AlCl3 Inorganic Mg2+ Electrolytes with

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2+

Tertiary Mg/MgCl/AlCl Inorganic Mg Electrolytes with Unprecedented Electrochemical Performance for Reversible Mg Deposition Jian Luo, Shuijian He, and T. Leo Liu ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 24 Apr 2017 Downloaded from http://pubs.acs.org on April 24, 2017

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ACS Energy Letters

Tertiary Mg/MgCl2/AlCl3 Inorganic Mg2+ Electrolytes with Unprecedented Electrochemical Performance for Reversible Mg Deposition Jian Luo,# Shuijian He,# T. Leo Liu* The Department of Chemistry and Biochemistry, Utah State University, Logan, UT #

Equally contributed

*Corresponding author: [email protected]

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Abstract: Due to the high reactivity and sensitivity of the Mg2+ electrolytes in organic solvents, developing facile methods of preparing high performance Mg2+ electrolytes is still challenging, and thus impedes the development of Mg batteries. In this study, a convenient method involving tertiary reactants, Mg powder, MgCl2, and AlCl3, was reported to prepare all inorganic Mg2+ electrolytes (named as MMAC electrolytes) in ethereal solvents. These MMAC electrolytes exhibited unprecedented performance for reversible Mg deposition, Coulombic efficiencies at 90% ~ 100%, overpotential of 125 ~ 215 mV, and anodic stability up to 3.5 ~ 3.8 V (vs Mg). A comprehensive fundamental study of the MMAC electrolytes showed that the electron transfer and mass transport kinetics during Mg deposition/stripping was affected by solvent, working electrode, and the composition of the electrolytes. In brief, these tertiary MMAC electrolytes represent the most facile and reliable inorganic Mg electrolytes known to date. TOC GRAPHICS

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ACS Energy Letters

Magnesium (Mg) batteries have received increased attention as promising battery systems alternative to Li ion, Li metal, and Na metal batteries for electrochemical energy storage.1-4 Mg batteries possess several attractive technical merits, being safe (less reactive compared to Na and Li metal), environmentally benign, inexpensive (ca. 24 times cheaper than Li), high capacity (2205 Ah/kg or 3832 Ah/L vs 3861 Ah/kg or 2062 Ah/L for Li), and high reduction potential (2.37 vs SHE). In the studies of Mg ion batteries, developing high performance Mg2+ electrolytes has been emphasized because of the pivot role of electrolytes for the rechargeable batteries.5 During the past few decades, a great effort has been made in developing reversible Mg2+ electrolytes,6-13 and their electrochemical performance has been remarkably improved.1-3 However, developing high performance and simple Mg2+ electrolytes like those used in Li ion batteries is still challenging.14

Particularly, inorganic Mg electrolytes are very scarce.14-19 In previous studies, the MgCl2/AlCl3

electrolytes (called Magnesium and Aluminum Chloride Complex electrolytes, abbreviated as MACC electrolytes), represent the first generation of all inorganic Mg2+ electrolytes and their simplicity is highly attractive for rechargeable Mg battery applications.14,15 The MACC electrolytes exhibited good reversibility of Mg deposition/stripping (up to 100% Coulombic efficiency), high anodic stability (up to 3.4 V vs Mg), and limited nucleophilic susceptibility (non-nucleophilic and sulfur compatible).14,15 In spite of the apparent merits, preparation of high performance MACC electrolytes is not straightforward. Aurbach et al.20 and Gewirth et al.

