Magnesium Anode Pretreatment Using a Titanium Complex for

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Magnesium Anode Pretreatment Using a Titanium Complex for Magnesium Battery Taeeun Yim, Sang-Gil Woo, Si-Hyoun Lim, Jong-Yeol Yoo, Woosuk Cho, Min-Sik Park, Young-Kyu Han, Young-Jun Kim, and Ji-Sang Yu ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 17 May 2017 Downloaded from http://pubs.acs.org on May 22, 2017

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ACS Sustainable Chemistry & Engineering

Magnesium Anode Pretreatment Using a Titanium Complex for Magnesium Battery Taeeun Yim,§ Sang-Gil Woo,†,* Si-Hyoun Lim,† Jong-Yeol Yoo,† Woosuk Cho,† Min-Sik Park,ǂ Young-Kyu Han,ф,* Young-Jun Kim,‡,* Jisang Yu†

§ Department

of Chemistry, Incheon National University, 119 Academi-ro, Incheon 22012,

Republic of Korea † Advanced

Batteries Research Center, Korea Electronics Technology Institute, Seongnam-si,

Gyeonggi-do 463-816, Republic of Korea ǂ

Department of Advanced Materials Engineering for Information and Electronics, Kyung Hee

University, 1732 Deogyeong-daero, Giheung-gu, Yongin 17104, Republic of Korea ф Department

of Energy and Materials Engineering and Advanced Energy and Electronic

Materials Research Center, Dongguk University-Seoul, Seoul 100-715, Republic of Korea ‡SKKU

Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Gyeonggi-

do, 16419, Republic of Korea

Corresponding author: [email protected] (for S.-G. Woo), [email protected] (for Y.-K. Han), and [email protected] (for Y.-J. Kim)

ACS Paragon Plus Environment

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ABSTRACT

Although magnesium batteries have received a great deal of attention as a promising power source, the native oxide layer on the Mg surface significantly impedes practical applications owing to the sluggish kinetic behavior of Mg-ion deposition and dissolution. Here, a new approach to improve electrochemical reactivity of Mg anode is proposed, based on chemical pretreatment of the Mg anode using a titanium complex, Ti(TFSI)2Cl2, that effectively removes the native oxide layer on the Mg anode surface. The pretreatment of the Mg anode by Ti(TFSI)2Cl2 remarkably decreases the binding affinity between Mg and O via the formation of a multi-coordinate complex (Mg–O–Ti). Thereafter, a series of chemical reactions cleave the Mg– O bonds, resulting in a fresh Mg surface. This creates a cell comprised of the Ti(TFSI)2Cl2pretreated Mg anode, glyme-based electrolytes, and cathode material that exhibits reversible electrochemical behavior at the electrode/electrolyte interface, resulting in practical applicability and good electrochemical performance.

KEYWORDS: titanium complex, magnesium battery, reaction mechanism, sluggish kinetics, surface chemistry


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Introduction Magnesium batteries (MgB) have received considerable attention as an advanced energy storage and conversion system based on a large theoretical and specific capacity of 2230 mA h g–1, low working potential (–2.36 V vs. the standard hydrogen electrode), and abundant reserves of Mg materials.1–5 However, widespread adoption of MgB is restricted because it is difficult to facilitate a reversible Mg-ion deposition and dissolution reaction at the interface between the Mg anode and electrolyte,6-10 which is attributed to insulating magnesium oxide (MgO) developed on the surface of the Mg. Improving the electrochemical reversibility of Mg-ions at the interface of the anode and the electrolyte is a critical challenge to creating practical MgB systems. Many attempts have been made to overcome the poor kinetic behavior on the surface of Mg, and most approaches have employed task-specific chemical components in electrolytes that effectively erode the insulating MgO layer on the Mg surface during cell operation. For example, the Gregory group reported that employing an organoboron and halide-combined electrolyte is effective for increasing the efficiency of Mg-ion deposition and dissolution in MgB system,11 while the Muldoon group demonstrated that organoboron complexes (R3B) combined with a Lewis acid (AlCl3) in an ether-based solvent improves the kinetic behavior of the Mg anode and increases the anodic stability of the MgB electrolyte.12–14 The Aurbach group provided an important breakthrough in the development of MgB electrolytes: they systematically optimized electrochemical properties for a family of Grignard reagents in ether-based solvents (RMgX, R: alkyl or aryl substituent, X: halide) by changing the classes of the substituent, halide, solvent, and Lewis acid.15–19 The proposed Grignard reagent-based electrolytes dramatically alleviate sluggish reactions from the Mg anode and research on MgB systems has increased dramatically to date. For


