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Magnesium Anthracene Systems Based-Electrolyte as promoter of High Electrochemical Performance Rechargeable Magnesium Batteries Seydou Hebié, Fannie Alloin, Cristina Iojoiu, Romain Berthelot, and Jean-Claude Leprêtre ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16491 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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ACS Applied Materials & Interfaces

Magnesium Anthracene Systems Based-Electrolyte as promoter of High Electrochemical Performance Rechargeable Magnesium Batteries Seydou Hebié†, Fannie Alloin†*, Cristina Iojoiu†, Romain Berthelot§, Jean-Claude Leprêtre† †

Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, Grenoble INP*, LEPMI, 38000

Grenoble, France * Institute of Engineering Univ. Grenoble Alpes §

Université de Montpellier, UMR-5253, CNRS, Institut Charles Gerhardt de Montpellier

(ICGM), Equipe Agrégats Interfaces Matériaux pour l’Energie (AIME), 2, place Eugène Bataillon – CC1502 – 34095 Montpellier cedex 5, France †,§

Réseau sur le Stockage Electrochimique de l'Energie (RS2E), CNRS, FR3459, 33 Rue Saint

Leu, 80039 Amiens Cedex, France

Keywords: Magnesium battery • π stacking stabilization • anthracene • electrolyte •electrochemistry Abstract The development of efficiently, inexpensive and safe rechargeable batteries for large-scale environmentally benign cells is one of the key requirements to accommodate and satisfy various technological applications. To date, the development of magnesium battery as a promising candidate for next-generation battery systems has been hindered by the lack of high performance and stable electrolyte. In this work, we have developed an original, safe, and high performance class of electrolytes based on a simple mixture of commercially available compounds i.e. Mg(TFSI)2, anthracene, MgCl2, and diglyme solvent. We have proven that anthracene induces stabilization of the reduced form of magnesium involving reversible magnesium plating/stripping with very high current density. The electrolyte investigated exhibits an unprecedented 1 ACS Paragon Plus Environment

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electrochemical stability window of up to 3.1 V whereas MgCl2 addition allows the improvement of the Mg/electrolyte interface properties and enables a large cyclability of Mg/ Mo6S8 Chevrel phase cell allowing to reach high performances. 1. Introduction Research in rechargeable magnesium (Mg) battery systems is considered a real alternative to lithium-ion technology and has increased its momentum in these last two decades.1-6 Although Mg exhibits higher oxidation potential (−2.3 V vs. SHE) compared to the Li+/Li system (−3 V vs. SHE), it is well compensated by its higher energy density than lithium due to its divalent nature leading to a volume capacity of 3833 mAh cm-3 against 2061 mAh cm-3 for lithium. More interestingly, Mg material is widely abundant on earth’s crust, easy to be produced as a metal and therefore less expensive. Moreover, Mg technology has additional advantages such as safety and dendrite-free Mg deposition.7-9 The search for reliable electrolyte and positive electrode material is the main challenge for the effective development and the commercial application of rechargeable Mg batteries. Thus the development of an electrolyte which enables the magnesium plating/stripping reversibility and exhibits a large electrochemical stability window is a key priority. Rechargeable magnesium battery is recognized as early as the 1990s by Gregory et al.11 Thereafter, the first prototype of a magnesium rechargeable cell was reported in 2000 by Aurbach et al.12. The most studied electrolytes were Mg(B(R)4)2 solutions (where R can be various organic groups)11 and a family of Mg organohaloaluminate salt solutions such as Mg(AlCl3R)2 and Mg(AlCl2RR’)2, where R and R’ are alkyl groups in tetrahydrofuran (THF) or glyme solvents.12 Magnesium can be reversibly deposited in THF using Grignard reagents (R-MgX, R = alkyl; X = Br, Cl), and amido-magnesium halides.13 However, due to the strong reducing character of 2 ACS Paragon Plus Environment

