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Cite This: Chem. Mater. XXXX, XXX, XXX−XXX
Zinc Blende Magnesium Sulfide in Rechargeable Magnesium-Sulfur Batteries Yuri Nakayama,*,† Ryuhei Matsumoto,† Kiyoshi Kumagae,† Daisuke Mori,† Yoshifumi Mizuno,† Shizuka Hosoi,† Kazuhiro Kamiguchi,† Naoki Koshitani,† Yuta Inaba,‡ Yoshihiro Kudo,‡ Hideki Kawasaki,§ Elizabeth C. Miller,∥ Johanna Nelson Weker,∥ and Michael F. Toney∥ †
Murata Manufacturing Co., Ltd., 1-10-1 Higashi-kotari, Nagaokakyo-shi, Kyoto 617-8555 Japan Sony Corporation, Atsugi Tec., 4-14-1 Asahi-cho, Atsugi-shi, Kanagawa 243-0014 Japan § KRI, Inc., Kyoto Research Park, 134, Chudoji Minami-machi, Shimogyo-ku, Kyoto 600-8813 Japan ∥ SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States
Chem. Mater. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/05/18. For personal use only.
‡
S Supporting Information *
ABSTRACT: Magnesium-sulfur batteries are one of the most promising next-generation battery systems due to their high energy density, low cost, and high level of safety. However, the reaction mechanisms are not well understood, and in particular, the discharge reaction products have not yet been identified. Here we show that zinc blende magnesium sulfide is observed as a reaction product after discharging in magnesium-sulfur batteries. When magnesium reacts electrochemically with sulfur in a sulfone-based magnesium electrolyte, sulfur becomes amorphous consisting of magnesium and sulfur in the cathode. In this study, it has been found that the amorphous material has an unusual local structure, which is not related to the most stable rock salt phase of magnesium sulfide but rather the metastable zinc blende phase. It was indicated that this material realizes the reversibility of magnesium-sulfur batteries.
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INTRODUCTION Magnesium is an attractive candidate for an anode material with high energy density, low cost, and increased safety owing to its lack of dendrite formation.1−3 These characteristics are especially relevant for the magnesium-sulfur (Mg-S) battery, composed of magnesium metal anodes and sulfur composite cathodes, which is potentially one of the most promising battery systems with high energy densities beyond the limit of lithium ion batteries.4−12 The rechargeable Mg-S battery was first demonstrated by Muldoon et al. in 2011 and was realized by the development of a non-nucleophilic Mg electrolyte.4 Regarding the reaction mechanism, they reported that the chemical conversion from crystalline sulfur (S8) to magnesium sulfide (MgS) was confirmed after discharging, together with a partially reversible reaction via charging, through analysis of cathode materials by X-ray photoelectron spectroscopy (XPS). Detailed analysis was reported by Fichtner et al. in 2015, where UV−vis spectroscopy of electrolytes was performed to probe the dissolved polysulfides together with the XPS analysis of cathodes, suggesting the three step sulfur reduction during discharge as S8 − MgS4 − MgS2 − MgS.7 Recently, thermodynamics and kinetics of these reactions have been thoroughly investigated by Wang et al.12 Although these reaction mechanisms are beginning to be elucidated, there have been few reports demonstrating the direct observation of © XXXX American Chemical Society
discharge products. One of the two exceptions was reported by Ha et al., stating that MgS with rock salt (RS) structure was a discharge product, which was suggested by X-ray diffraction (XRD).5 Another discharge product has been reported by Dominko et al., showing that MgS with wurtzite (WZ) structure was a final discharge product, which was observed by 25 Mg nuclear magnetic resonance (NMR) measurement.11 However, as both suggestions were based on broad peaks in each measurement, identification of the discharge product is still controversial, which is partially due to the amorphous phases obtained in Mg-S batteries, as well as the electrolyte dependence.8,11 Hence, in this study, we have investigated the reaction mechanism of Mg-S batteries in detail by electrochemical measurements, XPS, X-ray absorption near edge structure (XANES) spectroscopy, XRD, high-energy XRD (HE-XRD), transmission X-ray microscopy (TXM), solid-state NMR, and ab initio calculations. We chose an electrochemically active Mg electrolyte composed of magnesium chloride (MgCl2) and ethyl n-propyl sulfone (EnPS), which is the simplest, nonGrignard, Lewis acid-free, and room temperature operating Received: May 18, 2018 Revised: August 25, 2018
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DOI: 10.1021/acs.chemmater.8b02105 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 1. . Electrochemical activity of MgCl2/EnPS = 1/8(mol) and typical discharge and charge profiles of Mg-S batteries. (a) Cyclic voltammetry for a magnesium electrolyte MgCl2/EnPS = 1/8(mol). (b) Discharge and charge curves for the Mg-S battery with the electrolyte of MgCl2/EnPS = 1/8 (solid line). Dashed lines show the charge and discharge profiles for Mg-S battery using commercial MgS (rock salt structure) as chargestarting cathode material.