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example, Aurbach et al. suggested another convenient electrolyte system composed of a nonnucleophilic, glyme-based solvent combined with a Mg-salt (MgCl2) and Lewis acid (AlCl3) to enhance the electrochemical stability of MgB electrolytes.20 These electrolytes are effective for the removal of MgO via chemical or electrochemical reactions, and the resulting fresh Mg surface ensures ionic and electronic pathways and allows reversible electrochemical deposition and dissolution of Mg-ions on the Mg anode surface.3,21 Nevertheless, there is a critical limitation to expanding the use of high-voltage cathode materials, which can enhance the energy density of the MgB system. The relatively low anodic stability of the electrolyte is a bottleneck to employ high-voltage cathode materials in MgB.22–24 This restricts the overall energy density of state-of-the-art MgB systems. Therefore, promising results for simultaneously satisfying reversible electrochemical reactions in both Mg anode and cathode materials are necessary to achieve high performance of MgBs. This study suggests a new approach that allows reversible electrochemical reactions on Mg anode materials. A two-step strategy is applied in the suggested approach: 1) chemically pretreating the Mg anode to remove MgO before assembling the cell and 2) combining the pretreated Mg anode with a cathode material and compatible electrolyte during the cellassembling process (Figure 1). Strong binding affinities between Mg and O impede the removal of MgO from the Mg surface,25,26 and decreasing the binding affinities between Mg and O is necessary, for which oxophilic (Ox) and nucleophilic components are effective. If the Ox component binds with O, it can weaken the surface binding of Mg and O via the formation of an O-activated new local structure, Ox–O+–Mg, which is extremely unstable and can be easily decomposed by the nucleophilic component via chemical reactions even if the nucleophilic


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Figure 1. Scheme for Mg anode pretreatment by employing a Ti complex.

component has low nucleophilicity. This implies that the use of a material combined with oxophilic and nucleophilic components in the pretreatment of Mg anodes would be effective for the removal of MgO, leading to superior interfacial reactivity of the Mg anode. The pretreatment process before assembling the cell simultaneously allows use of high-voltage, stable, and glymebased electrolytes, which have wide anodic electrochemical windows demonstrated by previous studies.9,10 On the basis of these considerations, a titanium (Ti) complex functionalized by an oxophilic Ti element and a nucleophilic bis(trifluoromethanesulfonyl)imide (TFSI) ligand for Mg anode pretreatment is proposed. The Ti complex is synthesized by a simple one-step process, and the underlying reaction mechanism for the chemical reaction between the Ti complex and MgO is


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systematically demonstrated by experimental and computational analyses. The cell employing the Mg anode pretreated by the Ti complex represents practical applicability and good electrochemical performance. This is the first attempt to address controlling interfacial behaviors of the Mg anode via chemical pretreatment, to the best of our knowledge.

Experimental Sections Preparation of pretreatment solution and etching procedure. For pretreatment solution, 0.3 M Mg(TFSI)2 in DME/DGM (1:1) solution and 0.1 M TiCl4 (Aldrich) in DME/DGM (1:1) solution were prepared. They were then mixed with a ratio of 1:1 (mol%) and stirred for more than one day under Ar atmosphere at room temperature. After chemical reaction was completed, 12-pi Mg-disk was immersed in etching solution and thereafter they were stored with a variation of storage time (more than 12 h) under Ar atmosphere at room temperature. After etching process was completed, recovered Mg-disk was quickly washed with DME two times and they were analyzed by SEM (Quanta 3D FEG, FEI) to observe changes in surface morphology. The supernatants before and after etching process were analyzed by fourier transform infrared spectroscopy (FT-IR) (VERTEX 70, Bruker) and nuclear magnetic resonance (NMR) (Bruker, ASCENDTM400) to observe changes in chemical status for etching solution. Characterization of physicochemical and electrochemical properties of electrolytes. To determine the anodic limit of electrolytes, three-electrodes cells composed of stainless steel as a working electrode and Mg disk (Johnson Matthey Korea) as both counter and reference electrodes were assembled. The linear sweep voltammetry (LSV) for electrolyte, 0.8 M Mg(TFSI)2 (Kishida) in 1,2-dimethoxyethane (DME)/diglyme (DGM) (1:1 vol%, Aldrich), was measured by electrochemical workstation (Biologic, SP-300) with a scan rate of 1 mV s–1 in a