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Grignard reagents, and its limited anodic stability (EtMgBr, BuMgCl are oxidized from 1.5 V vs. Mg2+/Mg), it is not adapted for battery application.13 Thereafter, Liao et al.14 reported a synthetic strategy to enhance the anodic stability of Grignard reagents by replacing the alkyl groups with alkoxide ones. Unfortunately, electrolytes involving such salts, even modified with aromatic based ligands, do not entirely satisfy the expected criteria namely safety, chemical stability, and wide electrochemical stability window.15 To overcome problems associated with Grignard reagent, other Mg compounds such as MgCl21619

, ROMgCl9, Mg(BH4)220,21, Mg(PF6)222, Mg(CB11H12)223 , Mg[Z(ORF)4]2, Z = Al, B; RF =

fluorinated alkyl group24 and magnesium bistrifluoromethane sulfonyl imide (Mg[TFSI]2)25-27 have been investigated. The last salt is particularly attractive due to its high ionic conductivity and anodic stability. However, without incorporation of additional anion, low Mg plating/stripping kinetic is observed. A stable and reversible magnesium plating/stripping was reported for Mg(TFSI)2 in dimethoxyethane and Mg(TFSI)2 in glyme only after addition of MgCl2 or Mg(BH4)228,29, chloride addition helps to overcome the passivation of the Mg electrode30 and facilitate the Mg plating/stripping process due to the formation of binuclear complex [Mg2(µ-Cl)2]2+ as intermediate19. A cleaver approach was to use Mg coordinating agent such as cyclopentadienyl to form a magnesium stable complex i.e. magnesocene31 which enables to obtain reversible Mg plating/stripping process. However, the anodic stability of the magnesocene complex is low (1.5 V vs. Mg2+/Mg on platinum electrode) and is not adapted for Mg battery application. In this work, we propose for the first time the anthracene- Mg coordinating agent to enhance the Mg stripping/plating. We investigate the effect of anthracene with Mg(TFSI)2 glyme electrolyte. The magnesium anthracene complex formation was evidenced by Ramsden in 1965 and its first 3 ACS Paragon Plus Environment


studies focused on organometallic catalysis for hydrogenation began to develop in the 1980s32. It was reported that magnesium anthracene, MgC14H10(THF)3, can be prepared in high yield by reacting magnesium with anthracene in THF in the absence of air and moisture.33 The electrochemical characterizations of Mg(TFSI)2 + anthracene in glyme electrolyte for Mg battery were first performed in a three electrode cell configuration and Mg/ stainless steel coincell, in order to investigate the anthracene effect on Mg plating/stripping process. The use of a Mg(TFSI)2 + anthracene based electrolyte in a Mg battery using the Chevrel phase and metallic Mg as positive and negative electrodes respectively was also investigated. 2. Experimental section Anthracene (>98%, HPLC grade from Fluka) was sublimated at 60 °C under a vacuum and stored in an Ar filled glove box. Diethyleneglycol dimethyl ether (diglyme, anhydrous 99% from Acros Organics) was freshly distilled over calcium hydride (CaH2) at 60 °C under a vacuum. Then, it was stored in molecular sieves (diameter 1-2 mm, pore size of 3Å from Alfa Aesar) to reduce the residual trace of water content. Finally, the solvent was stored in an argon-filled glove box with less than 2 ppm of water. The magnesium salt, Mg(TFSI)2, was synthesized by an acid-base reaction of magnesium hydroxide (Mg(OH)2 BioUltra ≥99.0%) and acid bis(trifluoromethanesulfonyl)imide (HTFSI, 80% aqueous solution) in aqueous medium. After removing water under reduced pressure, the crude product was recrystallized in anisol:1,2-dichoroethane (2:8) mixture at 60°C. After cooling, the salt was filtered and washed with 1,2-dichoroethane at room temperature. The residual traces of solvent were removed under a vacuum and the white solid, Mg(TFSI)2 salt was dried under a vacuum at 240 °C for 72 h and finally stored in an Ar glove box.

