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The Effect of Concentration on the Electrochemistry and Speciation of the Magnesium Aluminum Chloride Complex Electrolyte Solution Kimberly See, Yao-Min Liu, Yeyoung Ha, Christopher J Barile, and Andrew A. Gewirth ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08088 • Publication Date (Web): 21 Sep 2017 Downloaded from http://pubs.acs.org on September 22, 2017

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ACS Applied Materials & Interfaces

The Effect of Concentration on the Electrochemistry and Speciation of the Magnesium Aluminum Chloride Complex Electrolyte Solution Kimberly A. See, Yao-Min Liu, Yeyoung Ha, Christopher J. Barile, Andrew A. Gewirth* Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 United States AUTHOR INFORMATION Corresponding Author *Andrew A. Gewirth, [email protected]

KEYWORDS Magnesium aluminum chloride complex, magnesium battery electrolyte, magnesium deposition, Mg dimer, electrolyte conditioning

ABSTRACT Magnesium batteries offer an opportunity to use naturally abundant Mg and achieve large volumetric capacities reaching over four times that of conventional Libased intercalation anodes. High volumetric capacity is enabled by the use of a Mg metal anode in which charge is stored via electrodeposition and stripping processes, however, electrolytes that support efficient Mg electrodeposition and stripping are few and are often prepared from highly reactive compounds. One interesting electrolyte solution that

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supports Mg deposition and stripping without the use of highly reactive reagents is the magnesium aluminum chloride complex (MACC) electrolyte. The MACC exhibits high Coulombic efficiencies and low deposition overpotentials following an electrolytic conditioning protocol that stabilizes species necessary for such behavior. Here, we discuss the effect of the MgCl2 and AlCl3 concentrations on the deposition overpotential, current density, and the conditioning process. Higher concentrations of MACC exhibit enhanced Mg electrodeposition current density and much faster conditioning. An increase in the salt concentrations causes a shift in the complex equilibria involving both cations. The conditioning process is strongly dependent on the concentration suggesting that the electrolyte is activated through a change in speciation of electrolyte complexes and is not simply due to the annihilation of electrolyte impurities. Additionally, the presence of the [Mg2(µ-Cl)3·6THF]+ in the electrolyte solution is again confirmed through careful analysis of experimental Raman spectra coupled with simulation and direct observation of the complex in sonic spray ionization mass spectrometry. Importantly, we suggest that the ~210 cm−1 mode commonly observed in the Raman spectra of many Mg electrolytes is indicative of the C3v symmetric [Mg2(µ-Cl)3·6THF]+. The 210 cm-1 mode is present in many electrolytes containing MgCl2, so its assignment is of broad interest to the Mg electrolyte community.


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1 INTRODUCTION Mg-based batteries employ abundant, inexpensive materials and enable diversification of the energy storage landscape.1,2 Through the use of a Mg metal anode, Mg batteries additionally enable high volumetric capacities reaching theoretical values of 3833 mAh cm−3,1 about 4.5 times higher than that of the graphite anodes used in conventional Li-ion batteries (graphite has a theoretical capacity of 818 mAh cm−3). The electrolytes that support efficient anode processes in Mg metal batteries, namely Mg electrodeposition and stripping, are difficult to develop due to the high reactivity of Mg metal toward common electrolyte components and the complex and poorly understood nature of Mg electrochemistry occurring at both the anode and cathode. The electrolyte is an important component of the cell as its composition and properties directly affect Mg deposition and stripping behavior. Targets for the ideal Mg electrolyte include high thermal stability, the ability to support 100% reversible Mg electrodeposition and stripping along with reversible Mg insertion and desertion reactions at the cathode, and a wide potential window to allow for high voltage battery operation. Most electrolytes developed to date, however, are extremely reactive due to the presence of highly reducing Grignards present in the electrolyte itself or during electrolyte preparation. It is therefore necessary to explore electrolytes that do not require the use of Grignards, including those based on carborane,3,4 borohydride,5,6 trifluoromethanesulfonyl imide (TFSI),7,8 cyclopentadienyl,9 and chloride anions.10–12 Of particular interest is the system based on simple chloride salts, namely the magnesium aluminum chloride complex (MACC) electrolyte, due to the ease of preparation and attractive Mg electrodeposition metrics.10,11 Additionally, MACC is a relatively simple Mg electrolyte in that it only contains one source of anions: Cl−. Therefore, understanding the MACC system provides a baseline of understanding that can be used as a starting point to better understand more


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complicated systems that contain additionally anionic species in addition to Cl−. Additionally, while the community would like to move away from Cl-containing electrolyte systems to less corrosive electrolytes, we suggest that Cl− plays a significant role in reducing the deposition overpotential and allowing for reversible deposition and stripping. Therefore, it is important to understand the mechanisms by which Cl-containing electrolytes function so that design principals may be developed to guide the discovery of new, less corrosive electrolyte systems. MACC is prepared by dissolving MgCl2 and AlCl3 in THF at a 2:1 ratio.10,11 The addition of the Lewis acid AlCl3 is necessary to solubilize MgCl2 in organic solvents.10 The MACC electrolyte supports efficient Mg deposition and stripping while maintaining a high anodic stability window (>3 V vs. Mg/Mg2+).10,11,13 To achieve these desirable characteristics, the MACC must undergo an electrolytic conditioning procedure in which the electrolyte is cycled above and below 0 V in a Pt | Mg cell.13 When a low concentration of MgCl2 and AlCl3 is conditioned (0.06 M and 0.03 M, respectively), the initial conditioning cycles are dominated by irreversible Al deposition.13 Electrodeposition of Al at the working electrode coincides with oxidation of Mg at the counter electrode, resulting in a net increase in Mg concentration in the form of the active dinuclear Mg complex [Mg2(µ-Cl)3·6THF]+, known colloquially as the “dimer,” and the formation of solvated Cl−.14 We showed that efficient Mg electrodeposition in the MACC system is correlated with a change in the composition of the electrolyte itself. The formation of solvated Cl− in solution likely plays a vital role in activating the electrolyte by either protecting the highly reactive Mg surface against the O-rich solvent or acting to stabilize intermediate species during deposition.14 Interestingly, preparation of the activated MACC electrolyte can be achieved by chemical means via stirring a solution of CrCl3 and AlCl3 in the presence of Mg metal,15 likely mimicking the