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range of 1.0–5.0 V (vs. Mg/Mg2+). For measurement of ionic conductivity, each electrolyte with a variation of molar concentration of Mg(TFSI)2 was prepared and thereafter ionic conductivity was measured by conductivity meter (TOA, cm30b) at room temperature. Evaluation of electrochemical performance. For evaluating electrochemical performance, Mo6S8 was synthesized as below. Cu2Mo6S8 was synthesized by solid-state reaction under Ar atmosphere. Cu (99.7%, Aldrich), Mo (> 99.9%, Aldrich), and MoS2 (99%, Aldrich) were used as starting materials, which were mixed by using mechanical ball milling at 480 rpm for 6 h to reduce the particle size. The mixture placed in a Swagelock cell was heated at 1050°C for 24 h, and then Cu2.5Mo6S8 was obtained. To prepare Mo6S8, chemical Cu extraction from Cu2Mo6S8 was performed with continuous stirring in 8 N HCl solution for 10 h with O2 bubbling. Mo6S8 was gained after the product was rinsed by de-ionized water several times and then was dried at 120°C for 12 h. The cathodes were prepared by coating a slurry containing the active materials (82 wt%), a conducting agent (Super P, carbon black, 10 wt%) and a binder (poly(vinylidene fluoride), Kureha, 8 wt%) dissolved in N-methyl pyrrolidinone (NMP) onto SUS foil substrates. After coating was completed, the electrode was pressed at a pressure of 200 kg cm–2 and dried at 120°C for 12 h. For comparison, the loading of active mass was fixed at 4.0 mg cm–2. To evaluate their electrochemical performance, coin-type half cells (CR 2032) were assembled in a dry room with the dew point controlled to less than –45°C. The Mg disk and a porous poly(ethylene) (PE) separator was used as a counter electrode and a separator, respectively, and the 0.8 M Mg(TFSI)2 in DME/DGM (1:1 vol% ratio) was used as electrolyte. Galvanostatic discharge–charge (i.e. Mg2+ insertion–extraction) experiments were performed in a range of


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0.50–1.95 V (vs. Mg/Mg2+) with a current of 6 mA g−1 (0.05 C) or 5 mA g–1 at room temperature, respectively. Density functional theory (DFT) calculations were performed with the Gaussian09 program.27 The geometry optimization calculations used Becke’s three-parameter exchange functional in combination with the Lee–Yang–Parr correlation functional (B3LYP)28,29 and the standard 631G(d) basis set. All stationary points were characterized as minima by analysis of the Hessian matrices. The absolute chemical shielding constants were computed using the gauge-independent atomic orbital (GIAO) approach.30 The single-point NMR calculations were performed with a large 6-311G(3df,3pd) basis set.

Results and discussion For pretreatment of the Mg anode, a Ti complex was synthesized using TiCl4 and Mg(TFSI)2 in a DME/DGM solution using a ligand-exchange reaction between TFSI and the halide (Figure 2a). The solution was yellowish at the beginning stage of the ligand exchange reaction, but the color of the solution continuously changed to dark brown as the ligand exchange reactions proceeded (Figure 2b). FT-IR and NMR spectroscopies of the solution after seven days confirmed the formation of a new Ti complex. In the FT-IR spectrum (Figure 2c), a new absorbance signal was observed at 850 cm–1, which corresponded to S–O connectivity31 in the TFSI ligand attached to the Ti. The

19

F-NMR analysis of the pretreatment solution provided informative spectroscopic

evidence supporting chemical binding between the Ti and TFSI; a new

19

F signal of the TFSI

ligand was found at –74.4 ppm, indicating a change in the chemical environment of the Mg(TFSI)2 (Figure 2d). This implies that the TiCl4 chemically reacted with Mg(TFSI)2, creating a new Ti complex.