16,17

reported an electrochemical conditioning method to improve the performance of the MACC electrolytes in THF and DME. However, the electrochemical conditioning process is tedious and difficult to scale up as it needs up to many cycles of cyclic voltammogram (CV) to get good

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reversibility for a small amount of electrolyte. In our own experience, very careful control of solvent quality with water content below 10 ppm was needed to achieve reliable electrochemical results free of electrochemical conditioning.14 Our recent practices suggest MACC electrolytes are also susceptive to unknown impurities introduced from solvents or reactants (see supporting information). Maintaining such high quality solvent is also not feasible. Oh et al. reported a conditioning-free MACC like electrolyte in THF.18 However, the conditioning-free MACC electrolyte only delivered low activity for Mg deposition (about 3.0 mA/cm2 at -0.5 V vs Mg) and also involves the use of highly toxic CrCl3. Herein, we report a convenient approach to prepare high performance all inorganic Mg/MgCl2/AlCl3 electrolytes in different ethereal solvents including THF, DME, and diglyme (DGM), abbreviated as MMAC electrolytes. For the new generation electrolytes, MMAC−THF, MMAC−DME, and MMAC−DGM, the Coulombic efficiency is up to 90% ~ 100%, the overpotential (defined as the difference between the onset potentials of Mg deposition and striping) was as low as 120 ~ 215 mV, the anodic oxidative stability was pushed to 3.5 ~ 3.8 V (vs Mg). To the best of our knowledge, these tertiary MMAC electrolytes represent the most facile and reliable inorganic Mg electrolytes known so far. We hypothesized that reductive Mg powder could function as a scavenger to remove deleterious species present in the reaction of preparing the MgCl2/ACl3−THF electrolyte (MACC−THF). Explorative studies revealed that the MACC−THF electrolyte (0.04 mol/L MgCl2 and 0.02 mol/L AlCl3) with Mg powder post treatment could significantly improve its electrochemical performance (Figure S1). After 20 hrs of Mg powder treatment (loading at 5.0 mg/mL Mg), -0.263 V (vs Mg) deposition onset potential, 165 mV overpotential, and 100% Coulombic efficiency were obtained, which is significantly improved over the untreated MACC–

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ACS Energy Letters

THF electrolyte that exhibited 400 mV overpotential, and 72% Coulombic efficiency (Figure 1A, S1, and S4).

Figure 1. (A) CV curves of MMAC–THF electrolytes with Mg powder post treatment (red) and in situ treatment (blue), and MACC–THF (black trace, without Mg treatment). Insert: CVs in the range of 2.5 ~ 4.0 V. (B) Plots of charge over time of the Mg deposition and stripping for the studied electrolytes. (C) The bulk electrolysis of MACC–THF electrolyte (upper) and MMAC– THF (in situ Mg treatment) (lower) at 3.2 V, 3.5 V, and 3.6 V for 60 min. (All tests were conducted on a glassy carbon working electrode.) Encouraged by the above preliminary results, then the three-component electrolyte, Mg/MgCl2/AlCl3–THF electrolyte (abbreviated as MMAC–THF) was directly prepared by mixing 5.0 mg/mL Mg together with MgCl2 (0.04 mol/L) and AlCl3 (0.02 mol/L) in THF. The Mg powder was removed by filtration; no further purification was needed. As shown in Figure 1A, after stirring at room temperature for 20 hrs, the MMAC–THF electrolyte displayed excellent reversibility and anodic stability. The overpotential of the MMAC–THF electrolyte yielded by in situ Mg treatment, 159 mV, was even slightly smaller than that of the post treated one, 165 mV. In Figure 1B, the plots of the charge (including both deposition and stripping) over time are shown for MACC–THF (black), post treated (red) and in situ treated (blue) MMAC– THF electrolytes. The equivalent charges of deposition and stripping processes for the MMAC– THF electrolytes (red and blue curve) indicate that the Coulombic efficiency of Mg2+ deposition/stripping for these MMAC–THF electrolytes is nearly 100%. In addition, the stripping process exhibited faster kinetics than the corresponding deposition process, e.g. in the case of the in situ treated MMAC–THF electrolyte, 0.140 and 0.296 C/(s·cm2) for deposition