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reactions occurring during electrochemical conditioning. Ultimately, the conditioned electrolyte achieves >99% Coulombic efficiency (CE).10,13 In addition to the electrolytic conditioning step required to activate the MACC, another disadvantage of MACC is its low deposition and stripping current densities of 1.8 − 2.0 mA cm−2.10 This low current density is obtained in a MACC electrolyte with 0.25 M MgCl2 in dimethoxyethane solvent.10 Current densities below 2 mA cm−2 are prohibitively small when compared with other Mg systems and are not viable for full cell application. The Grignard-based all-phenyl complex (APC) electrolyte, for example, achieves deposition and stripping current densities of 10 mA cm−2 and 13 mA cm−2, respectively.16 In order to increase the current density of Mg electrodeposition supported by MACC, we evaluated the effect of concentration on the Mg electrodeposition current densities using tetrahydrofuran (THF) as a solvent. Interestingly, the increase in concentration not only results in a notable change in the Mg electrodeposition metrics, but it also significantly affects the conditioning process and gives further insight into the mechanisms involved. Increasing the concentration of the component salts in the MACC results in significantly higher deposition and stripping current densities, much more facile deposition and stripping kinetics, considerably lower deposition overpotential, and faster electrolytic conditioning. 2 EXPERIMENTAL 2.1 Preparation of the MACC electrolytes The MACC electrolyte (MgCl2 and AlCl3 in THF at a 2:1 ratio) was prepared according to the procedure described by Barile et al.17 Anhydrous THF was purchased from Sigma Aldrich and further dried by refluxing over Na and benzophenone. After refluxing, the THF was degassed via three freeze-pump-thaw cycles and brought directly into an Ar-atmosphere glove


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box and stored over activated 3 Å molecular sieves. Anhydrous AlCl3 was purchased from Sigma Aldrich and opened in the glove box. Anhydrous MgCl2 was purchased from Alfa Aesar and further dried at 200 °C in a tube furnace for 2 hr under flowing HCl gas. The HCl gas was created by dropping concentrated HCl into concentrated H2SO4 (~1 drop every 3 sec) and carried through the tube furnace with Ar carrier gas purified by bubbling through two solutions of concentrated H2SO4. Once dried, the MgCl2 was cooled to room temperature and brought into the glove box without exposure to air. MACC electrolytes were prepared with varying quantities of MgCl2 and AlCl3 to achieve the final concentration targets. First, the AlCl3 was weighed into a glass vial and cooled to a final temperature of ~0 °C. THF (2.5 mL) was also cooled to ~0 °C in a separate glass vial and added dropwise to the cooled AlCl3. The AlCl3 solution was agitated until fully dissolved. Separately, MgCl2 was weighed into a vial and 2.5 mL of THF was added with stirring. The AlCl3 solution was then added dropwise to the stirring MgCl2/THF. The solution was stirred for 2 days until fully dissolved. 2.2 Conditioning protocol Once the MACC electrolytes were fully dissolved yielding a clear, colorless solution, the electrolytes were conditioned until reversible Mg electrodeposition and stripping at nearly 100% Coulombic efficiency (CE) was achieved. To condition the MACC electrolyte, 1 mL – 1.5 mL of the MACC was cycled with a Pt working electrode and a Mg foil counter/reference electrode from −1.2 V to 2.8 V (vs. Mg/Mg2+) at 5 mV s−1. The Mg foil (GalliumSource, LLC) was mechanically polished with a razor blade in an Ar atmosphere glove box and the 5 mm Pt wire (Alfa Aesar) was cleaned in concentrated HNO3 and dried in a H2 flame prior to use. The


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electrochemistry was controlled by a SP-150 BioLogic potentiostat or 600D, 620A, 760C, or 760D CH Instruments potentiostats. Potentials are reported vs. Mg/Mg2+ unless otherwise noted. 2.3 Cu electrodeposition experiments The electrolytes used for Cu electrodeposition and stripping experiments were prepared with 18.2 MΩ·cm Milli-Q water (Millipore). First, 0.4 M H2SO4 was prepared by diluting concentrated H2SO4 (Ultrex II Ultrapure, J.T.Baker). The CuSO4 (Sigma Aldrich) was dissolved in the 0.4 M H2SO4 at 0.3 M and diluted accordingly with 0.4 M H2SO4 to achieve the desired Cu concentrations. CVs were obtained with a 5 mm glassy carbon working electrode, Cu metal counter electrode, and a “no-leak” Ag/AgCl reference electrode (3.4 M KCl, eDAQ). The CVs were measured at 5 mV s‒1 from ‒0.26 V (vs. Ag/AgCl) to 0.6 V (vs. Ag/AgCl) on CH Instruments potentiostats. 2.4 Characterization details Raman spectroscopy was obtained with a 632.8 nm He/Ne laser using an instrument setup described previously.18 The Raman spectra of the electrolytes were measured in a quartz cuvette with a screw cap further sealed with Parafilm. 27

