge capacity is progressively close to but not higher than the discharge capacity. The hypothesis about capacity decay can be further verified by increasing the electrolyte concentration. It has been reported the increase of the electrolyte concentration could mitigate the polysulfide shuttling in the electrolytes.43 In a concentrated electrolyte, fewer S molecules in the form of either elemental S or polysulfides are dissolved into the electrolyte during cycles. As a result, it is beneficial to inhibit the loss of active material. In this work, the discharge-charge performance of S@MC|Mg coin cell with a concentrated electrolyte (0.25 M Mg(CF3SO3)2+0.5 M AlCl3+0.25 M MgCl2+0.025 M anthracene/THF+TG (1:1 volume ratio)) was measured. As shown in Fig. 6c, the specific capacities obviously decrease and the cycling performance is even worse than that with the dilute electrolyte. Especially, the Coulombic efficiency becomes greater than 100% upon stable cycling (Fig. 6d). This means the simple approach of increasing the concentration may not work to improve the cycling performance even if the concentrated electrolyte is soluble (Fig. S15). The comparison of Mg deposition-dissolution measurements (Fig. S16) demonstrates that the concentrated electrolyte shows a lower Coulombic efficiency during cycles, which is related to worse interfacial compatibility between electrode and electrolyte due to higher viscosity of the electrolyte.


25


Page 26 of 38

Figure 7. Discharge-charge profiles and cycling performance of S@MC|Mg coin cell with 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.25 M MgCl2+0.025 M anthracene/THF+TG (1:1 volume ratio) electrolyte adding 0.5 M LiCl (a, b) or LiCF3SO3 (c, d) between 0.5 and 1.7 V at 0.05 C.

To improve the discharge-charge performance of S@MC|Mg cell with as-prepared electrolyte, LiCl or LiCF3SO3 was further added in the solution of 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.25 M MgCl2+0.025 M anthracene/THF+TG (1:1 volume ratio). The presence of Li+ in the Mg electrolytes can reactivate MgS and MgS2 to reduce the kinetic barrier and increase their solubility by an ion-exchange reaction to form rechargeable Li2S and Li2S2 or a strong coordination of Li+ with S2- and S22- of MgS and MgS2.18 Using LiCl as an additive in


26

Page 27 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nucleophilic electrolyte

50, 51

and LiTFSI in non-nucleophilic electrolyte 18, the discharge-charge

reversibility and cycling stability of the Mg-S batteries have been significantly enhanced. Fig. 7a exhibits the first three discharge-charge curves of the S@MC|Mg coin cell at 0.05 C. The electrolyte is 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.25 M MgCl2+0.025 M anthracene+0.5 M LiCl/THF+TG (1:1 volume ratio). The discharge process corresponds to approximately 880.3 mAh g-1 capacity in the 1st cycle, which decreases to 640.7 mAh g-1 and 515.6 mAh g-1 in the 2nd and 3rd cycles, respectively. The main capacity contribution from the voltage plateau at 1.05 V exhibits higher retentivity than that without LiCl, along with obviously increased Coulomb efficiency. These results suggest that Li+ promotes Mg-ion dissociation in the electrolyte and reactivates MgS/MgS2 in the S cathode, thus increasing the specific capacity, improving the reaction reversibility and enhancing the capacity retention of the cell. The discharge capacity maintains stable during initial 20 cycles and then decreases to around 300 mAh g-1 at the 55th cycle (Fig. 7b). The improvement is more obvious when using LiCF3SO3 as the electrolyte additive (0.5 M), as exhibited in Fig. 7c and 7d. The capacity is 1193.8 mAh g-1 and 820.4 mAh g-1 in the 1st and 2nd discharge process, respectively. However, the capacity keeps at a comparatively stable value of approximately 420 mAh g-1 without large fluctuations and attenuation during 55 cycles. As mentioned above, CF3SO3- ions play the critical role in the Mg deposition-dissolution process. Adding LiCF3SO3 not only exerts the advantage of Li+, but also reduces the destructive effect to Mg deposition. A further increase of LiCF3SO3 to 1.0 M saturated concentration reduces the voltage polarization and improves the specific capacity (Fig. S17a), however, deteriorates the cycling performance of the cell (Fig. S17b). The discharge capacity suddenly decreases and the cell cannot cycle again, suggesting that the drop in the transport properties of ions, such as diffusivity and conductivity, become a hurdle. Unless


27


Page 28 of 38

employing an Mg-Li alloy anode to provide Li-resource, the electrochemical capacity of Lireaction cathode in Mg batteries greatly depends on the concentration of Li salts. Harmonizing ionic conductivity of the electrolyte, interfacial compatibility between sulfur cathode and electrolyte and/or interfacial resistance between Mg anode and electrolyte, there is an optimized Li-salt concentration.52 Additional work will focus on further enhancing the compatibility of the Mg(CF3SO3)2 based electrolyte with S cathode to develop practical Mg-S technology.

