Conditioning-Free Electrolytes for Magnesium Batteries UsingSufone−Ether Mixtures with Increased Thermal StabilityLaura C. Merrill and Jennifer L. Schaefer*

Department of Chemical and Biomolecular Engineering, University of Notre Dame, 205 McCourtney Hall, Notre Dame, Indiana46556, United States

*S Supporting Information

Increasing demands for higher performing and moresustainable batteries have led to research in beyond

lithium-ion battery chemistries. Magnesium metal serves as aviable option as a new anode material due to its widespreadabundance, about 1000 times more abundant in the earth’scrust compared to lithium, and its high volumetric capacity(3833 vs 2062 mAh/cm3 for lithium metal). The developmentof the practical magnesium metal battery requires developmentof electrolytes with several characteristics including non-corrosiveness with common inactive components, sufficientlywide electrochemical stability, sufficient thermal stability, abilityto electrodeposit and dissolve magnesium metal with highefficiency, and ability to facilitate reversible electrochemicalbehavior at a cathode.1−3

Recently, magnesium electrolyte research has focused almostexclusively on ethereal solvent formulations.4−6 In someelectrolyte formulations containing higher molecular weightand more thermally stable ethers such as glymes, irreversibilityis observed during cycling from electrolyte decomposition.7,8

This is hypothesized to be due to the chelating effect of longerchain glymes that leads to decomposition prior to desolvationduring charge transfer. This effect is reported in works thatidentify Mg2Cl3

+ and MgCl+ as the active species.7,8

A few recent reports demonstrated magnesium electrolytesbased on sulfones.9,10 Sulfones are attractive due to their highboiling points and low vapor pressures. Prior studies showedlow reversibility in early cycles, and very negative potentials(less than −1 V vs Mg2+/Mg0) were required to inducemagnesium deposition.9 Improvement in reversibility wasachieved only upon conditioning, or repeated cyclic voltam-metry until reversible behavior is achieved.We herein report that Lewis acid-free Mg(HMDS)2−MgCl2

containing electrolytes based on certain sulfone/tetrahydrofur-an (THF) mixtures, in contrast, exhibit magnesium depositionat low underpotentials with reversibilities above 90% on thefirst cycle, with much higher thermal stability than the pureTHF counterpart. The Mg(HMDS)2−MgCl2 formulation isparticularly attractive for its facile preparation from alreadycommercialized salts, but to date this system has shown highelectrochemical performance only in THF, and little to nosolubility in higher boiling point ethers such as glymes, thuslimiting practical application.11 This electrolyte formulation wasreported in glyme/THF mixtures; however, significantmagnesium deposition/stripping was not observed.11 Wereport here on insights into the sulfone/THF electrolytesolution speciation through NMR and mass spectrometrystudies and show that the relative population of Mg2Cl3

+ versus

MgCl+ cations is a determining factor in reversible anodicelectrochemistry.Electrolytes were prepared, as described in Supporting

Information, in different solvents (THF, di-n-butyl sulfone(BS), and/or sulfolane (SL)) with the salt ratios andmagnesium concentration kept constant (Mg(HMDS)2−4MgCl2, 1.25 M Mg). The thermal stability of the electrolyteswas measured using TGA.Figure 1 shows the resulting mass loss curves for electrolytes

based on BS/THF mixtures as compared to pure THF. The 30

THF/70 BS (v/v) electrolyte and 50 THF/50 BS (v/v)electrolyte demonstrated thermal stability to 100 and 80 °C,respectively. The volatile nature of the pure THF electrolyte isdemonstrated as the mass fraction is immediately decreasing; itwas impossible to get an accurate starting mass of the THFelectrolyte as the solvent was evaporating off at roomtemperature. The 50 THF/50 SL (v/v) electrolyte was similarto the pure THF electrolyte and had an immediate decrease inmass; however, the rate of mass loss was much lower (FigureS4).Both 50 THF/50 BS and 30 THF/70 BS electrolytes

facilitated magnesium electrodeposition at fairly low depositionunderpotentials (about −200 mV vs Mg2+/Mg0 referenceelectrode), as depicted in Figure 2. The voltammogram showshigh reversibility was observed for equal volume mixtures ofTHF and BS. Conversely, the electrolyte based on pure butyl

Received: February 1, 2018Revised: May 31, 2018Published: May 31, 2018

Figure 1. TGA of Mg(HMDS)2−4MgCl2 in THF (light blue), 50THF/50 BS (blue), and 30 THF/70 BS (green).