Al and 35Cl NMR spectra of the electrolytes were measured in the University of Illinois

at Urbana-Champaign School of Chemical Sciences NMR laboratory in 5 mm NMR tubes with screw caps as described previously.14 The time between removal of the conditioned solution from the electrochemical cell and measurement was minimized as much as possible. The

27

Al

NMR was measured on a 600 MHz Varian spectrometer with a 90 degree pulse width of 9.95 µs, an acquisition time of 0.2 s, and a relaxation delay of 0.5s. An external standard was introduced via a sealed capillary containing Al(NO3)3 in D2O.

35

Cl NMR was measured on a 750 MHz


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Agilent spectrometer with a 90 degree pulse width of 29.8 µs, acquisition time of 0.15 s, and relaxation delay of 0.15 s for 1.5 h. Sonic spray ionization mass spectrometry (SSI-MS)19 was performed by spraying the electrolyte solutions through a microdroplet sprayer described previously20 into the inlet of an LTQ-Orbitrap XL mass spectrometer (Thermo Fisher Scientific, San Jose, CA).21–23 The spray was generated without spray voltage using a pressure of N2 sheath gas at ~200 psi at a spray rate of 10 µL/min. Upon entering the inlet of the mass spectrometer, desolvation was achieved using a capillary temperature of 200 °C. The mass spectra were obtained in both positive and negative ionization modes with a single-stage m/z range of 50 m/z – 1000 m/z, a resolution setting of 100,000 at 400 m/z, a mass accuracy of 5 ppm, microscans set to 1, an ion injection time of 500 ms, and a tube lens voltage of 100 V. 2.5 Raman simulations Theoretical Raman spectra were calculated using Gaussian 0924 using the B3LYP functional and 6-31+G(d) basis set. Integral-equation-formalism polarizable continuum model (IEF-PCM) was used to model implicit THF solvent.25

3 RESULTS AND DISCUSSION 3.1 Electrochemical Characterization Fig. 1 shows deposition and stripping voltammetry obtained from the MACC electrolyte prepared with increasing concentrations of the component salts while maintaining a 2:1 Mg:Al mole ratio. The 1× MACC electrolyte is defined here as 60 mM MgCl2 + 30 mM AlCl3 in THF. As the concentration increases, the current density associated with Mg electrodeposition and stripping in the conditioned electrolytes increases. The 5× MACC (300 mM MgCl2 + 150 mM


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AlCl3) reaches cathodic current densities up to −7.5 mA cm−2 at −1.2 V, competitive with other Mg electrolytes. Table 1 lists the Mg electrodeposition metrics for the MACC electrolytes and compares these to those from other non-Grignard containing electrolyte solutions. The Mg(CB11H11)2 electrolyte exhibits higher deposition current densities at ‒0.5 V compared to the MACC system, however, the Mg(CB11H11)2 electrolyte maintains a relatively large deposition overpotential of around 250 mV and contains more than twice the concentration of Mg compared to the 5× MACC.3 Furthermore, the high concentration MACC electrolytes maintain a high anodic stability of >3.2 V (see SI for CVs up to 3.5 V) and high CE. When fully conditioned, the MACC at low, medium, and high concentrations exhibits CEs of >99%.

Figure 1. (a) Cyclic voltammograms of conditioned MACC electrolytes in THF with 60 mM MgCl2 + 30 mM AlCl3 (1× MACC), 120 mM MgCl2 + 60 mM AlCl3 (2× MACC), and 300 MgCl2 + 150 mM AlCl3 (5× MACC). The electrolytes are cycled at 5 mV s−1 from -1.2 V to 2.8 V with a Pt working electrode and Mg foil counter/reference. The inset shows the region around 0 V at higher resolution. (b) The evolution of Coulombic efficiency of the MACC electrolytes when cycled as described in (a).


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In addition to the high current density, the electrodeposition overpotential decreases as the MACC concentration is increased suggesting more facile electrodeposition (see the inset in Fig. 1a). The high concentration MACC exhibits a deposition overpotential of 60 mV which is among the lowest Mg deposition overpotentials observed for non-Grignard containing electrolyte solutions (see Table 1). Information on the oxidation kinetics can be obtained by comparing the shape of the stripping waves at each concentration. The slope of the stripping curve becomes steeper with increasing salt concentration suggesting more facile oxidation kinetics. The shape of the oxidation wave is also affected by the concentration. As the concentration of the electrolyte increases, the contribution from oxidation processes after the peak anodic current is greater.

