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Journal of Power Sources 362 (2017) 308e314

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Journal of Power Sources

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Improving halide-containing magnesium-ion electrolyte performancevia sterically hindered alkoxide ligands

Carl A. Nist-Lund a, b, Jake T. Herb a, b, Craig B. Arnold b, c, *

a Department of Chemistry, Princeton University, Princeton, NJ 08540, United Statesb Princeton Institute for the Science and Technology of Materials, Princeton University, Princeton, NJ 08540, United Statesc Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08540, United States

h i g h l i g h t s

� Sterically hindered Mg dialkoxides and AlCl3 in THF reversibly electrodeposit Mg.� Positive relationship between ligand steric size and electrolyte performance.� Larger ligands improve anodic stability, cycling efficiency, and deposition purity.� The triphenylmethoxide-based electrolyte enhances reactivity of metallic Mg.

a r t i c l e i n f o

Article history:Received 19 June 2017Accepted 13 July 2017Available online 20 July 2017

Keywords:Magnesium-ion batteriesElectrodepositionElectrolytesAlkoxides

* Corresponding author. Princeton Institute for thMaterials, Princeton University, Princeton, NJ 08540,

E-mail address: cbarnold@princeton.edu (C.B. Arn

http://dx.doi.org/10.1016/j.jpowsour.2017.07.0450378-7753/© 2017 Elsevier B.V. All rights reserved.

a b s t r a c t

While homoleptic magnesium dialkoxides (MgR2, R ¼ alkoxide) have shown promise as precursors formagnesium-ion electrolytes, the effect of ligand steric bulk on the performance of electrolytes based onthese compounds is not fully understood. Increasing steric hindrance, studied via R groups with addi-tional phenyl moieties, produces electrolytes with sequentially lower deposition overpotentials (lessthan �90 mV), higher purity Mg deposits (ca. 100% Mg), and lower overall cell impedances. The twolargest alkoxide ligands show consistent cycling behavior and low stripping and plating overpotentialsover 200 constant-current plating/stripping cycles. A deep-red visual change and the presence of largesolubilized magnesium particulates above 450 nm in size is observed in an electrolyte containingmagnesium bis(triphenylmethoxide) and aluminum chloride in contact with an abraded magnesiumanode. Further morphological and impedance characterization show that this electrolyte system rapidlyactivates the magnesium metal anode surface to produce low overpotentials and, as such, is a candidatefor further investigation.

© 2017 Elsevier B.V. All rights reserved.

1. Introduction

An efficient, reliable, and energy dense magnesium-ion battery(MIB) has the potential to accelerate the adoption of affordableelectric vehicles and grid-level energy storage systems [1]. One ofthe major hurdles in the creation of such a MIB has been thedevelopment of an effective electrolyte capable of reversible mag-nesium deposition and dissolution with high efficiency and lowplating and stripping overpotentials [2e4]. Fundamental to thesearch for promising electrolytes is an understanding of how

e Science and Technology ofUnited States.old).

various electrolyte components interact with one another and theireffects on the resulting electrochemistry. Most magnesium-ionelectrolytes contain halide ions in some form, either from themagnesium precursor itself (i.e. PhMgCl), or from a secondcomponent, often AlCl3 or MgCl2 [3,5]. Although the presence ofhalide ions limits the anodic stability of electrolyte systems, thebenefits of halides include, but are not limited to: activation of thetenacious Mg surface oxide layer, enabling close to 100% cyclingefficiencies, and low overpotentials for both the deposition andstripping processes [6,7].

Two classes of halide-containing electrolytes have emerged, assummarized in Fig.1. In the first class, themagnesium component isderived fromheteroleptic systems of the form RMgX, which includealkyl- [8], aryl- [9], amido- [10], and alkoxymagnesium [11e13]halide compounds. Quasi-Grignard or pseudo-Grignard species

Fig. 1. Classes of electrolyte precursors, some prototypical examples within each class, and the equilibria that form in such liquid electrolytes. In both cases, R is chosen from alkyl,amido, or alkoxy ligands, and X is a halide (usually Br or Cl). A special case on far right includes the combination of MgCl2 and AlCl3, canonically known as the Magnesium AluminumChloro Complex (MACC) [28,29].