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and stripping, respectively. Note that the anodic oxidation waves of the electrolytes, at which the oxidation current density is ~ 0.5 % of the peak current density, are observed at ca. 3.5 V vs. Mg for the MMAC–THF electrolytes (see the inset in Figure 1A). Linear scan voltammetry (LSV) (Figure S5) studies also confirmed a 0.3 V improvement of the anodic stability through Mg powder treatment (3.2 V for MACC–THF and 3.5 V for MMAC–THF). The anodic stability of the MMAC–THF electrolyte was further studied by the bulk electrolysis. As shown in Figure 1C, we first set up an electrolysis potential at 3.2 V, at which only non-Faradic background current was observed. When electrolysis potential was shifted to 3.5 V and 3.6 V, the long-term current density was stabilized in the range of 2.0 ~ 3.0 µA/cm2 and 5.0 ~ 9.0 µA/cm2, respectively, for the in situ treated MMAC–THF electrolyte (Figure 1C, lower). However, for the untreated MACC–THF electrolyte, the long-term current density was stabilized in the range of 15 ~ 25 µA/cm2 and 110 ~ 120 µA/cm2 under 3.5 V and 3.6 V, respectively (Figure 1C, upper). We further studied the electrochemical performance of the MMAC electrolytes in DME and DGM solvents using 1:1 and 1:2 ratios of MgCl2 and AlCl3, respectively, with an appropriate amount of Mg powder (Figure 2). Compared to the MACC–DME electrolyte, the Mg deposition onset potential of the MMAC–DME electrolyte was positively shifted about 0.2 V (from -0.429 V to -0.234 V vs Mg), the overpotential was dropped by 180 mV, from 354 mV to 171 mV, and the Coulombic efficiency was increased from 85% to 92% (Figure 2A and 2B). In the case of the MMAC–DGM electrolyte in comparison to the MACC–DGM electrolyte, the Mg deposition onset potential was positively shifted more than 0.3 V (from -0.667 V to -0.332 V vs Mg), the overpotential decreased 41 mV (from 253 mV to 212 mV), and the Coulombic efficiency was increased from 57% to 85% (Figure S6C and S6D). Compared to the MMAC–THF electrolyte, the MMAC–DME and MMAC–DGM electrolytes exhibited higher anodic stability, their

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irreversible oxidation wave was shown up to 3.7 V and 3.8 V vs. Mg, respectively (Figure 2A and 2C, insets). It is probably because of the improved stabilization effect of chain DME and DGM over THF for the AlCl4- anion. The anodic stability of MMAC–DME and MMAC–DGM electrolytes were further confirmed by the bulk electrolysis (Figure S6).

Figure 2. (A) CV curves of the MMAC–DME electrolyte with in situ Mg powder treatment (red) and the MACC–DME electrolyte (black trace, without Mg treatment). Insert: anodic LSV curves of MMAC–DME and MACC–DME electrolytes. (B) Plots of charge over time of the Mg deposition and stripping of the MMAC–DME and MACC–DME electrolytes. (C) CV curves of MMAC–DGM electrolytes with in situ Mg powder treatment (red), and the MACC–DGM electrolyte (black trace, without Mg treatment). Insert: anodic LSV curves of the MMAC–DGM and MACC–DGM electrolytes. (D) Plots of charge over time of the representative Mg deposition and stripping cycles of the MMAC–DGM and MACC–DGM electrolytes. (All tests were conducted on a glassy carbon working electrode.)

The electrochemical data of the MMAC electrolytes in different solvents tested with different working electrodes was summarized in Table 1 (Figure S7 ~ S13 in the supporting information for detail). In general, the MMAC electrolytes show better reversibility in THF and DME but higher anodic stability in DGM. Among the different working electrodes, GC and Pt working electrode displayed the best reversibility and stability in all of these three solvents.