Table 1. Mg electrodeposition and stripping properties of non-Grignard containing electrolyte solutions including current density (J), deposition overpotential (ηdep), and Coulombic efficiency (CE). electrolyte Solvent a CE ref. Mg conc. scan J at -0.5 V ηdep (mV vs. 2+ -2 ) (%) (M) rate Mg/Mg (mA cm ) (mV s-1) 0.06 M MgCl2 this work, THF 0.06 5 −0.31 228 99 13 + 0.03 M AlCl3 0.12 M MgCl2 THF 0.12 5 −1.5 135 99 this work + 0.06 M AlCl3 0.30 M MgCl2 THF 0.3 5 −4.3 59 99 this work + 0.15 M AlCl3 3 Mg(CB11H11)2 G4 0.75 5 −6.1 b < 250 94.4 3 Mg(CB11H11)2 G3 0.75 5 −13.7 b < 250 79.6 Mg(C2B10H11)2 4 THF NI c 5 −3.6 b 500 > 98 + MgCl2 8 Mg(TFSI)2 DME/G2 0.3 0.2 −0.8 b NI c NI c 5 Mg(BH4)2 DME 0.1 5 -0.08 b 340 67 5 Mg(BH4)2 THF 0.5 5 N/A 600 40 b b 5 Mg(BH4)2 + LiBH4 DME 0.18 5 -8.2 300 94 a

The indicated solvents include: tetrahydrofuran (THF), tetraglyme (G4), triglyme (G3), dimethoxyethane (DME), and diglyme (G2). b Values obtained from published CV curves. Citations can be found in the “ref.” column. c NI = values were not indicated in the reference.


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The reason for a change in peak shape can be understood from previous work studying the oxidative processes during stripping. Our group has shown with in situ electrochemical stress experiments coupled with density functional theory (DFT) calculations that the Mg stripping curve can be broken down into a few sequential oxidation processes.26 As oxidation begins on the positive CV sweep, the majority of the anodic processes involve oxidation of Mg(1010) planes to Mg2+ or MgOx followed by oxidation of Mg(0001) planes and removal of MgOx as evidenced by two distinct regions in the stress curve.26 Therefore, the change in the shape of the oxidation wave as a function of concentration seen in Fig. 1 could be caused by a change in the habit of the Mg electrodeposit resulting in a relatively higher fraction of Mg(0001) planes that oxidize at relatively higher potentials compared to the Mg(1010). The kinetics can also be probed by evaluating the Tafel plots for each concentration of MACC. The Tafel plots and linear fits to the high overpotential regions are shown in the SI with the Tafel slopes associated with the deposition and stripping processes plotted as function of Mg2+ concentration. The Tafel slopes associated with both the deposition and stripping processes decrease as Mg2+ concentration increases suggesting more facile deposition and stripping kinetics. Fig. 1b shows the CE as a function of cycle number for each concentration of MACC during the conditioning procedure. The concentration of the electrolyte affects the number of conditioning cycles required to reach a fully conditioned state with the 1×, 2×, and 5× MACC electrolytes plateauing at a CE of ~ 99% at cycle 45, 35, and 18, respectively. Thus, the concentration of the component salts has a significant impact on the conditioning process. It has recently been suggested that the activation of the MACC electrolyte using a diglyme solvent


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simply involves the removal of water from the electrolyte27 while we have suggested that the change in speciation as a result of conditioning is the cause for activation.14 If the conditioning process were simply removing water from the electrolyte, then we would expect that higher concentrations of salt would either (a) cause slower conditioning due to the introduction of more water present in the salts themselves or (b) cause no change in the conditioning process if most of the water is present in the solvent. Therefore, the conditioning process is more complicated and is related to the speciation of the Mg and Al complexes in the electrolyte as we have previously shown.14 The CE as a function of cycle number shown in Fig. 1b displays a local peak during the initial cycles of the 1× and 2× MACC electrolytes due to the activity of both Mg and Al species. To understand why the CE appears to peak early, we look to the CVs during conditioning. The evolution of the CVs as a function of cycle number is shown in Fig. 2. By evaluating the shape of the CV, we can conclude that the initial cycles are convoluted by Al deposition (negative current above 0 V) with minimal stripping that is gradually replaced by increasingly efficient Mg electrodeposition and stripping. Poor oxidation efficiency of the Mg-related processes in the initial cycles coupled with residual oxidation of Al-species above 2 V gives an apparently high CE before the electrolyte is actually conditioned. Then, the anodic current above 2 V drops to nearly 0 A causing the CE to decrease from the local maximum. The CE then increases to nearly 100% as a result of increasingly efficient Mg electrodeposition and stripping processes. Also worthy of note, the higher concentration electrolytes exhibit Mg electrodeposition and stripping character much earlier in the conditioning process as shown in Fig. 2, suggesting a different speciation compared to the low concentration electrolytes. Mg deposition and stripping was also


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observed

in

the

initial

Figure 2. CVs from the electrolytic conditioning of (a) 2× MACC and (b) 5× MACC. CVs are measured between −1.2 V to 2.8 V at 5 mV s−1 on a Pt working electrode with a Mg foil counter/reference. CV cycles of the MACC electrolyte in a diglyme solvent in which high concentrations of MgCl2 and AlCl3 were used.27 An intriguing aspect of the MACC system is the relationship between salt concentration and the measured current density. As the concentration of Mg in the electrolyte is increased, the current density expectedly increases, as shown by the CVs in Fig. 1, which corresponds to an increase in the quantity charge passed. The quantity of charge passed is directly related to the amount of Mg deposited and stripped, assuming no side reactions are occurring, and is dependent on the activity of the electroactive species in the electrolyte. In a system in which the activity of the electroactive complexes directly scale with the concentration of the active ion in solution, we would expect the ratio of active ion to charge passed to be 1:1. Fig. 3A shows the reductive


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charge passed during the electrodeposition of Cu2+, a well-studied divalent electrodeposition system.28–33