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Figure 2. (a) Synthetic scheme for Ti complex, (b) Monitoring change of solution as increasing reaction time, (c) FT-IR spectra for solution, and (d) 19F-NMR spectra before and after chemical reaction (black: bare solution and red: solution after reaction).


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To understand the TFSI to Cl substitution reactions of TiCl4, we optimized the structures of Mg(TFSI)2 and Ti(TFSI)nCl4−n, n = 1−4, and evaluated the

19

F NMR shielding constants of the

complexes via DFT methods (Table S1). Each of the Ti(TFSI)Cl3 and Ti(TFSI)2Cl2 complexes provided only one

19

F-NMR peak, whereas each of the Ti(TFSI)3Cl and Ti(TFSI)4 complexes

provided two distinct peaks. The F atoms of the TFSI ligands were exposed to the same chemical environment because two O atoms were bound to the Ti to create the TFSI ligand for both Ti(TFSI)Cl3 and Ti(TFSI)2Cl2. However, two different TFSI ligands, with two O−Ti and one O−Ti bond for a TFSI ligand, coexisted for Ti(TFSI)3Cl and Ti(TFSI)4, leading to two distinguishable F-NMR peaks. Thus, one or two substitutions of TFSI to Cl is possible because only one peak was observed in the NMR experiment. The DFT calculations suggest that the reaction energy profile for the reaction between Mg(TFSI)2 and TiCl4 (Figures S1–S3). The experimentally observed peak at −74.4 ppm is attributed to Ti(TFSI)2Cl2 due to the substitution reactions being terminated after the replacement of TFSI with Cl, leading to the production of Mg(TFSI)Cl. The DFT calculations predict that the ∆shift value was −1.5 ppm vs. Mg(TFSI)2, but such a peak was not observed in the experiment. Instead, the experimental value of ∆shift (–5.5 ppm) was similar to the theoretical ∆shift value of – 5.4 ppm for Ti(TFSI)2Cl2. The substitution reaction path was a moderately endothermic process, which agreed with the observation that the reactant Mg(TFSI)2 remained the major compound even after the substitution reactions. Favorable solvation of Ti(TFSI)2Cl2 by an excess amount of ether solvents detached the MgCl2 from the weakly coordinated Ti(TFSI)2Cl2·MgCl2 complex. After characterizing the chemical structure of the Ti complex, the Mg disk was then subjected to a pretreatment process by immersing it in a pretreatment solution composed of Ti(TFSI)2Cl2 to remove the native MgO that had developed. Then, surface morphologies of the pretreated Mg


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Figure 3. (a) SEM analyses for Mg-disk i) before (up) and ii) after pretreatment (down), (b) 19FNMR spectra for supernatant i) before (red) and ii) after pretreatment (blue).

disk and remaining solution are analyzed using SEM and

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F-NMR to clarify the reaction

mechanism for MgO removal in the presence of Ti(TFSI)2Cl2 (Figure 3). The SEM results indicated a chemical pretreatment effect of Ti(TFSI)2Cl2 on the surface morphologies of the Mg disk, which had a relatively rough surface with localized corrosion pits. The surface of the non-


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treated Mg disk was uniformly covered with a native MgO layer (Figure 3a). The EDS analyses supported this explanation (Figure S4) because most of the oxygen elements disappeared in the pretreated Mg disk, whereas the non-treated Mg disk displayed large amounts of oxygen coverage associated with MgO. The NMR analysis of the supernatant collected after pretreatment of the Mg disk also provided informative clues to investigate the specific role of Ti(TFSI)2Cl2. In 1H-NMR spectroscopy results, significant changes were not observed after Mg disk pretreatment (Figure S5), although 1

H-NMR is only detectable for DME and DGM (which have a 1H nucleus in their molecular

structure). It can therefore be concluded that DME and DGM do not participate in the removal of MgO,

only

serving

as

a

reaction

medium

In contrast, significant differences were observed in pretreatment (Figure 3b); a new

19

19

to

dissolve

Ti(TFSI)2Cl2.