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are special cases of heteroleptic systems that use non-alkyl or non-aryl ligands bonded to a magnesium halide species (e.g. amido-magnesium halides or alkoxymagnesium halides). In contrast toheteroleptic Mg compounds, homoleptic magnesium compoundsof the type MgR2 include compounds such as dialkyl Grignard re-agents [14], magnesium bisamide compounds [15] includingMg(HMDS)2 [16], and magnesium dialkoxides [17e19]. Whereas inthe first class, additional halide containing components enhance,but are not necessary for reversible electrochemistry [20,21], themajority of members within the second class often require a secondcomponent such as AlCl3 or MgCl2 in order to perform efficiently[22]. Many researchers have endeavored to remove halide ionsfrom solutionwith varied success. While the majority of halide-freeMg compounds do not reversibly plate magnesium by themselves,notable exceptions include Mg(TFSI)2 [23], Mg(CB11H12)2 [24],Mg(PF6)2 [25], and Mg[Al(HFIP)4]2 [26,27].

In previous work on magnesium dialkoxides composed of sim-ple ligands (MgR2, R ¼ OEt, OiPr, OtBu, OPh) in combination withAlCl3, the series of electrolytes performed with reasonable effi-ciencies, and relatively pure deposits [18]. The Mg(OPh)2:AlCl3 so-lution performs most favorably, and it is hypothesized that thecombination of electron-withdrawing ligands and/or increasedsteric hindrance enhance electrolyte figures of merit [13,30]. Toinvestigate this hypothesis further, a series of increasingly stericallyhindered magnesium dialkoxides is investigated in this work,including magnesium bis(benzyl methoxide) Mg(OCH2Ph)2, mag-nesium bis(diphenylmethoxide) Mg(OCHPh2)2, and magnesiumbis(triphenylmethoxide) Mg(OCPh3)2. After combining these pre-cursors with AlCl3, the resulting electrolytes are studied via a va-riety of canonical characterization methods including anodic linearsweep voltammetry, cyclic voltammetry, and galvanostatic depo-sition. Since research on alkoxide-based magnesium electrolyteshas already shown that these solutions enable reversible interca-lation into a variety of cathode materials [19,31,32], this work fo-cuses on the electrochemical parameters that are relevant to anodicprocesses. Scanning electron microscopy (SEM) and electrondispersive x-ray spectroscopy (EDX) analysis of the plated deposits,and electrochemical impedance spectroscopy (EIS) of symmetricMg-Mg cells is also presented. Long-term constant-current plating/stripping of the two most sterically hindered solutions is used toestablish reversible cell performance as determined by coulombicefficiency.

2. Experimental

2.1. Synthesis of precursors magnesium bis(phenolate), magnesiumbis(benzyl methoxide), magnesium bis(diphenylmethoxide), andmagnesium bis(triphenylmethoxide)

All reactions were carried out with rigorous exclusion of air and

water in an Ar filled glovebox (Vacuum Atmospheres) and Schlenkapparatus. All chemicals were purchased from Sigma-Aldrich andused as received unless otherwise specified. MgR2 compoundswere synthesized using a common reaction pathway as previouslydescribed [18,33] by charging a Schlenk flask with approximately5 g of the solid alcohol (phenol (99%), benzyl alcohol (99.8%),diphenylmethanol (99%), triphenylmethanol (97%)) and pullingvacuum on the flask in an effort to remove residual water.Approximately 30 mL of inhibitor-free THF (99%) was added tosolvate the alcohol, forming a clear solution that was chilled in anice bath to 0 �C. 0.8 equivalents of di-n-butylmagnesium (1 M inheptane) were added drop wise to the solution and the reactionflask was warmed to room temperature and left to react overnight.The solutions were gently refluxed for 30 min and then cooled toroom temp. The mixture was then vacuum distilled to form an oilywhite solid. Approximately 20 mL of THF was re-added to form asuspension, which was left to settle overnight and the supernatantsubsequently removed. This process was repeated three times.After removing the final THF aliquot, the remaining mixture washeated in an oil bath at 80 �C under vacuum to remove THF, hep-tane, and any residual alcohol. All resulting solids were white, free-flowing powders.