Figure 3. The quantity of cathodic charge passed, ΣQcathodic, during electrodeposition of (a) Cu2+ and (b) Mg2+ as a function of the divalent ion concentration in the electrolyte. Linear fits to the data along with the expected [X2+]:Q = 1:1 are shown in each plot. The data in (a) is obtained by integrating the negative current of the CV performed with a 5 mm glassy carbon working electrode, Cu counter electrode, and Ag/AgCl reference electrode at 5 mV s-1 from ‒0.26 V (vs. Ag/AgCl) to 0.6 V (vs. Ag/AgCl). The electrolyte contains varied concentrations of CuSO4 in 0.4 M H2SO4. The data in (b) is obtained by integrating the negative current in the CVs obtained with the MACC electrolytes shown in Figure 1. The sum of the charge passed vs. Cu2+ concentration is fit with a linear regression shown by the solid line in Fig. 3A. The slope of the fit is 30 mC cm‒2 M‒1 while the slope of the 1:1 correlation extrapolated from the 0.06 M Cu2+ data point is 31 mC cm‒2 M‒1 suggesting that electrodeposition of Cu2+ follows the expected 1:1 correlation of Cu2+ concentration with charge passed. Fig. 3b shows the sum of the cathodic charge passed, ΣQcathodic, between ‒1.2 V and 2.8 V at 5 mV s‒1 as a function of Mg concentration. The ΣQcathodic increases with increasing


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concentration, however, the increase is greater than would be predicted from the expected 1:1 ratio of [Mg]:Q, as was observed for Cu2+. The 1:1 relationship is shown in the blue dashed line extrapolated from the 0.06 M [Mg] (1× MACC) electrolyte. The observed ΣQcathodic is 2.4 times greater than the expected 1:1 relationship. Because of the mostly linear shape of the deposition curve, we can assume that the deposition is not diffusion limited which allows for the assumption that the quantity of charge passed is a proxy for the activity of the electroactive species in the electrolyte. Therefore, as the concentration of the component salts is increased, the concentration of the electroactive Mg complexes increases at a greater rate than would be expected from the nominal concentration. We suggest that this enhancement in due to a higher concentration of the electroactive species as a result of a shift in equilibrium of the Mg complexes. To confirm the effect of concentration on the current density and decouple effects other than concentration, such as changes in diffusion as a result of viscosity changes, etc., an effective exchange current density, J0, was extracted from the Tafel plots. The relationship between the effective exchange current density and Mg concentration is shown in the SI and indeed, the increase in the effective exchange current density also far exceeds the expected one-to-one correlation of current density with Mg concentration. We note, however, that it is difficult to determine the actual J0 from macroelectrodes and these numbers should only be used to compare the three electrolyte concentrations.

3.2 Raman Spectroscopy The conditioning process and Mg electrodeposition metrics are strongly dependent on the concentration of the MACC electrolyte suggesting that the equilibria in solution and therefore the speciation are affected by changes in concentration. The conditioned electrolytes were


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characterized by Raman spectroscopy, NMR spectroscopy, and sonic spray mass spectrometry to characterize the complexes in each MACC electrolyte. The Raman spectra comparing the conditioned 2× and 5× MACC, along with neat THF and the previously reported Raman for 1× MACC14 are shown in Fig. 4. Table 2 shows the peak assignments for the modes observed. The conditioned electrolytes exhibit Raman modes attributed to AlCl4− (modes a and e)34,35 and Mg complexes (mode b), similar to the spectra of the 1× MACC characterized previously.14 Mode b at 210 cm-1 is commonly ascribed to the symmetric Mg‒Cl stretch similar to that found in MgCl2.14,35,36 We note however, that the A1g stretch in solid MgCl2 is observed at 240 cm‒1,37,38 substantially higher in energy relative to what is observed in solution. Additionally, the local coordination environment of the Mg in the solubilized species is substantially different from the Mg-Cl octahedra found in the solid-state structure. The coordination of THF to the Mg centers, for example, has been confirmed by mass spectrometry.39 The large frequency disparity between the solid-state mode and the change in the local coordination environment suggests that the 210 cm‒1 peak is not purely a Mg-Cl symmetric stretch. The band instead likely involves modes related to solvent coordination or formation of a multinuclear complex. We note that, similar to the

1×


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Figure 4. Raman spectra of the conditioned 2× and 5× MACC electrolytes. The previously reported 1× MACC spectrum is shown for comparison.14 The peak assignments for modes a-g can be found in Table 2. Briefly, modes a and e are related to Al complexes, modes b and c are related to Mg complexes, and modes d, f, and g are THF modes. The small peak indicated by the * is an artifact introduced by the quartz cuvette. MACC we studied previously, the intensity of mode b increases as a result of conditioning the 2× MACC (see SI for the Raman spectra of the as-prepared electrolytes). The Raman spectrum of the as-prepared 5× MACC, however, suffers from a very large fluorescent background making it difficult to interpret. The conditioned 5× MACC exhibits a new, weak mode not observed in the other electrolytes at ~240 cm‒1 labeled mode c. A small shoulder is observed in the 240 cm‒1 region in the conditioned 2× MACC, however, it is too weak to confirm its association with mode c. Mode c is likely a signature of Mg complexes since all the modes related to the only Al complex, AlCl4‒ (vide infra), are assigned. Due to the low intensity, it is difficult to determine if this mode arises as a result of a new species only present in the 5× MACC or is simply a weak mode that is not visible in the other electrolytes due to the low concentration.


