Article pubs.acs.org/cm
Non-Grignard and Lewis Acid-Free Sulfone Electrolytes for Rechargeable Magnesium Batteries Sung-Jin Kang,† Sung-Chul Lim,† Hyeonji Kim,† Jongwook W. Heo,† Sunwook Hwang,† Minchul Jang,‡ Dookyong Yang,‡ Seung-Tae Hong,*,† and Hochun Lee*,† †
Department of Energy Systems Engineering, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 42988, South Korea ‡ Batteries R&D, LG Chem Ltd., Daejeon 34122, Republic of Korea S Supporting Information *
ABSTRACT: A major challenge for developing rechargeable Mg-ion batteries (MIB) is the lack of suitable electrolytes. We report herein dialkyl sulfones as non-Grignard and Lewis acidfree MIB electrolytes. In particular, a dipropyl sulfone (DPSO)/tetrahydrofuran (THF) (1/1, v/v) solution with MgCl2 salt exhibits high ionic conductivity (1.1 mS cm−1 at 30 °C), Mg cycling efficiency (>90%), and anodic stability (ca. 3.0 V vs Mg). As evidenced by single crystal X-ray diffraction analysis, a novel [Mg(DPSO)6]2+ cation complex balanced by two [MgCl3(THF)]− anions is identified in the DPSO/THF solution. The DPSO/THF electrolyte also enables excellent cycle performance (>300 cycles) of a Chevrel phase Mo6S8 cathode and displays a decent compatibility with an organic cathode (3,4,9,10-perylenetetracarboxylic dianhydride, PTCDA). Along with the superior electrochemical properties of the DPSO/THF electrolyte, its innate chemical stability and eco-friendly nature make it a promising MIB electrolyte.
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demonstrated the first prototypes of rechargeable MIBs employing Chevrel phase Mo6S8 as the cathode material.6 Since this pioneering work, various Mg electrolytes have been reported including amide-based, alkoxide/phenoxide-based, and magnesium aluminum chloride complex (MACC) electrolytes,7−9 but they are still based on the combination of Grignard reagents (or their derivatives) and strong Lewis acids. Unfortunately, the presence of strong Lewis acids raises safety concerns due to their toxic, corrosive, and combustible nature and may introduce unwanted side reactions such as Al deposition and solvent decomposition.6,8,10,11 Recently, there have been several efforts to establish a Mg electrolyte without using Grignard reagents and strong Lewis acids. Magnesium bis(trifluoromethane sulfonyl)imide (Mg(TFSI)2) in glyme solutions was introduced to provide a wide electrochemical window (>4.0 V vs Mg),12,13 but the electrolytes have been proven to suffer from a huge overpotential in the Mg deposition/dissolution reaction, which also heavily depends on water contamination.14 Later, the couse of MgCl2 and Mg(TFSI)2 salts was shown to notably enhance the Mg cycling performance in THF or glyme solutions.15−17 In MgCl2−Mg(TFSI)2 THF electrolytes, the electroactive complex responsible for the reversible Mg
INTRODUCTION Despite the recent progress, meeting all requirements for power sources of electrical vehicles (EVs) and large energy storage systems (ESSs) is a challenge for current lithium ion batteries (LIBs). In particular, the strict expectations for high specific energy density and cost competitiveness for next-generation applications have directed extensive attention toward postlithium batteries. Among them, the Mg-ion battery (MIB) has been recognized as a promising candidate because of the crucial merits of the Mg metal anode: suitable electrode potential (−2.37 V vs NHE), high specific capacity (3830 mAh cm−3), eco-benign nature, and low material cost (ca. $2,700/ton for Mg vs $64,000/ton for Li).1−5 Furthermore, due to the divalent nature of the Mg-ion, an appropriate anode and cathode if developed could generate twice the capacity of the best hosts available for the monovalent Li-ion. However, development of practical MIBs has so far stagnated mainly due to the limited performance of the current electrolyte system, which in turn leads to a lack of proper MIB electrode materials. Gregory et al. first reported that Mg can reversibly deposit from a mixture of Grignard reagent and strong Lewis acid in ether solutions.1 However, the Grignard-based electrolyte suffered from low oxidation stability (2.4 V vs Mg) and © 2017 American Chemical Society
Received: January 19, 2017 Revised: March 9, 2017 Published: March 9, 2017 3174
DOI: 10.1021/acs.chemmater.7b00248 Chem. Mater. 2017, 29, 3174−3180
Article