2.2. Preparation of electrolyte solutions

MgR2:AlCl3 electrolytes with concentrations of either 100 or250 mM by Mg were prepared by first mixing the magnesiumcompound in an appropriate amount of THF, followed by slowlyadding the corresponding stoichiometric amount of ultra-dryaluminum chloride (99.999%, Alfa Aesar) to form the desiredmolar ratio solution. Often the MgR2 solutions in THF were initiallycloudy, but turned clear and colorless after the addition of AlCl3.Solutions were filtered through Thermo Scientific Target 0.45 mmPTFE filters before use. Electrolytes were reacted for a minimum of24 h prior to electrochemical testing. NMR spectra were recordedusing a Bruker Avance III 500 MHz spectrometer. Solutions wereprepared in D8-THF (Cambridge Isotope Laboratories, Inc.). Spectraand peak assignments are included in the Supporting Information.

2.3. Preparation of coin-cells

Standard 2032 coin-cells were constructed in an Ar glovebox.Celgard 2400 separators were soaked in electrolyte solutions for aminimum of 2 h before use. Freshly abraded magnesium (99.95%GalliumSource) was used as the anode in either Mg/Mg symmetriccells orMg/Cu half cells. A 125 mm thick Teflon ring (McMaster Carr)with an outer diameter of 5/8” and inner diameter of 1/4” wasplaced between the anode and cathode to restrict the workingelectrode to a defined area.

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2.4. Electrochemical testing and imaging analysis

Cyclic voltammetry was performed at a scan rate of 10 mV s�1

and galvanostatic plating experiments in coin-cells were per-formed at a current density of �50 mA cm�2 for 7 h (Arbin SystemModel BT2000). Galvanostatic switching experiments were per-formed at current densities of ±0.5 mA cm�2. Cycles consisted of15 min during each plating step, and a stripping current wasapplied until the cell voltage reached þ1.0 V vs. Mg/Mg2þ. Anodicstability measurements were acquired in a flooded cell at a scanrate of 10 mV s�1 using either a polished platinum working elec-trode (CH Instruments, diameter ¼ 2 mm) or a polished copperworking electrode (diameter ¼ 1/8 inch, area ¼ 0.0792 cm�2) withan abraded magnesium reference and counter electrode. EIS mea-surements were taken using coin-cells at open circuit from a rangeof 1 MHz to 0.1 Hz (Solartron Analytical 1260 and 1287). SEM im-ages and EDX spectrawere acquired at a 15 keV accelerating voltage(FEI Quanta 200 ESEM with Oxford Instruments EDX).

3. Results and discussion

3.1. Electrolyte preparation and anodic stability measurements

The sterically hindered alkoxide ligands of MgR2 precursorsinvestigated in this work are shown in Fig. 2. When consideringelectrolyte preparation routes, earlier work on alkoxide electrolytesshows comparable performance among analogous precursors (e.g.combining Mg(OPh)2 and AlCl3 or PhOMgCl and AlCl3), suggestingthat similar electroactive solution species are formed in bothpreparations [18]. One practical benefit of using these magnesiumdialkoxide precursors is that they are easily manipulated powders.During electrolyte preparation, the dialkoxide precursors bythemselves exhibit minimal solubility in THF. Magnesium dia-lkoxides are known to readily form insoluble aggregates [33,34],and one of the main benefits of adding halides such as chloride isthe ability to form m-bridging atoms to form a variety of solublesolution species [22,35,36]. Additionally, for sterically hinderedalkoxides such as those studied herein, MgCl2 alone does notappear to increase solubility of sterically hindered alkoxides, butthe strong Lewis acid AlCl3 can act as both a halide source to breakup alkoxy aggregates and a means to extend the oxidative stabilityof the electrolyte [6].