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Table 2. Raman peak assignments for the conditioned MACC solutions. Raman shift (cm −1)

assignment

peak ID a

2× MACC, conditioned ~172

5× MACC, conditioned ~178

species

b

210

213

[Mg2(µ-Cl)3·6THF]+

A1 Mg2(µ-Cl)3 breathing mode + αCH2-βCH2 twist

this work

c

~241

~239

[Mg2(µ-Cl)3·6THF]+

CH2 in ligated THF

this work

d

284

285

THF

e

346

346

AlCl4−

f

595

590

THF

g

650

653

THF

AlCl4−

mode T2 bending

Ref. 34,35

βCH2 (bend + rock) + ring pucker

40,41

A1 symmetric stretch

34,35

αCH2 (bend + twist) + CαCβ asym. stretch + ring pucker αCH2 (bend + twist + rock) βCH2 (twist + rock) + ring bend

40,41 40,41

3.3 Calculated Raman Modes To determine the origin of modes b and c, we calculated the Raman spectra of the dinuclear complex [Mg2(µ-Cl)3·6THF]+ implicated in our previous studies14 along with the sixcoordinate and four-coordinate monomer structures: [MgCl·5THF]+ and [MgCl·3THF]+. Experimental structural parameters determined by refinement of single crystal diffraction data from Liu et al.11 were used to define the initial atomic coordinates for the dinuclear complex [Mg2(µ-Cl)3·6THF]+. Experimental data was also used for the initial coordinates of the sixcoordinate monomeric [MgCl·5THF]+ reported by Pour et al.35 Because there is no experimental data for the structure of the four-coordinate monomer [MgCl·3THF]+, the structure predicted by DFT reported by Wan et al.42 was used as the initial structure for the [MgCl·3THF]+ monomer.


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The exact structure and bond lengths of the Mg complexes are not known in the solution so the structures were perturbed to systematically determine how structural changes affect the calculated Raman spectrum. Three perturbations on each structure were achieved by minimizing the energy of the system and (a) allowing optimization of all bond lengths, (b) fixing the Mg‒Cl bond lengths to those found in the solid-state, or (c) fixing both the Mg‒Cl and Mg‒O bond lengths to those in the solid-state. We find changes Mg‒Cl and Mg‒O bond lengths result in shifts in the calculated Raman spectrum. The low wavenumber region of the calculated Raman spectra for the dinuclear [Mg2(µCl)3·6THF]+ complexes are shown in Fig. 5a. In all cases, the most intense peak in the low wavenumber region (>2.43 Å would be required to shift the Mg‒Cl symmetric stretch below 320 cm‒1. However, as we discussed above, it is unlikely that the Mg‒Cl bond length above 2.43 Å would be stable. It is therefore highly unlikely that the symmetric Mg‒Cl stretch would appear below 300 cm‒1 in the monomeric, THF coordinated complexes.

3.4 NMR Spectroscopy The 27Al NMR spectra of the 2× and 5× MACC as-prepared and conditioned along with the component AlCl3 solutions are shown in Fig. 6. The chemical shift is referenced to an Al(NO3)3 in D2O standard solution introduced to each tube via a sealed, coaxial capillary. The


22

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spectra are normalized using the resonance from the standard. Two peaks are observed in the AlCl3 component solutions corresponding to the dominate AlCl3·2THF at 64 ppm and a small amount of AlCl4− at 103 ppm. Despite the higher concentration of AlCl3 in the 5× AlCl3 component solution (0.15 M), substantial asymmetric cleavage of the Al species to AlCl2+ and AlCl4−, for example, is not observed with most of the Al present as neutral AlCl3·2THF. These ionic species have been reported at >0.3 M AlCl3 in THF.47 Upon addition of MgCl2, the Al speciation in both the 2× and 5× MACC shifts strongly to AlCl4‒ suggesting a cationic Mg complex for charge balance. The speciation in the as-prepared solutions closely mirrors that found in the as-prepared 1× MACC characterized previously.14 However, the change in speciation as a result of conditioning differs. In the case of 1× MACC, the

27

Al NMR of the

conditioned solution shows a significant decrease in the concentration of AlCl4‒ species due to irreversible Al deposition during the conditioning process.14 The concentration of AlCl4‒ in the 2× MACC solution does decrease after conditioning, as shown by the decrease in intensity of the 103 ppm resonance. However, this decrease is much less significant compared to the 1× MACC electrolyte. The conditioned 5× MACC exhibits a negligible decrease in AlCl4‒ concentration suggesting that activation is achieved without a significant decrease in Al concentration in the electrolyte. The negligible decrease in Al concentration after conditioning agrees well with the features observed in the conditioning CVs that suggest a smaller quantity of current related to Al deposition processes in the higher concentration MACC. In both cases, the AlCl4‒ peak narrows in width suggesting a more symmetric local environment or enhanced mobility.


23


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Figure 6. (a) 27Al NMR spectra of the (a) 2× and (b) 5× MACC as-prepared and conditioned with the component AlCl3 solutions at (a) 0.06 M and (b) 0.15 M AlCl3. The MgCl2 component solution is not shown due to the low solubility of MgCl2 causing preparation of homogenous solutions difficult. The solutions were also characterized by

35

Cl NMR shown in Fig. 7. The full width at

half max (FWHM), peak positions, and integrated intensities as determined by Lorentzian fits can be found in the SI. The two resonances observed in the

35

Cl NMR can be used to roughly

determine the quantity of Cl bound to Al3+ and Cl bound to Mg2+ with the more deshielded resonance (more positive chemical shift) attributed to the Al−Cl species.14 The 35Cl NMR spectra of the AlCl3 component solutions are shown to confirm the chemical shift region in which we would expect the Al−Cl species. In both concentrations, the resonance of Cl− associated with Al in the as-prepared MACC narrows relative to the component solutions due to the formation of AlCl4− complexes, which are either more mobile or symmetric relative to the AlCl3•2THF species in the component solutions. The 35Cl NMR of the as-prepared 2× MACC solution shows