F-NMR spectroscopies after Mg disk

F signal was observed at –78.2 ppm after Mg disk

pretreatment, which must originate from the TFSI structure. This implies that the chemical environment of Ti(TFSI)2Cl2 is changed by reacting with the MgO developed Mg disk. Based on this spectroscopic evidence, a reaction mechanism is proposed for the chemical role of Ti(TFSI)2Cl2 (Figure 4). Once the Mg anode is immersed in the pretreatment solution composed of Ti(TFSI)2Cl2, non-bonding electrons in the O chelate with oxophilic Ti in the Ti(TFSI)2Cl2, resulting in the formation of a bimetallic oxonium (O+) complex (Step 1: Oactivation). The resulting O+ intermediate is highly unstable as a result of electron deficiency in O+; therefore, it withdraws extra electrons from neighboring Mg, which creates a more electrophilic Mg than the initial Mg state. In this way, the TFSI ligand attached to Ti(TFSI)2Cl2 leads a preferential chemical attack of the most electrophilic site of the Mg, even if the TFSI ligand does not have sufficient nucleophilicity, to cleave Mg–O connectivity (Step 2:


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Figure 4. Proposed mechanism for chemical reaction between Ti(TFSI)2Cl2 and Mg-disk.

disconnection Mg–O). Next, additional O directly attached to the Mg further chelates with oxophilic Ti, and an intramolecular substitution of chloride (Cl–) yields a rearranged product 1, which can be easily dissolved into the reaction medium (Step 3: removal of Mg–O from the Mg anode). This intermediate can further activate O attached to Mg, and repetition of this process


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creates a di-TFSI-substituted soluble adduct 2 with two molar equivalents of Mg–O units. At this stage, the Cl– anion positioned on the Mg can be rearranged into Ti via intramolecular substitution and an elimination reaction, resulting in the formation of the final, soluble adduct 3. The final adduct is highly soluble in ether-based organic solvents because it has TFSI ligands, which have excellent solubility in typically used organic solvents. This is the main reason the TFSI ligand was used in this study for the Ti complex, because insoluble and rigid native MgO species can be conveniently removed using a chemical conversion reaction followed by an insitu washing process. A new for 19F-peak at –78.2 ppm (∆shift = −1.7 ppm) was observed in the supernatant after the Mg pretreatment, and first-principles calculations for the chemical shift value of adduct 3 (∆shift = −1.1 ppm) was consistent with the experimental results. Therefore, Ti(TFSI)2Cl2 plays a key role in removing insulating MgO species by activating the Mg surface through a chemical-etching process. After obtaining these results, a cell with the pretreated Mg anode, halide-free electrolyte, and cathode material was assembled to demonstrate the effectiveness and generality of the pretreatment approach. An electrolyte composed of Mg(TFSI)2 and a glyme-based solvent (DME and DGM) was used to exclude all corrosive species in the electrolyte, making it possible to employ a high-potential working cathode material. Anodic stability of Mg(TFSI)2/DME/DGMbased electrolytes is quite high—up to 4.9 V (vs. Mg/Mg2+; Figure S6a). In terms of ionic conductivities, Mg(TFSI)2/DME/DGM presents a moderate ionic conductivity value (Figure S6b); ionic conductivities gradually increased with increasing molar concentration of Mg(TFSI)2 and reached a highest value of 0.8 M Mg(TFSI)2 (6.5 mS cm–1), which is comparable to the ionic conductivities of conventional electrolytes.32,33 In addition, 0.8 M Mg(TFSI)2/DME/DGM is compatible with pretreated Mg anodes (Figure S7). Reversible Mg-ion deposition and dissolution


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Figure 5. (a) Voltage profile and (b) cycling performance of the cell assembled with pretreated Mg anode, DME/DGM-based electrolyte, and Mo6S8 cathode (black: Polished Mg, and blue: Pretreated-Mg).

as observed in cyclic voltammetry for the cell assembled with the pretreated Mg anode, while the oxidative current for Mg dissolution did not appear in the cell fabricated with bare Mg. The combination of the pretreated Mg anode and halide-free electrolyte (0.8 M Mg(TFSI)2/DME/DGM) was highly compatible with the Mo6S8 cathode that is typically used (Figure 5). The cell cycled with the pretreated Mg anode exhibited a moderate initial discharge specific capacity of 87.3 mA h g–1 and capacity retention for the initial 30 cycles (75.9%), while the cell cycled with the non-pretreated Mg anode did not work at all. For comparison, the cell assembled with TiCl4-pretreated Mg anode was also evaluated to determine the task-specific role of the TFSI ligand in the Mg pretreatment process (Figure S8). The cell assembled with TiCl4pretreated Mg only afforded 6.1 mA h g–1 of discharge specific capacity in the initial cycle, while the cell cycled with the Ti(TFSI)2Cl2-pretreated Mg anode exhibited much higher discharge specific capacity. This implies that Mg surface activation is primarily governed by chemical