Figure 2. Reversible changes of electronic states in sulfur cathodes for Mg-S batteries. (a, b) XPS spectra for S 2p and Mg 2p, respectively, of sulfur cathodes, where bulk means the spectrum obtained after sputtering. (c) S K-edge XANES spectra for sulfur cathodes showing the reversible peak decreasing and recovering at 2472 eV with regard to their discharging and charging states. (d) S K-edge XANES spectra for reference samples, such as S8, MgS (rock salt), MgCl2/EnPS = 1/8, and ethyl methyl sulfone (EMS).
electrolyte.13 Consequently, we have found that metastable zinc blende (ZB-) MgS appears in the cathodes after discharging, rather than RS-MgS or WZ-MgS. It was also indicated that ZB-MgS realizes the reversibility of Mg-S batteries, although they show structural irreversibility.
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of MgCl2 (99.99%, anhydrous) and EnPS (>99%, anhydrous) as MgCl2/EnPS = 1/8(mol). For the experiments of ex situ XPS, XANES, and XRD, in order to eliminate the Mg electrolytes on the surface, cathodes were rinsed with both EnPS and toluene before the measurements. In addition, in the case of XPS experiments, cathodes were sputtered by Ar (2 kV, 10 min) to investigate the electronic states of materials approximately 14 nm below the surface. For the bulk analysis by HE-XRD, TXM, and NMR experiments, cathodes were investigated just after drying without any surface treatment. Commercialized equipment was used for both XPS (ESCA 5800, ULVAC-Phi) and XRD (D-8, Bruker). XANES measurements were performed in fluorescence mode at BL-11B at the Photon Factory at High Energy Accelerator Research Organization, KEK, while HEXRD was done at BL04B2 at SPring-8 with an incident X-ray energy of 61.4 keV. TXM observations were performed using an 8 keV incident beam at BL6-2c at the Stanford Synchrotron Radiation Lightsource at SLAC National Accelerator Laboratory for both ex situ and operando measurements. 33S and 25Mg magic angle spinning (MAS) NMR spectra were collected by an ECA-800 (JEOL Resonance, 800 MHz) at the National Institute for Materials Science (NIMS), where 33S isotopes (99%) were used as sulfur cathodes to
EXPERIMENTAL SECTION
All chemical preparations and measurements were performed under inert atmospheres, pure argon or vacuum, and all the measurements were carried out at room temperature (25 ± 2 °C). Cathodes were prepared by mixing sulfur (99.999%, Wako), conductive carbon (acetylene black: AB or Ketjen black: KB), and binders (polytetrafluoroethylene: PTFE, or styrene-butadiene rubber: SBR, and carboxymethyl cellulose: CMC), then fabricated as either pellets or sheets. Pellets were prepared for XPS, XANES, and XRD whose weight ratio was sulfur/AB/PTFE = 10/84/6, and coated sheets were prepared for others with sulfur/conductive carbon/SBR/CMC = 48.8/48.8/2/0.4(wt) where KB was used for HE-XRD and NMR, while AB was used for TXM as conductive carbons. Sulfone-based magnesium electrolytes were used in this study, which were composed B
DOI: 10.1021/acs.chemmater.8b02105 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 3. Irreversible structural changes in sulfur cathodes. (a) XRD for sulfur cathodes where diffraction peaks of S8 disappeared after discharging and are not recovered after charging. (b) PDF for pristine, discharged, and charged sulfur cathodes observed by HE-XRD measurements. A peak at 2.0 Å corresponding to S−S bonding disappeared after discharging and does not reappear even after charging. The other peak corresponding to Mg-S was observed at 2.5 Å after discharging, while it disappeared after charging. enhance the relative sensitivity of 33S. Ab initio calculations were performed for the simulations of NMR spectra using the plane-wave DFT code CASTEP (BIOVIA).