4. CONCLUSION A class of safe and high-performance electrolytes was developed by simply reacting commercially available Mg(CF3SO3)2 with AlCl3 in tetraglyme and tetrahydrofuran mixed solvent. After modification with anthracene (π stabilizing agent) to stabilize the Mg2+ ions by coordination effect and MgCl2 to improve the interface properties by accelerating the reaction of Mg(CF3SO3)2 with AlCl3, this electrolyte exhibits a low overpotential, a relatively high Coulombic efficiency for overall Mg deposition-dissolution, a moderate anodic stability on non-insert metals (up to 2.5 V on SS, and 2.0 V on Cu and 1.85 V on Al, respectively), suitable ionic conductivity and available compatibility with S cathode. It could provide a new approach of simple electrolytes for the development of practical Mg-S batteries.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI:


28

Page 29 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


XXXX . The electronic conductivity (mS cm-1) of solutions; The photos of the solutions of 0.125, 0.25 and 0.5 M Mg(CF3SO3)2 in THF; 0.5 M Mg(CF3SO3)2 in THF, TG and DME; 0.125 M Mg(CF3SO3)2+0.25 M MgCl2 in THF and DME; 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.25 M MgCl2 in THF, TG and THF+TG (1:1 volume ratio); 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.25 M MgCl2+0.025 M anthracene in THF and THF+TG (1:1 volume ratio); 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.25 M MgCl2+0.025 M anthracene/THF+TG (1:1 volume ratio) and 0.25 M Mg(CF3SO3)2+0.5 M AlCl3+0.25 M MgCl2+0.025 M anthracene/THF+TG (1:1 volume ratio); The first fifteen discharge-charge curves via SS|Mg coin-cells with the electrolytes of 0.125 and 0.25 M Mg(CF3SO3)2/THF; 0.25 M Mg(CF3SO3)2/TG; 0.125 M Mg(CF3SO3)2+0.25 M MgCl2/DME; 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.25 M MgCl2/THF; 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.25 M MgCl2 in TG and THF+TG (1:1). SEM images of electrodeposits at 7.64 C cm-2 (0.1 mA for 24 h) from 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.25 M MgCl2+0.025 M anthracene/THF+TG (1:1 volume ratio) solution on SS, Cu and Ag substrates, respectively. The first discharge-charge profiles and cycling performance of S@MC|Mg coin cell with 0.125 M Mg(CF3SO3)2+0.25 M AlCl3+0.25 M MgCl2+0.025 M anthracene/THF+TG (1:1 volume ratio) electrolyte after adding 0.5 or 1.0 M LiCF3SO3 between 0.5 and 1.7 V at 0.05 C.

Corresponding Author E-mail: [email protected] Author Contributions


29


Page 30 of 38

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS We are greatly thankful the support from the National Natural Science Foundation of China (No. 21573146) and the Shanghai Municipal Science and Technology Commission (No. 11JC1405700).

REFERENCES 1. Muldoon J.; Bucur C. B.; Gregory, T. Quest for Nonaqueous Multivalent Secondary Batteries: Magnesium and Beyond. Chem. Rev. 2014, 114, 11683–11720. 2. Aurbach, D.; Lu, Z.; Schechter, A.; Gofer, Y.; Gizbar, H.; Turgeman, R.; Cohen, Y.; Moshkovich, M.; Levi, E. Prototype Systems for Rechargeable Magnesium Batteries. Nature 2000, 407, 724–727. 3. Saha, P.; Datta, M. K.; Velikokhatnyi, O. I.; Manivannan, A.; Alman, D.; Kumta, P. N. Rechargeable Magnesium Battery: Current Status and Key Challenges for the Future. Prog. Mater Sci. 2014, 66, 1–86. 4. Knight, J. C.; Therese, S.; Manthiram, A. On the Utility of Spinel Oxide Hosts for Magnesium-Ion Batteries. ACS Appl. Mater. Interfaces 2015, 7, 22953–22961.