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The electrolytes consisted of a Mg(TFSI)2 salt dissolved in distilled diglyme. By using molecular sieves at least three days, we successfully reduced the water content in the electrolyte to 4 ppm, determined by the Karl-Fisher titration. Cathode material, Chevrel phase Mo6S8 preparation. Mo6S8 was prepared as described in the literature.34 Electrodes were prepared by mixing together the active powder (Mo6S8) with carbon black (Y50A, 20wt%) and polyvinylidene difluoride (Solvay, Solef 5130, 10wt.%) in 1-methyl-2-pyrrolidone (NMP, Sigma-Aldrich, 99%) as solvent. The slurry was stirred in a planetary ball-mill for 1 hour, then subsequently casted onto copper foil, and dried at room temperature for one day and finally 2 hours more under a dynamic vacuum at 80°C. Disks of electrode were then cut off and stored in glovebox. The average active material mass loading was 0.5 mg cm-2. Physicochemical characterization The NMR tubes were prepared in glove box and carefully sealed with a tube cap. 1H spectra of different solutions were recorded on Bruker Advance300 MHz spectrometer at 25 °C. SEM images and EDS (Energy-dispersive X-ray spectroscopy) spectra of galvanostatically cycled Mg and SS electrodes were recorded using an FEG ZEISS GeminiSEM 500 with an energy-dispersive X-ray spectroscopy (EDS & EBSD) detector. The detection limit of EDS is 0.5 atom%. Before SEM measurements, the cycled cells were disassembled in an argon-filled glove box, and the resulting isolated Mg and SS electrodes were washed with distilled diglyme. Then the electrodes are dried in glove box antechamber under a vacuum for 6 hours before to be sealed in pouches for transport to the SEM. All the electrochemical experiments were performed under an argon atmosphere in a glove box at 25°C. Electrochemical measurements are performed by using a typical three-electrode Pyrex 5 ACS Paragon Plus Environment


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glass cell. The working electrode was composed of a platinum disk of 2 mm diameter. The counter and the reference electrodes were a protected ribbon of polished magnesium metal (thickness = 250 µm, Goodfellow, 99.9%), electrically connected with a platinum wire and immersed in electrolyte. Prior to each experiment, the magnesium electrodes were thoroughly polished successively with three different sandpaper (coarse, medium, and extra fine grit) in order to remove the surface layer such as MgO and/or Mg(OH)2 species. Pt electrode was polished with diamond paste (purchased from ESCIL Company), followed by several cleanings by distilled THF and diglyme solvents, respectively. Magnesium plating/stripping process was evaluated by cyclic voltammetry technique by using a potentiostat VSP- Bio-Logic - Science Instruments. In order to investigate the properties of the electrolyte, cell tests were carried out in coin-cell system. The Celgard® 2500 was used as a separator. The cycling experiments for the cells were galvanostatically conducted at 25 °C using a multi-potentiostat VMP-300-BioLogic. The cells were cycled at constant current density at C/20, with voltage cut-offs of 0.5 V and 1.7 V respectively. The coulombic efficiency along cycling was defined as discharge (cycle n)

(cycle n+1)/charge

ratio.

3. Results and discussion In the first step, the impact of the presence of anthracene on the electrochemical response of Mg(TFSI)2-based electrolyte has been studied. Figure 1a exhibits typical reduction CV profile on Pt electrode for Mg(II) anthracene electrolyte. Whereas no redox signal, in this potential region, is observed when both anthracene35 and Mg(TFSI)2 are investigated solely, (the reduction of Mg2+ to Mg occurring at lower potential), a reversible oxidation/reduction couple Ox1/Red1 is 6 ACS Paragon Plus Environment

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observed at E1/2= -0.08 V vs Mg2+/Mg preceded by pre-peak at 0.10 V. One can suggest that, since the intensity of the pre-peak does not depend on the amount of anthracene, it can be attributed to some adsorption phenomenon. Figure 1b shows the evolution of the current intensity of the peaks at -0.08 vs Mg2+/Mg as function of the amount of anthracene up to 0.06 mol L-1 (anthracene solubility in diglyme is close to 0.07 mol L-1). This shows that the intensity of the peaks increases linearly with the amount of added anthracene for both the reduction and the oxidation processes. These results seem to indicate that the anthracene is involved in the reduction process and it can be supposed the formation of the magnesium anthracene complex. Additionally, it can point out that the peak current ratio (iOx1/iRed1) is nearly 1 whatever the concentration of anthracene demonstrating the reversible character of the redox process and the absence of side chemical reaction coupled to the electron transfer. Most importantly, the reversibility of the Ox1/Red1 couple, whatever the potential scan rate (Figure S1), indicates that the reduced specie formed during the electrochemical process is stable at the time scale of the voltammetry. In the anodic region (figure 1a), an irreversible anodic peak is observed at E2= 3.3 V vs Mg2+/Mg characteristic