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These electrolytes showed a higher Columbic efficiency with Pt working electrode than GC working electrode while GC working electrode delivered a better anodic stability than Pt working electrode. For example, MMAC–DME exhibited 100% Columbic efficiency and 3.3 V anodic stability on Pt compared to 92% Columbic efficiency and 3.7 V anodic stability on GC. When testing the MMAC–THF electrolyte using Mg strip as the working electrode, the Mg deposition onset potential is 0 V vs Mg and the overpotential is 0 V (Figure S14, SI). The result indicates that the observed additional onset potential and the overpotential on other working electrodes are attributed to the heterogeneous junction of the Mg metal and the applied working electrodes. Table 1. Electrochemical performance data of the MMAC electrolytes prepared in different solvents tested with different working electrodes, glassy carbon (GC), Pt, Al, and stainless steel (SS). The data given in the table is the deposition onset potential (V vs Mg)/overpotential η (mV)/ Coulombic efficiency (%)/potential of anodic stability (V vs Mg). GC

Pt

Al

SS

MMAC– THF

-0.245/159/100/3.5

-0.220/125/100/3.0

-0.245/220/---/1.1

-0.267/118/94/2.0

MMAC– DME

-0.234/164/92/3.7

-0.225/126/100/3.3

-0.127/56/---/0.8

-0.149/105/94/2.2

MMAC– DGM

-0.332/212/85/3.8

-0.359/203/94/3.4

-0.284/65/---/0.9

-0.333/195/77/2.1

To test if there was a composition difference between the as-prepared MACC electrolytes and the MMAC electrolytes, the concentration of Mg2+ and Al3+ in the solution after the Mg powder treatment was tested by ICP–MS. As shown in Table S1, in the presence of Mg powder, the Mg2+ : Al3+ ratio in the solution was increased from 2 : 1 for MACC–THF to 2.39 : 1 for MMAC–THF, from 1 : 1 for MACC–DME to 1.45 : 1 for MMAC–DME, and from 1 : 2 for MACC–DGM to 1 : 1.22 for MACC–DGM. The Mg2+ concentration was increased by 8%, 23%,

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and 41%, respectively. The Al3+ concentration was correspondingly decreased due to the reduction by Mg. The change amount of Mg2+ and Al3+ is consistent with the conversion of 3Mg + 2Al3+ → 3Mg2+ + 2Al. This means that during the Mg powder treatment, Al3+ from AlCl3 was partially reduced by metallic Mg. Which was further confirmed by the Mg deposition/stripping signal shown in the THF solution of AlCl3 treated by Mg powder (note: MgCl2 was not added. Figure S15 in the supporting information). Even free of CrCl3, the observed activity of the combined AlCl3 and Mg powder echoes the results of the AlCl3/Mg/CrCl3 electrolyte reported by Oh et al.18 As the reaction temperatures of MMAC–DME and MMAC–DGM were higher than that of MMAC–THF, there were more Al3+ ions being replaced in DME and DGM than in THF. It should be noted that Mg powder is mainly applied as a scavenger to remove impurities in this study. The large surface area (325 mesh, particle size < 45 µm) and excess use of Mg powder ensure its reactivity with impurities but also lead to the partial reduction of AlCl3. In contrast, in the study reported by Oh et al., Mg powder was used a sole Mg source (no MgCl2 used) and thus CrCl3 was needed as catalyst to facilitate the dissolution of Mg powder. The EDX studies confirmed the deposition of metallic Mg on the surface of the GC using the MMAC–DME electrolyte (Figure S20). There is 1.0 % of Cl shown in the EDX spectrum; this is consistent with Gewirth’s report.21 When the Al3+ was partly replaced by Mg, there was free Clanion generated, and it was proposed by Gewirth et al. that the free Cl- anion would replace the solvent to coordinate on the surface of electrode to avoid passivation. It is noted that no Al deposition was observed. As shown in the SEM image (Figure S20), a dendrite free, smooth, and uniform Mg film was deposited on the GC. It is known that water strongly jeopardizes the performance of Mg electrolytes.22,23 As shown in Figure S16, another control experiment confirmed that Mg powder could effective