30

Page 31 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5. Choi, S. -H.; Kim, J.-S.; Woo, S.-G.; Sun, W. C.; Choi, Y.; Choi, J.; Lee, K.–T.; Park, M.-S.; Kim, Y.-J. Role of Cu in Mo6S8 and Cu Mixture Cathodes for Magnesium Ion Batteries. ACS Appl. Mater. Interfaces 2015, 7, 7016–7024. 6. Aurbach, D.; Gizbar, H.; Schechter, A.; Chusid, O.; Gottlieb, H. E.; Gofer, Y.; Goldberg, I. Electrolyte Solutions for Rechargeable Magnesium Batteries Based on Organomagnesium Chloroaluminate Complexes. J. Electrochem. Soc. 2002, 149, A115–A121. 7. Aurbach, D.; Suresh, G. S.; Levi, E.; Mitelman, A.; Mizrahi, O.; Chusid, O.; Brunelli, M. Progress in Rechargeable Magnesium Battery Technology. Adv. Mater. 2007, 19 ， 4260– 4267. 8.

Du, A. B.; Zhang, Z. H.; Qu, H. T.; Cui, Z. L.; Qiao, L. X.; Wang, L. L.; Chai, J. C.; Lu, T.; Dong, S. M.; Dong, T. T.; Xu, H. M.; Zhou, X. H.; Cui, G. L. An Efficient Organic Magnesium

Borate-Based

Electrolyte

with

Non-Nucleophilic

Characteristics

for

Magnesium–Sulfur Battery. Energy Environ. Sci. 2017, 10，2616–2625. 9.

Zhang, Z. H.; Cui, Z. L.; Qiao, L. X.; Guan, J.; Xu, H. M.; Wang, X. G.; Hu, P.; Du, H. P.; Li, S. Z.; Zhou, X. H.; Dong, S. M.; Liu, Z. H.; Cui, G. L. Novel Design Concepts of Efficient Mg-Ion Electrolytes toward High-Performance Magnesium-Selenium and Magnesium-Sulfur Batteries. Adv. Energy Mater. 2017, 7, 1602055.

10. Xu, H. M.; Zhang, Z. H.; Cui, Z. L.; Du, A. B.; Lu, C. L.; Dong, S. M.; Ma, J.; Zhou, X. H.; Cui, G. L. Strong Anion Receptor-Assisted Boron-Based Mg Electrolyte with Wide Electrochemical Window and Non-Nucleophilic Characteristic. Electrochem. Commun. 2017, 83, 72–76. 11. Zhao-Karger, Z.; Bardaji, M. E. G.; Fuhr, O.; Fichtner, M. A New Class of Non-Corrosive, Highly Efficient Electrolytes for Rechargeable Magnesium batteries. J. Mater. Chem. A


31


Page 32 of 38

2017, 5, 10815–10820. 12. Tutusaus, O.; Mohtadi, R.; Arthur, T. S.; Mizuno, F.; Nelson, E. G.; Sevryugina, Y. V. An Efficient Halogen‐Free Electrolyte for Use in Rechargeable Magnesium Batteries. Angew. Chem. Int. Ed. 2015, 54, 7900–7904. 13. Carter, T. J.; Mohtadi, R.; Arthur, T. S.; Mizuno, F.; Zhang, R.; Shirai, S.; Kampf, J. W. Boron Clusters as Highly Stable Magnesium‐Battery Electrolytes. Angew. Chem. Int. Ed. 2014, 53, 3173–3177. 14. Zhao-Karger, Z.; Zhao, X. Y.; Fuhr, O.; Fichtner, M. Bisamide Based Non-Nucleophilic Electrolytes for Rechargeable Magnesium Batteries. RSC Adv. 2013, 3, 16330–16335. 15. Wall, C.; Zhao-Karger, Z.; Fichtner, M. Corrosion Resistance of Current Collector Materials in Bisamide Based Electrolyte for Magnesium Batteries. ECS Electrochem. Lett. 2015, 4, C8–C10. 16. Zhao-Karger, Z.; Zhao, X. Y.; Wang, D.; Diemant, T.; Behm, R. J.; Fichtner, M. Performance Improvement of Magnesium Sulfur Batteries with Modified Non-Nucleophilic Electrolytes. Adv. Energy Mater. 2015, 5, 1401155. 17. Vinayan, B. P.; Zhao-Karger, Z.; Diemant, T.; Chakravadhanula, V. S. K.; Schwarzburger, N. I.; Cambaz, M. A.; Behm, R. J.; Kübel, C.; Fichtner, M. Performance Study of Magnesium–Sulfur Battery Using a Graphene Based Sulfur Composite Cathode Electrode and a Non-Nucleophilic Mg Electrolyte. Nanoscale 2016, 8, 3296–3306. 18. Gao, T.; Noked, M.; Pearse, A. J.; Gillette, E.; Fan, X. L.; Zhu, Y. J.; Luo, C.; Suo, L. M.; Schroeder, M. A.; Xu, K.; Lee, S. B.; Rubloff, G. W.; Wang, C. S. Enhancing the Reversibility of Mg/S Battery Chemistry through Li+ Mediation. J. Am. Chem. Soc. 2015, 137, 12388–12393.