of

anthracene

oxidation

in

accordance

with

previous

electrochemical

investigation35. The high anodic stability of the electrolyte allows the use of high potential cathode, and consequently a notable improvement of the Mg battery energy density could be consequently obtained. Figure 2 shows the CV up to -1 V vs Mg2+/Mg in 0.5 mol.L-1 Mg(TFSI)2 diglyme (black curve) on Pt electrode. The Mg plating/stripping is weakly reversible as previously reported.3,36 The Mg is deposited at relatively high over-potentials of -580 mV vs Mg2+/Mg whereas the dissolution process occurs only at 2.1 V vs Mg2+/Mg. The quite low coulombic efficiency (i.e. 64 %), 7 ACS Paragon Plus Environment


indicates the consumption of freshly deposited magnesium metal by reaction with the electrolyte. The anodic stability is higher than 3 V vs Mg2+/Mg (limited by the oxidation of the diglyme solvent) which is significantly improved when compared to that of the wide potential window of even the best organohaloaluminates.10, 37-38 The addition of anthracene in the range of 0.0075 to 0.07 mol.L-1 (Figure 2 and S2) significantly reduces the overpotential for plating/stripping process (blue and red curves on Figure 2). The overpotential for Mg plating decreases from 0.58 V to 0.47 V vs Mg2+/Mg while oxidation of formed Mg highly decreases from 2.1 V to 0.01 V vs Mg2+/Mg. This result illustrates unambiguously the drastic change in the kinetic/mechanism aspect of the Mg plating/stripping which exhibits a more pronounced reversibility. In addition this high reversibility is preserved for subsequent cycles (Figure S3). Concerning the current density, anthracene addition leads to a 46 mA cm-2 value for Mg plating/stripping process which is the highest reported value to the best of our knowledge (ionic liquids1, Grignard reagents13 and organoaluminates39-40). This large enhancement could be assigned to the formation of stable intermediate complex between Magnesium and anthracene which improves Mg plating/stripping kinetic. The high conductivity of the electrolyte 0.5 M Mg(TFSI)2 + anthracene in diglyme i.e. 3.9 mS cm-1 at 25°C could also significantly contribute to the observed weak polarization. It has to be outlined that the Mg plating is preceded by a peak at close potential to Red1. One can suggest that the Mg/anthracene interaction leads to some anthracene magnesium complex involving the electron carrier properties of anthracene, as previously evoked41, which contributes to improve the reversibility of the overall MgII/Mg system. Indeed, Rieke et al.41 reported the preparation route of highly reactive magnesium by alkali metal reduction of magnesium salts. Typically, magnesium metal can be reduced by an alkali metal as potassium or lithium to form activated magnesium metal. It was found, in the synthesis process of Grignard reagents, that the 8 ACS Paragon Plus Environment

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use of π rich molecules such as anthracene (or naphthalene)32 (as an electron carrier to increase the kinetic of the reaction) in the presence of lithium and magnesium chloride salt increases notably the reaction kinetic at room temperature. In the case of our electrolyte, to prove the formation of a magnesium anthracene complex, we investigated the reactivity of Mg and anthracene through UV-visible characterization. After the adding of a Mg foil on a electrolyte glyme + Mg(TFSI)2 + 0.07 M anthracene, it can be quickly observed an important solution color change from light yellow (color of anthracene at 0.07 M) to more intense yellow then to black. Although at low wavelength some saturation is observed in the 600-200 nm region, to better characterize the Mg/anthracene reaction (Figure S4), we preferred to avoid dilution which could modify the sensitive content of the solution (e. g. due to the presence of residual moisture). The spectrum shows undoubtedly that Mg reacts with anthracene as judged by some broad signatures in the visible region. 1