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reactions with TFSI– and not Cl– attached to Ti. Thus, the pretreatment process using Ti(TFSI)2Cl2 is effective for allowing reversible Mg-ion deposition and dissolution. The use of Mg(TFSI)2/DME/DGM electrolytes enhances interfacial stability of the Mg anode. Ha et al.9 removed insulating MgO from the Mg surface using an Mg(TFSI)2/DME/DGM-based electrolyte via an electrochemical process at a low current. The electrolyte components used in this work were based on Mg(TFSI)2/DME/DGM combinations, which also contribute to Mg surface activation during electrochemical cycling. All halide species from the electrolyte were excluded by introducing a pretreatment process for the Mg anode before cell assembly. In this way, a reversible electrochemical reaction of the Mo6S8 cathode was successfully achieved, which implies that the two-step approach based on pretreatment of the Mg anode facilitates a reversible ion-conversion reaction in the Mg anode.

Conclusions A pretreatment approach for an Mg anode was proposed using a new Ti complex to improve reaction kinetics at the Mg anode interface. The Ti complex facilitates reversible Mg-ion deposition and dissolution at the Mg interface because it effectively removes the insulating MgO layer through a series of chemical reactions. The chemical structure of the Ti complex was Ti(TFSI)2Cl2, and its reaction mechanism was determined based on experimental results and first-principles calculations. The Ti(TFSI)2Cl2-pretreated Mg anode was highly compatible with glyme-based electrolytes, resulting in practical applicability and good electrochemical performance. This two-step approach could be expanded to many types of metals, such as aluminum, zinc, and titanium, that suffer from poor kinetic behaviors caused by a native oxide layer developed on the metal surface.


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ASSOCIATED CONTENT Supporting Information. First-principles calculation results for shielding constants of 19F NMR is presented in Table S1, and reaction enthalpy for the reaction between Mg(TFSI)2 and TiCl4 is described in Figure S1. Optimized chemical structures of Ti complexes are exhibited in Figure S2, and its relative energies are shown in Figure S3. EDS analyses for bare and pretreated Mg anode are presented in Figure S4, and 1H NMR analyses for supernatants are included in Figure S5. LSV and ionic conductivity of glyme-based electrolyte are displayed in Figure S6 and CV results are revealed in Figure S7. Potential profile for the cell assembled with TiCl4-pretreated Mg anode, Mo6S8 cathode, and Mg(TFSI)2/DME/DGM electrolyte is exhibited in Figure S8.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (S.-G. Woo), [email protected] (Y.-K. Han), and [email protected] (Y.-J. Kim)

ACKNOWLEDGMENT The work was supported by the Technology Innovation Program (Project No. 10049170) and the Industrial Technology Innovation Program (No. 20152020104870) funded by the Ministry of Trade, Industry and Energy (MI, Korea). This work was also supported by the National Research


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Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF2016R1C1B1009452 and 2016R1A2B4013374).

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(8) Nuli, Y.; Yang, J.; We, R. Reversible deposition and dissolution of magnesium from BMIMBF4 ionic liquid, Electrochem. Commun. 2005, 7, 1105-1110. (9) Ha, S.-Y.; Lee, Y.-W.; Woo, S. W.; Koo, B.; Kim, J.-S.; Cho, J.; Lee, K. T.; Choi, N.-S. Magnesium(II)

Bis(trifluoromethane

sulfonyl)

Imide-Based

Electrolytes

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TOC with synopsis

A new approach to improve electrochemical reactivity of Mg anode is proposed, based on chemical pretreatment of the Mg anode using a titanium complex, Ti(TFSI)2Cl2, that effectively facilitates reversible Mg-ion deposition and dissolution at the Mg interface.


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Graphical abstract 87x73mm (600 x 600 DPI)


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Magnesium Anode Pretreatment Using a Titanium Complex for

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