1H and 25Mg NMR spectroscopy provide valuable insights intothe composition of these electrolytes. 1:1 MgR2:AlCl3 electrolytesolutions in D8-THF were prepared and spectra from this series ofelectrolytes are shown in Table S1 and Figs. S1eS5. In the 1H NMRspectra, peaks corresponding to di-n-butyl magnesium, the syn-thetic starting compound, were not observed in any of the solu-tions. In the magnesium spectra, the chemical shifts observed forthese solutions are in the same range as halide-containing mag-nesium electrolytes reported previously [37,38]. Additionally, bymoving from phenoxide to larger ligand moieties, the 25Mg peakwidths broaden, which suggests greater asymmetry in the ligands

Fig. 2. Ligand moieties of magnesium precursor compounds used in this work. Thestoichiometry of the solid magnesium alkoxide precursors is assumed to be MgR2. Theprecursors were combined with AlCl3 in THF to produce electroactive magnesium-ionelectrolytes.

surrounding the Mg metal center, likely as the result of stericcrowding [37].

Various methods have been employed to extend oxidative sta-bilities of magnesium-ion electrolytes [13,30]. Since the develop-ment of the first magnesium-ion electrolytes, oxidative stability hasbeen successfully increased by modifying the magnesium ligandmoieties from alkyl to amido or alkoxy groups [5]. Regardless of theligand moiety bonded to the magnesium precursor, the stability ofchloride-containing electrolytes plateau at approximately 3.2 V onplatinum [3,5]. Within the alkoxide ligand family, it has also beenshown that either increasing electronic withdrawing effectsthrough the use of fluorine substituents [13,19,30,39] or steric bulkthrough tert-butyl or phenyl groups [13] have a positive effect onextending anodic stability. Consistent with these trends, as shownin Fig. 3, the electrolytes formed from ligands with more steric bulkthan phenoxide exhibit slightly higher anodic stabilities on Pt up-ward of 3.1 V vs. Mg/Mg2þ. On copper, stabilities reach a maximumof 1.5 V vs. Mg/Mg2þ (Fig. S6), indicating that copper metal disso-lution readily occurs at low potentials.

3.2. Performance of reversible Mg deposition/dissolution via cyclicvoltammetry

Fig. 4 shows cyclic voltammograms for the series of electrolytesin Mg/Cu half cells. On the initial cycle, for the three smallest li-gands, there is a large deposition overpotential for plating ofabout �750 mV. During long-term cyclic voltammetry, this over-potential progressively decreases to just below �300 mV on sub-sequent cycles. For the Mg(OCPh3)2:AlCl3 electrolyte, however, thedeposition overpotential on the first cycle is �215 mV, and by the50th cycle the overpotential is �150 mV. The triphenylmethoxidesystem also notably shows stripping overpotentials near zero onfirst cycle, whereas other electrolytes show overpotentials upto þ600 mV. During continued cycling, the Mg(OCPh3)2:AlCl3continues to show deposition and stripping overpotentialsnear �95 mV and 0 mV vs. Mg/Mg2þ over hundreds of cycles,respectively.

For the entire electrolyte series, the deposition overpotentialdecreases with continued cycling. It does not appear that thesesystems form passivation layers that prevent useful electrochem-istry during cycling. An initial conditioning process for the elec-trolytes does appear to be necessary for voltammograms to reachsteady state. The groups with more steric hindrance increase therate of the electrolyte conditioning process via a combination ofexposure of fresh Mg on the anode surface, decreasing residualunoxidizable Mg on the cathode, and increasing the concentrationof electroactive species in the electrolyte [29].

Fig. 3. Anodic stabilities of 100 mM 1:1 MgR2:AlCl3 electrolytes in THF. Linear sweepvoltammograms obtained at a scan rate of 10 mV s�1 on platinum.