H NMR spectra (Figure S5) performed on Mg(TFSI)2 + anthracene diglyme based electrolytes

show no significant chemical shift of anthracene protons, whereas the CH2 protons of diglyme are downfield shifted and broadened, evidencing that MgII interacts preferentially with diglyme. This behavior is due to its high donor number of 19.5 kcal.mol-1 and to its relative high dielectric constant of 7.23. Thus, by integration of all the results obtained on the electrochemistry analysis, UV-visible and NMR investigations and the state of the art, one can propose that anthracene (π stabilizing agent) and diglyme act conjunctly as coordinating ligands to stabilize MgII reduced form (scheme 1). Galvanostatic measurements were carried out in Mg/stainless steel (SS) coin-cells using 0.5 mol L-1 Mg(TFSI)2 + 0.07 mol L-1 anthracene as electrolyte (Figure 3a). During the first fifteen cycles, which is so-called pretreatment cycles, a high polarization of about 2 V is observed similarly to 9 ACS Paragon Plus Environment


those reported for Mg(TFSI)2 or Mg(TFSI)2/MgCl2 based electrolytes3,5 (Figure S6). This initial over-potential indicates a rather difficult Mg plating/stripping which could be occurring in both Mg electrode and SS one. This over voltage could originate from the presence of i) the native passive layer on Mg electrode and of ii) the reduction of impurities (water, oxygen, protic species, etc.) which could induce the formation of insoluble species at the SS electrode and impedes Mg plating/stripping as previously proposed by I. Shterenberg et al.3 In addition the coulombic efficiency is rather low during these first circles indicating some reactivity between fresly deposit Mg and electrolyte impurity traces. Thereafter, the over-potential was significantly lowered suddenly to about 0.25 V in both reduction and oxidation processes. This suggests that the native passivating layer on Mg foil has been at least partially removed along cycling or that impurities were consumed. The voltage profiles exhibit good stability versus time with weak Mg reversibility efficiency, which may be due to freshly deposited magnesium reactivity (Figure 3b). Afterward, large modifications were noticed with a sudden decrease of the polarization (from 0.25 V to 0.04 V) with a coulombic efficiency equal to 100%. Moreover, the shape of the voltage profile became rectangular and symmetric, which may correspond to the response of a pure resistance. SEM micrographs obtained from electrodes after cycling showed large morphological changes. Onto the Mg electrode (Figure 3 c-c’), one can observe areas with a homogeneous deposition of Mg on Mg electrode but also cavities of about 1 µm in diameter with homogeneous distribution at the surface without connection, which corresponds to local Mg dissolution. Regarding the SS electrode, some deposited Mg in the form of aggregates can be observed, evidencing some irreversible reduction processes (Figure 3 d-d’), and leading to the weak coulombic efficiency


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obtained. Small magnesium outgrowth with a diameter of 1 µm are observed on the surface of stainless steel electrolyte which can explain short-circuit observed during cycling. The inhomogeneous reactivity of the electrode can be attributed to the dense and resistive character of the native passivating layer. The electrolyte composed of 0.5 M Mg(TFSI)2 + 0.07 M anthracene is not reactive enough (probably due to the limited solubility of anthracene) in order to enable a complete dissolution of the native passivating layer and to tune the passivation of the Mg deposition of the SS electrode. In order to tune the interface properties and to improve the reversibility of the process, we have added to Mg(TFSI)2 + anthracene based electrolyte a slight amount of magnesium chloride (0.1 M MgCl2). Figures 4 a-b show the selected voltage profiles and the cycling efficiency for 0.5 M Mg(TFSI)2 + 0.07 M anthracene + 0.1 M MgCl2 cell. The addition of MgCl2 on the anthracene/Mg(TFSTI)2 based electrolyte exhibits unique properties with lower overpotential of 250 mV for both plating/stripping, good coulombic efficiency ranging from 80% to 86%, and stable cycling performance (during 300 cycles) (Figure S7). No polarization during the first cycles was noticed. For comparison, we have also performed the same experiment on 0.5 M Mg(TFSI)2 + 0.1 M MgCl2 electrolyte in Mg/stainless steel (SS) coin-cells (Figure S8). Even at low current density, a high polarization without any Mg deposition was observed, even if the cutoff potential was extended to very low potential. Indeed, it was recently showed that only at high concentration of MgCl2,3,19 the addition of MgCl2 to a Mg(TFSI)2 in dimethoxyethane (DME) solution decreases the overpotential and enhances the coulombic efficiency for Mg plating/stripping. Figure 4 c shows the SEM images of magnesium electrode after cycling. One can observe a homogeneous surface which means that the entire magnesium surface participates in the reaction. No Mg particles are observed on SS surface after the oxidation step (Figure S9). 11 ACS Paragon Plus Environment