Chemistry of Materials deposition was revealed to be [Mg2(μ-Cl)3(THF)6]+, which is commonly observed in previous electrolyte systems based on a mixture of Grignard reagent and strong Lewis acid.17 In contrast, a MgCl2−Mg(TFSI)2 monoglyme electrolyte was identified to contain a novel active cation complex [Mg2(μCl)2(monoglyme)4]2+, where one Mg atom is coordinated through the four oxygen atoms of two monoglyme molecules.15 More recently, Mg cycling was demonstrated in an acetonitrilebased electrolyte with Mg(PF6)2 salt where the labile acetonitrile ligand forms a [Mg(CH3CN)6]2+ complex coordinated by two PF6− anions.18 These results suggest that the nature of electrolyte solvent plays a crucial role in determining the structure of the active Mg species via coordination to the Mg atom and thus the overall electrochemical properties of the Mg deposition process. While candidate solvents for current Mg electrolytes have been limited to THF, glymes, and their derivatives that are known to be electrochemically inert toward metallic Mg, some recent studies have reported the use of conventional solvents including sulfones and ionic liquids as the Mg electrolyte components,19−21 which could provide a great opportunity for new categories of Mg electrolytes. In particular, sulfone compounds are known to be chemically stable and eco-friendly unlike Grignard reagents and Lewis acids. They were also investigated as 5 V-class LIB electrolytes because of their outstanding anodic stability (>5.0 V vs Li).22,23 Moreover, electrolytes consisting of dialkyl sulfones (R1R2SO2, R1, R2 = methyl, ethyl, propyl, and butyl) with AlCl3 salt have been reported to enable reversible Al deposition/dissolution.24,25 In addition, sulfone-based electrolytes with Mg(TFSI)2 salt exhibited the Mg deposition/dissolution behavior, although their performances are inferior to other established Mg electrolytes.19 To extend the work of previous studies, we herein investigated sulfone-based non-Grignard and Lewis acid-free Mg electrolytes. We found that the proper selection of dialkyl sulfone, the use of MgCl2 salt, and the addition of THF as a cosolvent enables a highly reversible Mg deposition/dissolution reaction. Then, the unique structures of the active Mg species in these novel electrolytes were examined through single crystal X-ray diffraction analysis and Raman measurements. In addition, the feasibility of the new Mg electrolytes was assessed by evaluating the cycle performance of Chevrel phase Mo6S8 and 3,4,9,10-perylenetetracarboxylic dianhydride organic cathode materials.
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Electrochemical Measurements and Characterization. A three-electrode configuration was employed for cyclic voltammetry and linear sweep voltammetry. A Pt disk electrode (area = 0.02 cm2), Mo foil (area = 0.8 cm2), Al foil (area = 0.04 cm2), SUS foil (area = 0.04 cm2), Chevrel (area = 0.25 cm2), or PTCDA (area = 0.28 cm2) was used as the working electrode, and Mg strips served as the reference and counter electrodes. The Pt electrode was polished with alumina (0.3 μm diameter) slurry on a polishing pad before use. The Mg strip electrodes were polished on an emery-paper (600 grit) inside an Ar-glovebox. Cyclic voltammograms (CV) were performed in the glovebox using a potentiostat/galvanostat (Bio-Logic Science Instruments, VSP-300). Ionic conductivity measurements were carried out using an ionic conductivity meter (Thermo Scientific, Orion VSTAR52) from 10 to 80 °C. For the battery tests, 2032-coin cells were used with a cathode electrode (Pt, Mo, Chevrel, or PTCDA), a glass fiber separator, and a Mg foil anode. Coin cells were galvanostatically cycled between −1 and 1 V for Pt/Mg cells, 0.3 and 1.8 V for Mo6S8/Mg cells, and 0.4 to 2.5 V for PTCDA/Mg cells. The elemental and morphological analyses were carried out using X-ray diffraction (XRD) at 25 °C on an X-ray diffractometer (Rigaku, Miniflex 600) and high-resolution field-emission scanning electron microscopy (HR FE-SEM) (Hitachi, SU-8020) with an energydispersive X-ray spectroscopy (EDX) attachment. Single crystal X-ray diffraction (Bruker, APEXII CCD) was carried out using monochromatized Mo−Kα radiation. Structures were solved by direct methods using SAINT software and refined using full-matrix least-squares using CRYSTALS. Raman spectra of the electrolyte solutions were recorded using a Raman spectrometer (Thermo Scientific, Nicolet almeca XR). Excitation was carried out with the 780 nm line from an argon ion laser. The electrolyte samples were contained in glass vials and measured using the 180-degree beam path accessory. Each spectrum consisted of 64 scans averaged together, and their resolution was 0.5 cm−1.