formation of MgS (RS), as shown in Figure 2d for reference, is observed in this measurement. These results may be due to the ex situ nature of these experiments, where species soluble in the electrolyte cannot be detected after prewashing treatment of cathodes. As a consequence, as far as probing the electronic states of sulfur cathodes, the near reversibility of the redox reactions has been confirmed by XPS and XANES measurements. To investigate the structural responses of sulfur cathodes during the redox reactions, ex situ XRD measurements were performed, and these results are presented in Figures 3a and S4a. While the crystal structure of sulfur S8 was observed both in a pristine cathode and in an assembled battery with electrolyte, they are not observed after either discharging or charging. This result shows an irreversible structural change regarding S8, and the formation of an amorphous phase in the first discharging without recrystallization after charging. To better understand the nature of the amorphous phases, we investigate these sulfur cathodes by ex situ HE-XRD to extract the atomic pair distribution function (PDF, presented as G(r)), revealing the distribution of interatomic distances, especially in the amorphous phase.18 These results are presented in Figure 3b and Figure S4b. The pristine cathode shows a sharp peak at the interatomic distance of 2.0 Å corresponding to a distance of S−S bonding in ordered S8 clusters.19 On the other hand, the discharged cathode shows a pair of peaks at 1.4 and 2.5 Å, where the former corresponds to that of C−C bonding either in electrolyte or from the conductive carbon and the binders,20 while the latter corresponds to the distance of Mg−S bonding in the discharged product MgSx. Note that 2.5 Å is slightly shorter than that of typical MgS with a rock salt structure.21 This 2.5 Å peak disappears (or the intensity decreases to zero) after charging without showing any obvious peak reappearing at 2.0 Å for S8, indicating that the charged cathode does not return to the ordered S8 structure (S8 clusters or crystals); instead, it becomes amorphous or so-called polysulfide with some variability of interatomic distances for S−S bonding. Hence, the S8 cathode in Mg-S batteries changes to another amorphous phase after discharging that involves Mg−S bonding with interatomic distances of 2.5 Å, while this disappears with charging, presenting an irreversible structural change in spite of the electrochemical reversibility. In order to better understand the structural irreversibility, we observed the morphological change by both ex situ and operando TXM, with which we can observe the sulfur particles
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RESULTS AND DISCUSSION Cyclic voltammetry was performed for the Mg electrolyte MgCl2/EnPS = 1/8(mol), described in Figure 1a. Mg deposition and dissolution was observed at room temperature with high Coulombic efficiency of 96%. Oxidation decomposition potentials with various working electrodes were investigated (Figure S1a), and it has been found that the electrochemical window is larger than 3 V with respect to the standard electrode potential of Mg. The electrolyte shows ionic conductivity of 1.7 × 10−4 S/cm at room temperature with the activation energy of 23 kJ/mol (Figure S1b), confirming that it is appropriate for the electrolyte of a Mg-S battery. Figure 1b shows a typical Mg-S rechargeable battery profile as solid lines. Here, we used Mg metal anodes, sulfur/carbon composite cathodes, and sulfone-based Mg electrolytes composed of MgCl2 and EnPS. The observed polarization is due to the overpotential at the anode as shown in Figure S2. In Figure 1b, in order to investigate the electrochemical activity of RS-MgS that has been considered as a discharge product, we also plot in dotted lines the charge and discharge properties of the Mg-S battery involving commercially available RS-MgS as starting cathode material (note that this cell was started from charging). As seen here, the RS-MgS cathode showed a much smaller capacity (corresponding to that of electric double layer) than the Mg-S battery, indicating that RS-MgS is not electrochemically active, nor is it the discharge product of the Mg-S battery. Ex situ XPS spectra for various cathode materials of Mg-S battery are presented in Figure 2a,b, where we can see that both sulfur (S 2p, Figure 2a) and magnesium (Mg 2p, Figure 2b) show reversible changes of electronic states with discharge and charge reactions. It should be noted that discharged cathodes show similar spectra to those of RS-MgS after sputtering, while charged ones show identical spectra to that of assembled cathodes composed of S8 and Mg electrolytes, implying completely reversible electrochemical reactions. Figure 2c shows the ex situ S K-edge XANES spectra for sulfur cathodes with various states of discharge, where peak intensities at 2472 eV corresponding to S8 decreased with discharging, and then increased with charging reversibly. On the other hand, neither a pre-edge peak corresponding to polysulfides as reported in Li-S batteries14−17 nor obvious C
DOI: 10.1021/acs.chemmater.8b02105 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 4. Ex situ and operando TXM images of sulfur composite cathodes in Mg-S rechargeable batteries. (a) Ex situ TXM mosaic images of pristine, (b) fully discharged, and (c) fully charged cathode. Particles observed in (c) are understood to be unreacted ones. (d) Operando TXM images of a sulfur composite cathode together with its discharge and charge profile. Numbers in the images describe the corresponding specific capacities presented by colored dots in the discharge and charge profile. (e) Operando TXM micrographs of a sulfur composite particle during first discharge together with their specific capacities. The particle corresponds to the circled one in (d). Outlines of the initial particle are presented in all images to compare the particle sizes at each capacity, describing the gradual particle swelling by 200 mAh/g, followed by the rapid dissipation after 250 mAh/g. The latter suggests sulfur dissolution into the electrolyte.