Fig. 4. Cyclic voltammograms performed at 10 mV s�1 in Mg/Cu coin cells for the series of electrolytes studied. A) Initial cycles for each respective ligand. B-E) 50th, 100th, and 150th

cycles for 100 mM 1:1 MgR2:AlCl3 in THF, R ¼ OPh, OCH2Ph, OCHPh2, OCPh3, respectively. The area of the electrode is 0.317 cm�2.

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3.3. Mg deposition purity and morphology

The purity and morphology of the magnesium deposits fromthese systems were studied using a series of galvanostatic platingexperiments in symmetric Mg/Mg and half-cell Mg/Cu coin-cells.As shown in Fig. 5, EDX analysis of the deposits on Mg show

relatively pure deposits among all systems, with extremely puredeposits for the two most sterically hindered electrolytes. SEMimaging reveals that each electrolyte's deposit has a non-dendritic,crystal-like morphology that is consistent on both substrates(Fig. S7).

Increased steric hindrance of the alkoxide ligands may yield

Fig. 5. EDX analysis of Mg deposits produced via galvanostatic depositionat �50 mA cm�2 for 7 h on a Mg substrate. Percentages adjusted after removal of ox-ygen peak (formed during transfer of sample from glovebox to sample chamber) andcarbon (double sided C tape to adhere sample to SEM/EDX stub).

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aluminate solution species that prevent the co-deposition ofaluminum. In solution equilibria, various transmetallation re-actions occur that place ligands that originate on the Mg species onthe Al metal center [20]. For the electrolytes studied here, asaluminum centers gain more alkoxide ligands, the more stericallyhindered ligands may prevent cationic Al species from co-depositing under anodic conditions. In other Mg-ion electrolytes,such as halide-containing ionic liquids, the use of larger ligandssuch as iodide instead of chloride, similarly leads to less Al code-position [40,41]. In effect, ligands with steric bulk might preventcharged Al atoms from approaching the anode surface and canrestrict charge transfer to favor deposition of magnesium cations.

3.4. Rapid reactions of Mg metal with Mg(OCPh3):AlCl3 electrolyte

During the construction of coin-cells for electrochemical testing,the Mg(OCPh3)2:AlCl3 solution produces a striking visual phe-nomenon. As shown in Fig. 6a and b, as well as Video S1, within afew seconds of adding a separator that had been soaked in elec-trolyte to a freshly abraded Mg disk, the anode develops a brightred color. In a separate experiment, an abraded Mg anode wasplaced in the 100 mM Mg(OCPh3)2:AlCl3 electrolyte for 15 min and

Fig. 6. Phenomena and observations during the preparation of Mg(OCPh3)2:AlCl3 coin-cells:disk; b) Mg disk 7 s after placement of soaked separator; c) SEM image of Mg surface aftershows abraded magnesium before soaking in electrolyte.

then removed and rinsed with THF, and the treated electrode sur-face appeared dull instead of shiny. SEM images of the surface showthat contact with the electrolyte leads to rapid surface pitting, asnoted in Fig. 6c, and is the cause of the dull macroscopic appear-ance. Color changes in nonaqueous Mg electrolyte solutions are notoften observed. We hypothesize that in the specific case of theMg(OCPh3)2:AlCl3 electrolyte system, a ligand-Mg0 adduct isformed. Of the electrolytes studied, only this electrolyte systemproduces this unique set of physicochemical reactions. Furtherelectrochemical implications of this reactivity are explored in thefollowing section.

Supplementary data related to this article can be found online athttp://dx.doi.org/10.1016/j.jpowsour.2017.07.045.

Halide ions in solution enable Mg dialkoxide electrolyte pre-cursors to form a variety of possible electroactive species [6,22,35].For example, an initial magnesium alkoxide dimer can react withMgCl2 or AlCl3 to form heteroleptic species [6]. Once the RMgXspecies is formed, it can autoionize to form MgRþ and RMgX2

- , orvarious bridged species. These ions and their various chargedequilibrium products can serve as active Mg-ion species. For theMg(OCPh3)2:AlCl3 electrolyte solution, the unique interaction be-tween the heteroleptic species and the Mg anode appears to allowfor facile Mg reduction and oxidation during cycling.