32

Page 33 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


19. Xu, Y.; Li, W. F.; Zhou, G. M.; Pan, Z. H.; Zhang, Y. G. A non-nucleophilic mono-Mg2+ electrolyte for rechargeable Mg/S battery. Energy Storage Mater. 2018, 14, 253–257. 20. Liao, C.; Sa, N. Y.; Key, B.; Burrell, A. K.; Cheng, L.; Curtiss, L. A.; Vaughey, J. T.; Woo, J.-J.; Hu, L. B.; Pan, B. F.; Zhang, Z. C. The Unexpected Discovery of the Mg(HMDS)2/MgCl2 Complex as a Magnesium Electrolyte for Rechargeable Magnesium Batteries. J. Mater. Chem. A 2015, 3, 6082–6087. 21. Yu, X. W.; Manthiram, A. Performance Enhancement and Mechanistic Studies of Magnesium–Sulfur Cells with an Advanced Cathode Structure. ACS Energy Lett. 2016, 1, 431–437. 22. Lu, Z.; Schechter, A.; Moshkovich, M.; Aurbach, D. On the Electrochemical Behavior of Magnesium Electrodes in Polar Aprotic Electrolyte Solutions. J. Electroanal. Chem. 1999, 466, 203−217. 23. Liu, T. B.; Shao, Y. Y.; Li, G. S.; Gu, M.; Hu, J. Z.; Xu, S. C.; Nie, Z. M.; Chen, X. L.; Wang, C. M.; Liu, J. A Facile Approach Using MgCl2 to Formulate High Performance Mg2+ Electrolytes for Rechargeable Mg Batteries. J. Mater. Chem. A 2014, 2, 3430–3438. 24. Zhao-Karger, Z.; Mueller, J. E.; Zhao, X. Y.; Fuhr, O.; Jacob, T.; Fichtner, M. Novel Transmetalation Reaction for Electrolyte Synthesis for Rechargeable Magnesium Batteries. RSC Adv. 2014, 4, 26924–26927. 25. Doe, R. E.; Han, R. B.; Hwang, J.; Gmitter, A. J.; Shterenberg, I.; Yoo, H. D.; Pour, N.; Aurbach, D. Novel Electrolyte Solutions Comprising Fully Inorganic Salts with High Anodic Stability for Rechargeable Magnesium Batteries. Chem. Commun. 2014, 50, 243– 245. 26. See, K. A.; Chapman, K. W.; Zhu, L. Y.; Wiaderek, K. M.; Borkiewicz, O. J.; Barile, C. J.;