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remove water impurity in the electrolyte solution during the treatment. After increasing the water content in the fresh MMAC–DME electrolyte by 50 ppm, the Coulombic efficiency was dropped to 86% and the overpotential was increased to 350 mV. After retreated with Mg powder, both Coulombic efficiency and overpotential were completely recovered. The results so far suggests two functions of Mg powder in the MMAC electrolytes, (A) functioning as a scavenger for impurities as proposed, (B) increasing [Mg2+] by partially replacing Al3+. It is believed that the benefits of Mg as a reactant can be applied to other Mg electrolytes. To get an in-depth understanding of the electrochemical

reaction

during

the

Mg

deposition/stripping process, a systematic kinetic study was conducted. The kinetic study can provide fundamental information of how solvents, electrodes, and the structure of the active species affect the performance of the electrolytes. The current density corresponds to the reaction rate of Mg deposition/stripping on the electrode surface, which is mainly controlled by two factors: the electron Figure 3. The exchange current density measurement of (A) the MMAC–THF electrolytes in different working electrodes and (B) the MMAC electrolytes in different solvents and the MMBC–DME electrolyte using a glassy carbon electrode.

transfers on the interface between electrode and electrolyte, and the mass transport from the bulk solution to the electrode surface.

The exchange current density i0 and electron transfer rate constant k0 can be calculated from the Butler-Volmer equation (eq. 1 and 2, see all equations in the supporting information).24 As

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shown in Figure 3, a linear relationship of i-η was obtained in a η range of -40 mV ~ +40 mV. The i0 of each electrolyte was determined by fitting i versus η with eq. 3 derived from the ButlerVolmer equation.24 k0 was calculated from eq. 4. The measured exchange current density i0 and electron transfer rate constant k0 of the MgCl2-based electrolytes were listed in Table 2A and 2B. In the case of MMAC–THF electrolyte, the exchange current density (i0 = 0.0317 mA/cm2) and electron transfer rate constant (k0 = 7.64 × 10-6 cm/s) shown the highest values on GC. And the electron transfer rate of the MMAC–THF electrolyte on the Al working electrode displayed one order of magnitude slower than that on other working electrodes, which was in agreement with the larger overpotential η (220 mV) of the Al working electrode. As shown in Figure S18, to obtain the same current density (-0.15 mA/cm2), the Al working electrode need a more negative potential (-1.2 V vs Mg). In different solvents, the MMAC electrolytes showed very different electron transfer rate constants. Besides the nature of working electrode, the structure of the electroactive species3 and physical properties of the solvent, such as viscosity and polarity,25,26 also highly affect the electron transfer process. With different Mg2+ : Al3+ ratios and coordinate solvent molecules, the Mg2+ carrying species in different solvents would have different activities. As shown in Table 2B, the slower electron transfer rate of MMAC–DGM electrolyte was in agreement with the higher viscosity of DGM (1.06 mPa/s at 20 oC). The largest k0 of MMAC electrolytes was obtained in DME (2.06 × 10-5 cm/s). With changing the electrolyte Lewis acid component from AlCl3 to BCl3, the electron transfer rate constant of the MMBC–DGM electrolyte displayed a one order of magnitude slower (k0 = 1.02 × 10-6 cm/s). It further confirmed the structure of electroactive species affected the electron transfer of Mg deposition. Table 2A. Exchange current densities (i0) and electron transfer rate constants (k0) of the MMAC–THF electrolytes measured by different working electrodes.