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RESULTS AND DISCUSSION Characterization of Dialkyl Sulfone Electrolytes. The chemical structure and melting points of the dialkyl sulfones (R1R2SO2, R1, R2 = methyl, ethyl, propyl, and butyl) investigated in this study are listed in Table 1.24,28,29 While the melting point of dimethyl sulfone (DMSO) is too high (107−109 °C) for use at ambient temperature, the other sulfones with longer alkyl chains show much lower melting points (99.0%), ethyl methyl sulfone (EMSO, >99.0%), dipropyl sulfone (DPSO, >99.0%), dibutyl sulfone (DBSO, >99.0%), and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA, >99.0%) were purchased from Tokyo Chemical Industry (TCI) Co. Tetrahydrofuran (THF) was purchased from Aldrich. All chemicals were used after treatment with molecular-sieves to keep the water content below 20 ppm. MgCl2 (99.9%) was purchased from Alfa Aesar and used as received. The electrolyte solutions were prepared and stored in an Arfilled glovebox (90% CE), and innate oxidation stability (ca. 3.0 V vs Mg). The unique structures of the active Mg species in this novel electrolyte were identified using single crystal X-ray diffraction analysis and Raman spectroscopy. DPSO/THF also enabled excellent cycle performance of a Chevrel phase Mo6S8 cathode and good compatibility with an organic cathode (PTCDA) material. Moreover, along with its superior electrochemical properties, the simple preparation procedure, intrinsic chemical stability, and eco-friendly nature of the DPSO/THF electrolyte make it a promising MIB electrolyte.
Afterward, the Mo6S8/Mg cell displayed excellent cycle performance up to the 300th cycle with a constant specific capacity of 70−75 mAh g−1 and high CE value (98−99%), as shown in Figure 5c. Next, we examine the compatibility of DPSO/THF with an unconventional organic cathode material, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA, inset in Figure 6a), with a
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b00248. Additional CV data, Raman spectra, and crystallographic data of single crystal XRD (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Figure 6. (a) Second CV cycle of the PTCDA in DPSO/THF at a temperature of 25 °C and a scan rate of 0.05 mV s−1 and (b) the charge/discharge profile of a PTCDA/Mg cell with a constant current rate of 0.116 mA cm−2 (0.73 C).
Seung-Tae Hong: 0000-0002-5768-121X Hochun Lee: 0000-0001-9907-5915 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was financially supported by the Ministry of Education, Science, and Technology (MEST) and the National Research Foundation of Korea (NRF) through the Human Resource Training Project for Regional Innovation (No. 2015035858).
theoretical capacity of 68.3 mAh g−1 when assuming a oneelectron reaction.38 Figure 6a presents a typical cyclic voltammogram of the PTCDA cathode, which shows a distinct pair of reduction and oxidation peaks at ca. 1.6 and 1.9 V, respectively. The galvanostatic charge/discharge cycle of a PTCDA/Mg cell also shows a reversible capacity of 65.0 and 44.8 mAh g−1 at the first and 10th cycles, respectively (Figure 6b), confirming the decent compatibility of DPSO/THF with the organic cathode material. The rather rapid capacity fading of the PTCDA cathode shown in Figure 6b does not seem to be due to the anodic decomposition of DPSO/THF because the oxidation potential of DPSO/THF (ca. 3.0 V vs Mg) was much higher than the upper voltage end in the galvanostatic cycle test (2.5 V vs Mg). Therefore, the poor cyclability of a PTCDA/Mg cell is more likely due to the degradation of PTCDA itself.39 While this study confirmed the compatibility of DPSO/THF with only two cathode materials due to the lack of available candidate Mg cathode materials, combining all the aspects considered, DPSO/THF can likely be applied to the other cathodes as well, which deserves further study. Furthermore, although the combination of DPSO and THF was proven as a highly promising Mg electrolyte, DPSO/THF may possibly not be the best match, and other combinations of dialkyl sulfone and cosolvent are currently under survey in our group.
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DOI: 10.1021/acs.chemmater.7b00248 Chem. Mater. 2017, 29, 3174−3180