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© 2018 American Chemical Society 3971 DOI: 10.1021/acs.chemmater.8b00483Chem. Mater. 2018, 30, 3971−3974

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sulfone did not facilitate magnesium electrodeposition. FigureS5 shows the voltammograms on a narrower current scale. Inaddition, the 50 THF/50 SL electrolyte showed reductivecurrents only at very low potentials and little reversibility,suggesting electrolyte decomposition.Magnesium was electrochemically deposited onto a platinum

wire from the three solvent mixtures using a constant potentialhold. The SEM/EDS of the deposits (Figure 3) showed thatthe two butyl sulfone containing electrolytes produced depositswith high magnesium content and low amounts of carbon,sulfur, chlorine, and silicon. The magnesium that was depositedfrom the butyl sulfone containing electrolytes did show some

decomposition products, primarily carbon and chlorine. Giventhe low amounts of sulfur present in the deposit, it is unlikelythat the decomposition products are predominantly due to thebutyl sulfone species.The deposit from the sulfolane containing electrolyte had a

lower amount of magnesium with higher carbon and chlorinecontents along with a spongey morphology as opposed to thedense, spherical morphology of deposits present for the othertwo electrolytes. This confirms that the lowered reversibility ofthe sulfolane electrolyte is related to solvent decomposition.Despite the high reversibility of the 50 THF/50 BS

electrolyte, it still has lowered current densities and greaterdeposition underpotentials as compared to the pure THFelectrolyte, as shown in Figure S6a. This is, in part, reflected inthe ionic conductivity of the electrolyte, shown in Figure S6b.The addition of butyl sulfone to THF increases the viscosity ofthe electrolyte and can slow the ion transport in solution.Furthermore, the butyl sulfone has a stronger coordination tothe magnesium cation, as discussed later, and therefore mayrequire a stronger driving force for the magnesium ion to shedits solvent shell and electrochemically deposit. The THF-onlyelectrolyte has a greater amount of the active species, aselaborated upon later in the text, and it is hypothesized that thisalso contributes to the greater current density.The oxidative stability limit of the 50 THF/50 BS electrolyte

we find to be 2.6 V (vs Mg2+/Mg0); this value is nearly identicalto the oxidative stability of the electrolyte in pure THF. Thisvalue is low but sufficient for cathodes such as sulfur andMo6S8. Figure S8 shows the electrolyte stability limits of theTHF and 50 THF/50 BS electrolyte versus Pt, Cu, Al, andstainless steel.Galvanostatic cycling at 20 and 50 °C of symmetric cells

containing the 50 THF/50 BS electrolyte shows that interfacialresistance is fairly stable with repeated deposition anddissolution at either temperature (Figure S11). However, aslight overpotential appears to be necessary for magnesiumdeposition, especially at 50 °C, given the shape of potentialprofiles. It is hypothesized that this is due to the formation ofan adsorbed layer at the magnesium-electrolyte interface.Galvanostatic cycling of magnesium/copper coin cells was

used to more quantitatively measure the reversibility of theelectrolyte (Figure 4a). Coulombic efficiencies around 92%were maintained during room temperature cycling (Figure 4b),compared with 95% measured for the THF only electrolyte.The efficiency for the mixture at 50 °C averaged 2% less thanthe efficiency at 20 °C. The exact cause of the nonunityCoulombic efficiency is not known, but could be due to traceimpurities, trace moisture, and the formation of uneven, highsurface area deposits. Trace water can alter the structure of thedouble layer at the magnesium electrode; this changes theinterfacial chemistry and can lead to slow deposition kinetics,affecting the reversibility.12 Large amounts of magnesium stillpresent on cycled copper electrodes after stripping indicate thatthese deposits are electronically isolated, thus some of theirreversibility is not correlated with side reactions (Figure S12).The nonuniformity could be related to adsorptive layerformation; this is beyond of the scope of this work but isunder investigation. We hypothesize that the depositionmorphology of the magnesium is the primary cause of thegradual polarization over cycling and related to the nonunityefficiency.X-ray photoelectron spectroscopy (XPS) was completed on

magnesium electrodes after cycling in Mg/Cu cells (see Figures

Figure 2. Cyclic voltammograms of Mg(HMDS)2−4MgCl2 at 1.25 MMg in (a) 50 THF/50 BS, (b) 30 THF/70 BS, (c) BS, (d) 50 THF/50SL. The voltammogram of panel d was collected at 80 °C as thisformulation is solid at room temperature. Data for the first cycle isshown, data beyond the first cycle is appended in Figure S7. Theworking electrode was platinum wire and the reference and counterelectrodes were magnesium ribbon. A scan rate of 5 mV/s was used.

Figure 3. SEM of magnesium deposits from Mg(HMDS)2−4MgCl2 in(a) 50 THF/50 BS, (b) 30 THF/70 BS, and (c) 50 THF/50 SL; and(d) the corresponding EDS data.