This indicates Mg reversible plating/stripping on SS electrode using this electrolyte (Figure S9). In addition, it has to be added that the incorporation of MgCl2 has no detrimental effect on the large electrochemical stability window on Pt electrode (Figure S10). Thus, supported by electrochemistry in solution (vide infra) we expect anthracene assisted by MgCl2 in diglyme solution forms a highly reactive species involving magnesium/anthracene and chloride ligands solvated by diglyme molecules as previously evoked by Bogdanovic et al.43. This could be well supported by the works of Kim et al.42 and Muldoom et al10 which showed some synergy effect due to the formation of specific complexes based on bromine or chloride magnesium allowing the improved the Mg plating/stripping process. To evaluate the performance of this new electrolyte, Mo6S8 was selected as the positive electrode and Mg foil as the negative one. Figure 5 shows the discharge/charge profiles and cycling stability of this cell (the capacity is based on the weight of Mo6S8). During the discharge process, two plateaus are observed at 1.1 V and 0.95 V corresponding to Mg2+ insertion into the so-called inner and outer sites in the Chevrel phase material, respectively. During the charge process, two plateaus are also observed at 1.28 and 1.57 V. The charge/discharge plateaus are consistent with the literature with conventional electrolyte indicating that Mo6S8 and Mg electrodes are compatibles with our new electrolyte. No corrosion has been observed on the copper current collector after cycling which is presumably due to its passivation. Such current collector was previously used in literature as mentioned by Murgia et al.44 and Hebié et al.29 without negative impact. The capacity for the first discharge is equal to 80 mAh g-1 and stabilizes to 55 mAh g-1, at the rate of C/20. These capacity values are quite similar to those obtained for 0.5 M Mg(TFSI)2 + 0.25 M MgCl2 electrolyte in THF (55 mAh g-1)28 but with lower polarization. Indeed, in our study, the first discharge plateau is observed at 1.1 V 12 ACS Paragon Plus Environment

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compared to 0.6 V with 0.5 M Mg(TFSI)2 + 0.25 M MgCl2 in THF. In addition, the capacity reached is higher than that of conventional electrolytes such as magnesium tetraphenylaluminate based electrolyte (Al(OPh)3/PhMgCl) in THF (35 mAh g-1)45 and fluorinated alkoxyaluminate electrolyte (40 mAh g-1)46 in the same experimental conditions. 4. Conclusion We have developed an original and easy tailoring approach to yield a new class of efficient electrolyte. This approach is simple since it only involves the addition of a π rich molecule and allows to access to a reversible Mg plating/stripping with very high current density, indicating a fast redox reaction. The Mg(TFSI)2/anthracene electrolyte permits a good plating/stripping reversibility and has a large electrochemical stability window greater than 3V. Due presumably to the low anthracene concentration used, an non-uniform reactivity of the Mg electrode is noticed and the addition of 0.1 M MgCl2 overcomes this drawback and permits to obtain a high cyclability of a Mg / Mo6S8 cell. This work is the first example of the use of π stabilizing agent such as anthracene as electrolyte additives for Mg battery. We believe this opens up a new strategy to the development of high performance electrolyte for magnesium batteries. Supporting Information. Electrochemical characterization of different electrolyte solutions, galvanostatic cycling of Mg/SS, digital photographs and SEM images of the cycled SS electrode. 5. Acknowledgment This work was financially supported by CNRS and the “Réseau sur le Stockage d’Électrochimique de l’Énergie” (RS2E). The authors thank Frederic Charlot, research engineer at “Consortium des Moyens Technologiques Communs” (CMTC) for performing SEM images.