33


Page 34 of 38

Chupas, P. J.; Gewirth, A. A. The Interplay of Al and Mg Speciation in Advanced Mg Battery Electrolyte Solutions. J. Am. Chem. Soc. 2016, 138, 328 –337. 27. Barile, C. J.; Barile, E. C.; Zavadil, K. R.; Nuzzo, R. G.; Gewirth, A. A. Electrolytic Conditioning of a Magnesium Aluminum Chloride Complex for Reversible Magnesium Deposition. J. Phys. Chem. C 2014, 118, 27623–27630. 28. Barile, C. J.; Nuzzo, R. G.; Gewirth, A. A. Exploring Salt and Solvent Effects in ChlorideBased Electrolytes for Magnesium Electrodeposition and Dissolution. J. Phys. Chem. C 2015, 119, 13524–13534. 29. Ha, J. H.; Adams, B.; Cho, J.-H.; Duffort, V.; Him, J. H.; Chung, K. Y.; Cho, B. W.; Nazar, L. F.; Oh, S. H. A Conditioning-Free Magnesium Chloride Complex Electrolyte for Rechargeable Magnesium Batteries. J. Mater. Chem. A 2016, 4, 7160–7164. 30. Li, W. F.; Cheng, S.; Wang, J.; Qiu, Y. C.; Zheng, Z. Z.; Lin, H. Z.; Nanda, S.; Ma, Q.; Xu, Y.; Ye, F. M.; Liu, M. N.; Zhou, L. S.; Zhang, Y. G.; Synthesis, Crystal Structure, and Electrochemical Properties of a Simple Magnesium Electrolyte for Magnesium/Sulfur Batteries. Angew. Chem. Int. Ed. 2016, 128, 6406–6410. 31. Pan, B. F.; Zhang, J. J.; Huang, J. H.; Vaughey, J. T.; Zhang, L.; Han, S.-D.; Burrell, A. K.; Zhang, Z. C.; Liao, C. A Lewis Acid-Free and Phenolate-Based Magnesium Electrolyte for Rechargeable Magnesium Batteries. Chem. Commun. 2015, 51, 6214–6217. 32. Zavorotynska, O.; El-Kharbachi, A.; Deledda, S.; Hauback, B. C. Recent Progress in Magnesium Borohydride Mg(BH4)2: Fundamentals and Applications for Energy Storage. Int. J. Hydrogen Energy, 2016, 41, 14387–14403. 33. Mohtadi, R.; Matsui, M.; Arthur, T. S.; Hwang, S. J. Magnesium Borohydride: From Hydrogen Storage to Magnesium Battery. Angew. Chem. Int. Ed. 2012, 51, 9780–9783.


34

Page 35 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


34. Shao, Y. Y.; Liu, T. B.; Li, G. S.; Gu, M.; Nie, Z. M.; Engelhard, M.; Xiao, J.; Lv, D. P.; Wang, C. M.; Zhang, J. G.; Liu, J. Coordination Chemistry in Magnesium Battery Electrolytes: How Ligands Affect Their Performance. Scientific Reports 2013, 3, 3130. 35. Tuerxun, F.; Abulizi, Y.; NuLi, Y. N.; Su, S. J.; Yang, J.; Wang, J. L. High Concentration Magnesium Borohydride/Tetraglyme Electrolyte for Rechargeable Magnesium Batteries, J. Power Sources 2015, 159, 255−261. 36. Tran, T. T.; Lamanna, W. M.; Obrovac, M. N. Evaluation of Mg[N(SO2CF3)2]2/Acetonitrile Electrolyte for Use in Mg-Ion Cells. J. Electrochem. Soc. 2012, 159, A2005−A2009. 37. Singh, N.; Arthur, T. S.; Ling, C.; Matsui, M.; Mizuno, F. A. High Energy-density Tin Anode for Rechargeable Magnesium-Ion Batteries. Chem. Commun. 2013, 49, 149−151. 38. Terada, S.; Mandai, T.; Suzuki, S.; Tsuzuki, S.; Watanabe, K.; Kamei, Y.; Ueno, K.; Dokko, K.; Watanabe M. Thermal and Electrochemical Stability of Tetraglyme-Magnesium Bis(trifluoromethanesulfonyl)amide Complex: Electric Field Effect of Divalent Cation on Solvate Stability. J. Phys. Chem. C 2016, 120, 1353−1365. 39. Ha, S.-Y.; Lee, Y.-W.; Woo, S. W.; Koo, B.; Kim, J.-S.; Cho, J.; Lee, K. T.; Choi, N.-S. Magnesium(II) Bis(trifluoromethane sulfonyl) Imide-Based Electrolytes with Wide Electrochemical Windows for Rechargeable Magnesium Batteries. ACS Appl. Mater. Interfaces 2014, 6, 4063–4073. 40. Ma, Z.; Kar, M.; Xiao, C. L.; Forsyth M.; MacFarlane D. R. Electrochemical Cycling of Mg in Mg[TFSI]2/Tetraglyme Electrolytes. Electrochem. Commun. 2017, 78, 29–32. 41. Hebié, S.; Ngo, H. P. K.; Leprêtre, J.-C.; Iojoiu, C.; Cointeaux, L.; Berthelot, R.; Alloin, F. Electrolyte Based on Easily Synthesized, Low Cost Triphenolate-Borohydride Salt for High Performance Mg(TFSI)2-Glyme Rechargeable Magnesium Batteries. ACS Appl. Mater.