24

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two resonances while the as-prepared 5× MACC converges to what appears to be a single peak. We would expect two resonances in the as-prepared 5× MACC as Mg complexes are present in the solution. Consequently, the reason a single peak is observed not clear. One possible explanation is that the high concentration of the Mg-related complexes results in some clustering causing significant local asymmetry and low mobility. Such a scenario would both affect the relaxation time and broaden the quadrupolar 35Cl NMR signal significantly to the point where it could be lost in the baseline. After conditioning the 5× MACC, however, two resonances area again observed in the same region as those found in the 1× and 5× MACC. Upon conditioning, the

35

Cl NMR of the 2× MACC solution reveals a shift in the

distribution of Cl associated with Al vs. Mg in favor of Mg complexes. The shift is as a result of the electrochemical reaction occurring during the conditioning process in which Al complexes in the electrolyte are reduced to form Al(s) while Mg(s) is oxidized at the counter electrode to form [Mg2Cl3·6THF]+.14 The

35

Cl NMR of the conditioned 5× MACC, however, reveals a larger

contribution of Cl− associated with Al3+ than Mg2+ with a distribution similar to that observed in the 1× MACC and 2× MACC as-prepared solutions suggesting that the conditioning process for the 5× MACC results in negligible Al reduction. This result agrees well with the lack of features associated with Al deposition in the CVs during conditioning and with both the Raman and 27Al NMR data. The

35

Cl NMR spectra only report on the behavior of Cl whose chemical environments

give rise to relaxation dynamics that are on relevant timescales of the NMR experiment. The most useful interpretation of the data is to evaluate changes in spectra for the Cl we do observe before and after conditioning. It is important to note, however, the activity of the electrolyte


25


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cannot be directly inferred from the 35Cl NMR spectra alone as the activity is determined by the speciation of the entire electrolyte in the bulk and at the interface.

Figure 7. 35Cl NMR spectra of the (a) 2× and (b) 5× MACC as-prepared and conditioned with the respective component AlCl3 solutions at (a) 0.06 M and (b) 0.15 M.

3.5 Mass Spectrometry The MACC is a multi-cation system containing both Al and Mg related species. The Al species are well characterized by Raman spectroscopy and

27

Al NMR. However, the identity of

the Mg complexes is more difficult to determine. Previously, we suggested that the Mg complexes in the 1× MACC are very likely the dinuclear Mg complex [Mg2(µ−Cl3)·6THF]+, commonly referred to as the “dimer,” using a combination of Raman spectroscopy, NMR, and X-ray pair distribution function data.14 However, we were not able to observe [Mg2(µ−Cl3)·6THF]+ directly and the stability of the dinuclear complex in solution


26

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[Mg2(µ−Cl3)·6THF]+ is a subject of discussion in the literature.43,48 In an attempt to directly characterize the Mg complexes, we used sonic spray ionization mass spectrometry (SSI-MS) which employs a soft ionization technique19 allowing for the interrogation of fragile complexes carrying a native charge in the electrolyte. Fig. 8 shows the mass spectrum of the conditioned 1× MACC electrolyte in both positive ion (Fig. 8a) and negative ion (Fig. 8b) mode. The mass spectra for both positive and negative mode out to 1000 m/z are shown in the SI. The spectrum in Fig. 8a shows prominent signals at m/z = 296.991 and 369.049 with satellite peaks at ±2 m/z. The m/z of the majority species exactly fit to the expected m/z for the dinuclear complex [Mg2Cl3·xTHF]+ where x=2 (m/z calculated = 296.99167) or 3 (m/z calculated = 369.04918). The expected isotope pattern confirming the presence of 3 chlorine atoms with natural abundancies of 35

Cl and 37Cl isotopes (Fig. 8c and 8d). The expected 6THF dinculear complex is not observed

likely due to the dissociation of the solvent ligands in the ion chamber or during the desolvation process in SSI. A small contribution is observed at the m/z = 275.126 and 462.971 due to the presence of the monomer [MgCl·3THF]+ (calculated m/z = 275.12643) and the trimer [Mg3Cl5·3THF]+ (calculated m/z = 462.97193) suggesting that these species exist in equilibrium, similar to the Schlenk equilibrium found in Grignard solutions.49 Although the formation of these species could be as result of the ionization process, the sonic spray ionization used here is very soft resulting in detection of only native ions. We therefore suggest the various Mg species are present in solution itself. The dominate species, however, is the dinculear [Mg2Cl3·xTHF]+ complex. The mass spectrum in Fig. 8a is the first direct evidence for the presence of a dinuclear Mg complex in the MACC electrolyte. Techniques such as electrospray mass spectrometry result in minimal signal39 which is likely a result of the significant fragmentation of the complexes due to the high energy ionization source. Subambient pressure ionization, which also employs a high


27


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Page 28 of 36

voltage ionization process, only observes Mg monomers [MgCl·3THF]+.39 Therefore, a very soft ionization technique such as sonic spray ionization is required to preserve the dinuclear complex [Mg2Cl3·6THF]+. The dinuclear complex has also been observed with coldspray ionization mass spectrometry of Grignard solutions.50 The negative mode reveals a single peak at m/z of 168.856 (Fig. 8b) that corresponds to the AlCl4− (calculated m/z = 166.85694) observed in the

27

Al NMR. The isotope pattern

corresponding to four Cl atoms in AlCl4− is observed and is shown in Fig. 8e. The mass spectrum of the conditioned 5× MACC was also measured to evaluate speciation in the higher concentration electrolyte. However, sonic spray ionization is difficult to employ on solutions with a high salt concentration as these salts precipitate in the column and inhibit entry of the analyte

into

the

Figure 8. Sonic spray ionization mass spectrometry of the conditioned 1× MACC in (a) positive ion mode and (b) negative ion mode. The peaks marked with “*” are impurities present in the instrument. The peak patterns for (c) [Mg2Cl3·2THF]+, (d) [Mg2Cl3·3THF]+, and (e) AlCl4‒.