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Page 18 of 25

Page 19 of 25

Figure captions

b)

8

10 8

Ipeak Ox

0

1

Ipeak Red

j / mA cm -2

4 j / mA cm -2

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


1

6 4

Ipeak Ox

2

-4

2

0 -8 0.00

0.04

-2

0.08 -1

0.00

0.04

0.08 -1

[anthracene] / mol L

[anthracene] / mol L

Figure 1. (a) Cyclic voltammetry on Pt electrode of 0.5 mol L-1 Mg(TFSI)2 in the presence of anthracene in diglyme solution, V= 50 mV s-1 at 25 °C. Mg foils served as reference and counter electrodes. (b) Plot of peaks current densities as function of the concentration of anthracene.



40 20 j / mA cm -2

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 20 of 25

0 -20 -1

0.5 mol L Mg(TFSI)2 -1

-1

0.5 mol L Mg(TFSI)2+ 0.0075 mol L anthracene

-40

-1

-1

0.5 mol L Mg(TFSI)2+ 0.07 mol L anthracene

-60

-1

0

1

2

3

E / V vs. Mg2+/Mg

Figure 2. Cyclic voltammetry on Pt electrode of diglyme solution containing i) 0.5 mol L-1 Mg(TFSI)2 ii) 0.5 mol L-1 Mg(TFSI)2 + 0.0075 mol L-1 anthracene/ iii) 0.5 mol L-1 Mg(TFSI)2 + 0.07 mol L-1 anthracene. V= 50 mV s-1 at 25 °C.


Page 21 of 25

E / V vs. Mg 2+/Mg

a) 1

0

1-10th cycles

-1 0

1000

2000

3000

4000

Time /sec 1

0

58-66th cycles

-1 67000

68000

69000

70000

71000

Time /sec b) 100 Coulombic efficiency (%)

E / V vs. Mg 2+/Mg

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


80 60 40 20 0 0

20

40 Cycle number

60


72000


a)

E / V vs. Mg2+/Mg

Figure 3. (a) Reversibility measurements for Mg/ 0.5 mol L-1 Mg(TFSI)2 + 0.07 mol L-1 anthracene in diglyme /SS coin-cell. Galvanostaticaly cycling was performed by applying a current density of ± 44 µA cm-2 for 300 s. The voltage profiles correspond to the first ten (after pretreatment) and 58-66th cycles of this experiment. (b) Cycling efficiencies. SEM images of cycled Mg (c-c’) and SS (d-d’) electrodes.

1

0

1-10th cycles

-1

E / V vs. Mg 2+/Mg

0

2000

156000

158000

4000

Time /sec

1

0

-1

290-300th cycles 160000

Time /sec

b)

100 Coulombic efficiency (%)

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 22 of 25

80 60 40 20 0

0

50

100

150

200

Cycle number


250

300

Page 23 of 25

c)

1.6

1 st 5 th

1.2

10th 20th 30th

0.8

100

100

80

80

60

60

40

40 Discharge

20

0.4 0.0

-1

b)

2.0

Capacity / mAh g

a)

0

20

40

60

0

80

20

Coulombic efficiency

Rate = C/20 0

-1

10

20

30

Coulombic efficiency (%)

Figure 4. (a) Reversibility measurements for Mg/ 0.5 mol L-1 Mg(TFSI)2 + 0.07 mol L-1 anthracene + 0.1 mol L-1 MgCl2 in diglyme/SS coin-cell. Galvanostaticaly cycling was performed by applying a current density of ± 44 µA cm-2 for 300 s. The voltage profiles correspond to the first ten and 290-300th cycles of this experiment. (b) Cycling efficiencies. (c) SEM images of cycled Mg

Voltage / V

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


0

Cycle number

Capacity / mAh g

Figure 5. (a) Discharge/charge profiles, and (b) cycling stability and discharge capacities of an Mg−Mo6S8 cell. Cell configuration: Mg/ 0.5 mol L-1 Mg(TFSI)2 + 0.07 mol L-1 anthracene + 0.1 mol L-1 MgCl2 in diglyme/Mo6S8.



Scheme 1: Suggested Mg(II) reduction mechanism in the presence of anthracene.


Page 24 of 25

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Abstract graphic:


Magnesium Anthracene Systems Based-Electrolyte as promoter of

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