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Working Electrodes

i/η

i0 (mA/cm2)

ko (cm/s)

GC

2.51

0.0317

7.64 × 10-6

Pt

1.99

0.0257

6.19 ×10-6

SS

1.22

0.0158

3.81 × 10-6

Mg

0.917

0.0115

2.77 ×10-6

Al

0.112

1.45 x 10-3

3.49 × 10-7

Table 2B. Exchange current densities (i0), electron transfer rate constants (k0) and the diffusion coefficient (D) of the MMAC and MMBC electrolytes using GC. Electrolytes

i/η

i0 (mA/cm2)

ko (cm/s)

D (cm2/s)*

MMAC– THF

2.51

0.0317

7.64 × 10-6

1.77 × 10-7

MMAC– DME

75.2

0.972

2.06 × 10-5

6.05 × 10-8

MMAC– DGM

1.24

0.016

1.18 × 10-6

2.80 × 10-8

MMBC– DME

0.305

0.00394

1.02 × 10-6

----

Mass transport is a fundamental property of electrolytes. Chronoamperometry (CA) was used to evaluate the diffusion coefficient (D) of the electroactive species in the MMAC electrolytes that is solvent and composition dependent. In the presence of supporting electrolyte, 250 mV overpotential for the Mg deposition was applied on the GC working electrode. The consumption of the active species near the working electrode appears as the current decays, and leads to an inverse of t-1/2 function, suggesting a typical diffusion controlled process. The

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diffusion coefficient D of each electrolyte was determined by fitting i vs t-1/2 using the Cottrell equation (eq. 5 in the supporting information, Figure S19).24 As reported in Table 2B, the MMAC–THF exhibited the fastest diffusion (D = 1.77 × 10-7 cm2/s). The diffusion in DME and DGM is relative slower (6.05 × 10-8 cm2/s for MMAC–DME and 2.80 × 10-8 cm2/s for MMAC– DGM). In summary, we report a convenient and reliable method involving Mg powder treatment to prepare high performance MMAC electrolytes in ethereal solvents. The presented MMAC electrolytes in THF, DME, and DGM exhibited unprecedented electrochemical performance for reversible Mg deposition/stripping including Coulombic efficiency up to 100%, the overpotential below 220 mV, and the anodic oxidative stability up to 3.8 V, respectively. It is believed that the beneficial effects of the Mg powder reactant can be extended to other Mg electrolytes. A systematic kinetic study of the MMAC electrolytes revealed that the electron transfer and mass transport behaviors during the Mg deposition/stripping process were affected by solvents, working electrodes, and the composition of the electrolytes. Our preliminary studies have shown these electrolytes are suitable choices for Mg-S batteries. It is anticipated that these reliable high performance inorganic MMAC electrolytes will find wide applications in Mg rechargeable batteries. Supporting Information contains experimental details and additional figures and tables. Supporting Information is available free of charge from the ACS Publications website or from the author. Acknowledgements We thank Utah State University for providing faculty startup funds to the PI (T. Leo Liu) and the Utah Science Technology and Research initiative (USTAR) UTAG award for supporting this study. J. L. and S.H. contributed to this work equally.