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S13−S15). Because of limitations of the facility instrumenta-tion, ambient air exposure was unable to be avoided.Decomposition products present on the surface includedcarbon, sulfur, chlorine, and silicon. The sulfur content wasvery low in comparison to the other elements, thus it is unlikelythat the low efficiencies are primarily due to the decompositionof the sulfone.To probe the differences between the sulfolane and butyl

sulfone containing electrolytes, ESI-MS and NMR spectroscopywere used to investigate speciation. Cation coordination withboth THF and the sulfone is confirmed via proton NMRspectroscopy. The 1H NMR spectra of the 50 THF/50 BSmixture shows that the protons on the α-carbon of THF and ofBS both shift downfield in the presence of the salt, 0.070 and0.072 ppm, respectively (Figure S10). Given that time scale ofNMR is slow in comparison with the solvation shell exchangekinetics, 1H peak shifts are representative of the averageenvironment of the proton.13

Table 1 summarizes the major cation speciation as identifiedfrom the ESI-MS. Table S1 details all identifiable peaks. The

ESI-MS for both sulfone electrolyte mixtures showed the mostintense peak being an MgCl+ cation solvated by two sulfonemolecules and one water molecule. The water molecules areassumed to come from residual moisture within the equipment.Both spectra show an MgCl+ species as the most intense peak,yet full spectral analysis of the THF/BS electrolyte shows aratio of MgCl+ to Mg2Cl3

+ of about 1:1. The THF/SLelectrolyte, on the other hand, exhibits a ratio of about 4:1MgCl+ to Mg2Cl3

+. This suggests that the formation of themagnesium cation dimer at adequate concentration must berequired for reversible magnesium deposition. Furthermore, theTHF-only electrolyte had a ratio of MgCl+ to Mg2Cl3

+ of 1:3.The mass spectra for both sulfone-based electrolytes show

solvation shells with varying numbers of sulfone molecules,ranging from one to three; however, no THF molecules areobservable. This, as similarly demonstrated by Xu andGreenbaum for lithium-ion electrolytes, is due to the weakercoordination of the ether oxygen as compared to the sulfoneoxygen.13 THF molecules are likely stripped from the cation asit enters the mass spectrometer. This gives rise to anappearance of an incomplete solvation shell. Thus, the massspectra cannot give a complete picture as to the coordination ofthe magnesium cation. Magnesium cations are expected to have4 or 6 coordination sites.14 As each sulfone contains twooxygens that may coordinate the cation, steric constraints areexpected to be a contributing factor in the average solvent shellpopulation. As mentioned previously, NMR spectroscopy doesconfirm THF incorporation in the cation solvent shell.In summary, we have developed a conditioning-free,

reversible electrolyte for magnesium batteries that is thermallystable up to 80 °C and can be facilely prepared by mixingcommercially available materials. Sulfone/THF mixtures greatlyincrease the operating temperature range of the electrolyte.MgHMDS2−4MgCl2 50 THF/50 BS electrolytes supportmagnesium metal cycling with reversibilities above 90% at 20and 50 °C. Similar formulations based on THF/SL do notfacilitate reversible magnesium electrodeposition and dissolu-tion. Through mass spectrometry, it is observed that Mg2Cl3

+

and MgCl+ form in the solutions, and that higher performing

Figure 4. (a) Galvanostatic cycling data of Mg(HMDS)2−4MgCl2 in50 THF/50 BS in a Mg/Cu coin cell, (b) zoom-in of early cyclingprofiles, (c) zoom-in of later cycling profiles, and (d) correspondingCoulombic efficiencies. A current of 0.25 mA was applied for 1000 s,beginning with a negative current to induce magnesium depositiononto the copper electrode. The upper voltage limit was 1.5 V.

Table 1. Mass Spectra Speciation of Major Cation Speciesa

Cationic Species %

THF ElectrolyteMgCl+·THF 17.6MgCl+·2THF 9.4Mg2Cl3

+·THF 10.6Mg2Cl3

+·2THF 56.6Mg2Cl3

+·3THF 5.8Butyl Sulfone/THF Electrolyte

MgCl+·BS 3.1Mg2Cl3

+·BS 4.9MgCl+·2BS 45.1Mg2Cl3

+·2BS 27.5MgCl+·3BS 6.7Mg2Cl3

+·3BS 12.8Sulfolane/THF Electrolyte

MgCl+·SL 7.0MgCl+·2SL 69.6Mg2Cl3

+·2SL 13.2MgCl+·3SL 4.9Mg2Cl3

+·3SL 5.2aSee Supporting Information for spectra and further detailed tables.

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electrolytes have a higher magnesium dimer content. Evidencefrom NMR and mass spectrometry shows that both thesulfones and THF are solvating the magnesium cation. Thisresearch suggests that solution speciation is critical toelectrochemical activity and that nonethereal solvents cansupport highly efficient magnesium metal electrodeposition anddissolution when occupying a partial cation solvation shell.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.chemma-ter.8b00483.

Detailed experimental information and procedures,supplementary electrochemical and thermal stabilitydata, detailed mass spectrometry information, NMRspectroscopy, and X-ray photoelectron spectra (PDF)

■ AUTHOR INFORMATIONCorresponding Author*J. L. Schaefer. E-mail: Jennifer.L.Schaefer.43@nd.edu, Tel:15746315114.ORCIDJennifer L. Schaefer: 0000-0003-4293-6328NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors gratefully acknowledge financial support from theNational Science Foundation via award number CBET-1706370. Additional acknowledgement to the University ofNotre Dame Integrated Imaging Facilities, the MaterialsCharacterization Facility, and the Mass Spectrometry andProteomics Facility for use of their facilities and instrumenta-tion.

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