3.5. Impedance analysis and long-term galvanostatic cycling

Cell impedance after a single galvanostatic deposition step insymmetric Mg/Mg cells was analyzed through EIS to elucidate thecharge transfer (CT) resistance of the various electrolyte solutionsas described [42]. The CT resistance for Mg(OCPh3)2 system afterone deposition cycle is almost two orders of magnitude lower thanthe impedance of the other systems (Fig. S8) and lower thanimpedance values for Grignard-based systems [43,44]. The low CTresistance in the sterically hindered systems is evidence of theformation of non-passivatedmagnesium deposits that can be easilyaccessed in subsequent stripping steps [45].

Extended galvanostatic plating/stripping was performed withonly the two most sterically hindered electrolytes due to theirhighly pure Mg deposits. Three initial cyclic voltammograms wereperformed before the first plating step to activate the anode sur-face. As shown in Fig. 7, the Coulombic efficiency for the Mg(OC-Ph3)2:AlCl3 system reaches approximately 80% efficiency while theMg(OCHPh2)2:AlCl3 system rises to 96.6% after 200 cycles. We hy-pothesize that the low efficiency of the Mg(OCPh3)2:AlCl3 system isassociated with the roughened anode surface as seen previously inFig. 6c. Even at open circuit, magnesium metal quickly dissolvesinto solution when in contact with this electrolyte. While thissystem allows for low deposition overpotentials and low cell

a) initial conditions before placement of electrolyte-soaked separator onto abraded Mg15 min of soaking an abraded anode in 100 mM 1:1 Mg(OCPh3)2:AlCl3 solution. Inset

Fig. 7. Coulombic efficiency for the first 200 cycles of the Mg(OCHPh2)2:AlCl3 andMg(OCPh3)2:AlCl3 systems during galvanostatic plating and stripping at ±0.5 mA cm�2

in Mg/Cu coin-cells. Cycles consisted of 15 min during each plating step, and a strip-ping current was applied until the cell voltage reached þ1.0 V vs. Mg/Mg2þ.

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impedances, parasitic loss of plated magnesium appears to be thesource of the low Coulombic efficiency in this system. While theexact nature of the metal-electrolyte interaction is unclear andwarrants further study, it appears that in this case, several surfaceactivating properties arise that are of benefit for some figures ofmerit at the expense of others.

4. Conclusion

Each component of magnesium-ion electrolytes, including themagnesium-containing compound, the halide-containing com-pound, and the solvent, work in concert on the microscopic scale toproduce macroscopic effects on the resulting electrochemistry inthe electrolyte. The entire series of magnesium dialkoxide/AlCl3mixtures studied here show reversible Mg electrochemistry.Additional steric hindrance appears to enhance electrochemicalproperties of interest, such as low deposition overpotentials andhigh purity deposits. This work illustrates that increased ligandsteric hindrance could be a promising avenue of discovery for avariety of electrolyte systems.

The most sterically hindered Mg(OCPh3)2:AlCl3 electrolyte isespecially reactive. This composition imparts a suite of beneficialcharacteristics, including low deposition overpotentials, activationof the Mg anode surface, low cell impedance, and pure Mg deposits.Given that alkoxide compounds can be used as additives and havebeen shown to increase anodic stability and enhance depositionmorphologies, a small amount of Mg(OCPh3)2 could thus be used asan additive in other Mg-ion electrolytes to help enhance variouselectrochemical figures of merit.

Acknowledgements

The authors acknowledge funding from the Addy/ISN NorthAmerican Low Carbon Emission Energy Self-Sufficiency Fund atPrinceton University, an NSF Graduate Research Fellowship underNSF - DGE 1148900, and the PRISM Imaging and Analysis Center,with funding from NSF - DMR 1420541.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.jpowsour.2017.07.045.

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