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References: (1) Yoo, H. D.; Shterenberg, I.; Gofer, Y.; Gershinsky, G.; Pour, N.; Aurbach, D. Mg rechargeable batteries: an on-going challenge. Energy Environ. Sci. 2013, 6, 2265-2279. (2) Muldoon, J.; Bucur, C. B.; Gregory, T. Quest for Nonaqueous Multivalent Secondary Batteries: Magnesium and Beyond. Chem. Rev. 2014, 114, 11683-11720. (3) He, S.; Nielson, K. V.; Luo, J.; Liu, T. L. Recent advances on MgCl2 based electrolytes for rechargeable Mg batteries. Energy Stor. Mat., http://dx.doi.org/10.1016/j.ensm.2016.12.001. (4) Xu, K. Chem. Rev. 2014, 114, 11503. (5) Tarascon, J. M.; Armand, M. Issues and challenges facing rechargeable lithium batteries. Nature 2001, 414, 359-367. (6) 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. (7) Beh, E. S.; De Porcellinis, D.; Gracia, R. L.; Xia, K. T.; Gordon, R. G.; Aziz, M. J. Neutral pH Aqueous Organic–Organometallic Redox Flow Battery with Extremely High Capacity Retention. ACS Energy Letters 2017, 639-644. (8) Guo, Y.; Zhang, F.; Yang, J.; Wang, F.; NuLi, Y.; Hirano, S. Boron-based electrolyte solutions with wide electrochemical windows for rechargeable magnesium batteries. Energy Environ. Sci. 2012, 5, 9100-9106. (9) Liu, T.; Cox, J. T.; Hu, D.; Deng, X.; Hu, J.; Hu, M. Y.; Xiao, J.; Shao, Y.; Tang, K.; Liu, J. A fundamental study on the [(µ-Cl)3Mg2(THF)6]+ dimer electrolytes for rechargeable Mg batteries. Chem. Commun. 2015, 51, 2312-2315. (10) Liao, C.; Sa, N.; Key, B.; Burrell, A. K.; Cheng, L.; Curtiss, L. A.; Vaughey, J. T.; Woo, J.-J.; Hu, L.; Pan, B.; Zhang, Z. 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. (11) Canepa, P.; Jayaraman, S.; Cheng, L.; Rajput, N. N.; Richards, W. D.; Gautam, G. S.; Curtiss, L. A.; Persson, K. A.; Ceder, G. Elucidating the structure of the magnesium aluminum chloride complex electrolyte for magnesium-ion batteries. Energy Environ. Sci. 2015, 8, 3718-3730. (12) Watkins, T.; Kumar, A.; Buttry, D. A. Designer ionic liquids for reversible electrochemical deposition/dissolution of magnesium. J. Am. Chem. Soc. 2016, 138, 641-650. (13) Brouillet, E. V.; Kennedy, A. R.; Koszinowski, K.; McLellan, R.; Mulvey, R. E.; Robertson, S. D. Exposing elusive cationic magnesium-chloro aggregates in aluminate complexes through donor control. Dalton Trans. 2016, 45, 5590-5597. (14) 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. (15) Doe, R. E.; Han, R.; 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. (16) Barile, C. J.; Barile, E. C.; Zavadil, K. R.; Nuzzo, R. G.; Gewirth, A. A. A. Electrolytic conditioning of a magnesium aluminum chloride complex for reversible magnesium deposition. J. Phys. Chem. C 2014, 118, 27623-27630. (17) 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, 1352413534. (18) Ha, J. H.; Adams, B.; Cho, J. H.; Duffort, V.; Kim, 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.

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ACS Energy Letters

(19) Keyzer, E. N.; Glass, H. F. J.; Liu, Z.; Bayley, P. M.; Dutton, S. E.; Grey, C. P.; Wright, D. S. Mg(PF6)2-based electrolyte systems: understanding electrolyte–electrode interactions for the development of Mg-ion batteries. J. Am. Chem. Soc. 2016, 138, 8682-8685. (20) Shterenberg, I.; Salama, M.; Gofer, Y.; Levi, E.; Aurbach, D. The challenge of developing rechargeable magnesium batteries. MRS Bull. 2014, 39, 453-460. (21)See, K. A.; Chapman, K. W.; Zhu, L. Y.; Wiaderek, K. M.; Borkiewicz, O. J.; Barile, C. J.; 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. (22) 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. (23) Yagi, S.; Tanaka, A.; Ichikawa, Y.; Ichitsubo, T.; Matsubara, E. Effects of water content on magnesium deposition from a Grignard reagent-based tetrahydrofuran electrolyte. Res. Chem. Intermed. 2014, 40, 3-9. (24) Bard, A. J.; Faulkner, L. R. Electrochemical methods - fundamentals and applications, 2nd Edition, Wily, Hoboken. 2001. (25) Zhang, X.; Leddy, J.; Bard, A. J. Dependence of rate constants of heterogeneous electrontransfer reactions on viscosity. J. Am. Chem. Soc. 1985, 107, 3719-3721. (26) Maroncelli, M.; Macinnis, J.; Fleming, G. R. Polar-solvent dynamics and electrontransfer reactions. Science 1989, 243, 1674-1681.

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AlCl3 Inorganic Mg2+ Electrolytes with

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