35


Page 36 of 38

Interfaces 2017, 34, 28377–28385. 42. Shterenberg, I.; Salama, M.; Yoo, H. D.; Gofer, Y.; Park, J.-B.; Sun, Y.-K.; Aurbach, D. Evaluation of (CF3SO2)2N-(TFSI) Based Electrolyte Solutions for Mg Batteries. J. Electrochem. Soc. 2015, 162, A7118–A7128. 43. Gao, T.; Hou, S.; Wang, F.; Ma, Z. H.; Li, X. G.; Xu, K.; Wang, C. S. Reversible S0/MgSx Redox Chemistry in a MgTFSI2/MgCl2/DME Electrolyte for Rechargeable Mg/S Batteries. Angew. Chem. Int. Ed. 2017, 56, 13526–13530. 44. Cheng, Y. W.; Stolley, R. M.; Han, K. S.; Shao, Y. Y.; Arey, B. W.; Washton, N. M.; Mueller, K. T.; Helm, M. L.; Sprenkle, V. L.; Liu, J.; Li, G. S. Highly Active Electrolytes for Rechargeable Mg Batteries Based on a [Mg2(μ-Cl)2]2+ Cation Complex in Dimethoxyethane. Phys. Chem. Chem. Phys. 2015, 17, 13307–13314. 45. Hebié, S.; Alloin, F.; Iojoiu, C.; Berthelot R.; Leprêtre, J.-C. Magnesium Anthracene System-Based Electrolyte as a Promoter of High Electrochemical Performance Rechargeable Magnesium Batteries. ACS Appl. Mater. Interfaces 2018, 10, 5527–5533. 46. Scheers, J.; Fantini, S.; Johansson P. A Review of Electrolytes for Lithium-Sulphur Batteries. J. Power Sources, 2014, 255, 204–218. 47. Connell, J. G.; Genorio, B.; Lopes, P. P.; Strmcnik D.; Stamenkovic, V. R.; Markovic N. M. Tuning the Reversibility of Mg Anodes via Controlled Surface Passivation by H2O/Cl– in Organic Electrolytes. Chem. Mater. 2016, 28, 8268–8277. 48. Vardar, G.; Sleightholme A. E. S.; Naruse, J.; Hiramatsu, H.; Siegel D. J.; Monroe, C. W. Electrochemistry of Magnesium Electrolytes in Ionic Liquids for Secondary Batteries, ACS Appl. Mater. Interfaces 2014, 6, 18033−18039. 49. Gregory, T. D.; Hoffman R. J.; Winterton, R. C. J. Electrochem. Soc. 1990, 137, 775–780.


36

Page 37 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50. Zeng, L. Q.; Wang, N.; Yang, J.; Wang, J. L.; NuLi, Y. N. Application of a Sulfur Cathode in Nucleophilic Electrolytes for Magnesium/sulfur Batteries. J. Electrochem. Soc. 2017, 164, A2504–A2512. 51. Wang, W. Q.; Yuan, H. C.; NuLi, Y. N.; Zhou, J. J.; Yang, J.; Wang, J. L. A Sulfur@microporous Carbon Cathode with a High Sulfur Content for Magnesium-sulfur Batteries with Nucleophilic Electrolytes. J. Phys. Chem. C 2018, 122, 26764−26776. 52. Wu, N.; Yang, Z.-Z.; Yao, H.-R.; Yin, Y.-X.; Gu, L.; Guo, Y.-G. Improving the Electrochemical Performance of the Li4Ti5O12 Electrode in a Rechargeable Magnesium Battery by Lithium–Magnesium Co-Intercalation. Angew. Chem. Int. Ed. 2015, 54, 5757–5761.


37


Page 38 of 38

High Active Magnesium Trifluoromethanesulfonate-Based Electrolytes for Magnesium-Sulfur Batteries

A new class of efficient electrolyte based on Mg(CF3SO3)2-AlCl3 dissolved in tetrahydrofuran and tetraglyme mixed solvents was developed for rechargeable Mg-S batteries. After modification with anthracene, MgCl2 and LiCF3SO3, a low over-potential for overall Mg deposition-dissolution, high Coulombic efficiency, reduced corrosive nature and good compatibility with S cathode could be achieved readily.


38

High Active Magnesium Trifluoromethanesulfonate-Based Electrolytes

Recommend Documents