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spectrometer. Because of this issue, the mass spectrum of the 5× MACC is difficult to obtain. However, a peak at m/z = 369.049 associated with the dinuclear complex was observed. Taken together, the characterization above strongly supports that conditioning leads to the formation of the Mg dinuclear complex, [Mg2(µ−Cl3)·6THF]+, which we suggest is the primary active complex for Mg electrodeposition. As suggested previously, Al-related electrodeposition as a result of conditioning both increases the concentration of [Mg2(µ−Cl3)·6THF]+ complexes and results in the formation of solvated Cl‒.14 We show here that as the concentration is increased in the electrolyte, the equilibrium of equation (1) is shifted to the right to increase the concentration of [Mg2(µ−Cl3)·6THF]+ prior to conditioning consequently reducing the required Al electrodeposition during equation (2): AlCl3·2THF

+

2MgCl2

+

4THF

AlCl4−

+

[Mg2(µ−Cl3)·6THF]+

(1) 4AlCl4− + 6Mg(s) + 6THF → 4Al(s) + 3[Mg2(µ−Cl3)·6THF]+ + 7Cl− (2) The electrodeposition of Al in equation (2), however, is still necessary to form the metastable Cl−. Mass spectrometry shows that the [Mg2(µ−Cl3)·xTHF]+ complex is the majority species in the conditioned solution, although a small signal associated with the cationic monomer is observed. Note we cannot observe the neutral Mg complexes with mass spectrometry. The shift in equilibria causes higher Coulombic efficiencies at the beginning of the conditioning process due to the increased activity of the Mg complexes.

4 CONCLUSION


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The conditioning and electrochemical behavior of the MACC electrolyte are strongly dependent on the concentration of the component salts in the as-prepared solution. The kinetics of the Mg electrodeposition and stripping processes are greatly enhanced by increasing the electrolyte concentration along with the current density associated with deposition. Although it is not surprising to observe an increase in current density due to a higher concentration of Mg2+, the enhancements seen here cannot be explained by the nominal concentration of Mg2+ in the solutions. Instead, we show that increases in initial salt concentration change the speciation in the electrolyte by shifting the complex equilibria of solvated species. By increasing the concentration of the MgCl2 and AlCl3 in the as-prepared solution, the equilibrium associated with the formation of the active complexes in solution is shifted to favor the dinuclear Mg complex, [Mg2(µ−Cl3)·6THF]+, resulting in much faster electrolyte activation during electrolytic conditioning. Higher concentrations of the [Mg2(µ−Cl3)·6THF]+ also result in higher deposition currents that reflect the equilibrium of the species in solution and not just the nominal concentration of Mg. The speciation is confirmed by Raman spectroscopy, NMR spectroscopy, and mass spectrometry. The presence of [Mg2(µ−Cl3)·6THF]+ in solution is confirmed by direct observation of [Mg2(µ−Cl3)·xTHF]+ in the mass spectra and careful analysis of the Raman spectra. We compliment the experimental Raman data with theoretical predictions of the vibrational modes that are expected from various Mg complexes. The experimental and theoretical data show that the ~210 cm-1 mode commonly observed in MgCl2-containing Mg electrolytes is very likely due to the symmetric stretch of the C3v [Mg2(µ−Cl3)·6THF]+ unit. We suggest that the conditioning process involves the formation of free Cl‒ as was demonstrated previously14 and the shift in equilibrium of the electrolyte species at higher concentrations


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supports the formation of free Cl‒ in fewer conditioning cycles. This study demonstrates that the seemingly simple MACC electrolyte is a very complex system that is sensitive the concentration of the salts. Additionally, we show that the Mg electrodeposition and stripping metrics of the MACC electrolyte is strongly dependent on the electrolyte speciation, not just nominal salt concentrations.

ASSOCIATED CONTENT Supporting Information. CV of the 5× MACC electrolyte up to 3.4 V, Tafel analysis of CVs obtained with a macroelectrode, CVs of Cu electrodeposition and stripping, simulated Raman spectra of the monomeric Mg complexes, Table with Lorentzian fit values of

35

Cl NMR data,

sonic spray ionization mass spectrometry out to 1000 m/z, sonic spray ionization mass spectrometry of conditioned 5× MACC. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was partially supported by the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. K. A. See acknowledges postdoctoral funding from the St. Elmo Brady Future Faculty


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Postdoctoral Fellowship. Y.-M. Liu acknowledges access to the Orbitrap-MS in the Mass Spectrometry Laboratory at UIUC. C. J. Barile acknowledges funding from the National Science Foundation Graduate Research Fellowship (No. NSF DGE-11444245) and a Springborn Fellowship. The authors thank Dr. Taras Pogorelov and Mike Hallock for assistance with Gaussian calculations. REFERENCES (1)

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