in the cathode directly.22,23 Figure 4a−c shows representative mosaic images of sulfur in pristine (Figure 4a), discharged (Figure 4b), and charged cathodes (Figure 4c) observed by ex situ TXM measurements. We can see that sulfur particles whose diameters are micrometers to several tens of micrometers disappear after discharging and do not reappear again after charging. Since the cathode stored at open circuit voltage for the same time required for particle disappearance during discharging still showed sulfur particles (Figure S5), it is clear that the discharge reaction promoted the particles’ disappearance. Thus, the ex situ XPS, XANES, and HE-XRD results confirmed the reaction products in prewashed cathodes are amorphous, and the TXM (Figure 4b,c) showed that (except for unreacted sulfur particles) the electrodes are almost uniform after discharging and charging. Taken together, this shows that amorphous solid products of both discharged MgSx and charged sulfur are uniformly redistributed on carbons in the cathodes.
Figure 4d shows sulfur cathodes of the Mg-S battery observed by operando TXM measurement, together with its discharge and charge profile. Note that it is different from Figure 1b due to the different cathode preparation (details are shown in Table S1 and Figure S3). Sulfur particles are observed during the first plateau and disappear rapidly by the point where the discharged capacity is about 400 mAh/g. After the disappearance, particles are not observed again during the second plateau or even after charging. In Figure 4e, enlarged images for a sulfur particle are given to investigate the morphological changes in detail. Here we can see that the sulfur particle slightly expands by the point where discharged capacity is 200 mAh/g, and then shrinks rapidly after the point of 250 mAh/g, which is well consistent with ex situ XRD measurements in Figure S4a. This behavior can be explained such that, in the first discharge reaction of the Mg-S battery, sulfur particles expand during the reaction of Mg + S8 → MgS8, whose capacity is expected to be 200 mAh/g, while the D
DOI: 10.1021/acs.chemmater.8b02105 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 5. Observed and calculated NMR spectra and crystal structures of MgS. (a) 33S NMR and (b) 25Mg NMR observed spectra for discharged cathode (blue) and charged cathode (red) of Mg-S batteries. Observed spectra for RS-MgS (black) are also plotted as references. Calculated NMR spectra for both RS-MgS (gray) and ZB-MgS (green) are plotted in the same figures to identify the observed material. (c) Crystal structures of RSMgS and (d) ZB-MgS.
Table 1. Calculated nucleus 33
S
25
Mg
33
S and 25Mg NMR Parameters for MgS with Rock Salt, Zinc Blende, and Wurtzite Structures
MgS structure rock salt (Fm3m) zinc blende (F4̅3m) wurtzite (P63mc) rock salt (Fm3m) zinc blende (F4̅3m) wurtzite (P63mc)
δisocalc (ppm)a
Δσ (ppm)b
−173 −265 −274 −2.39 70.3 51.5
0.00 0.00 2.68 0.00 0.00 6.63
ηc N/A N/A 0.00 N/A N/A 0.00
CQ (MHz)d −14
1.48 × 10 −4.01 × 10−14 −5.49 × 10−1 1.70 × 10−12 2.08 × 10−12 −3.08
ηQe N/A N/A 0.00 N/A N/A 0.00
δisocalc: isotopic chemical shift, obtained as δisocalc = σref − σiso, where σiso corresponds to isotropic chemical shielding obtained by calculations, while σref was set at ZnS (F4̅3m) for 33S,24 and set at MgO for 25Mg.26 bΔσ: chemical shift anisotropy cη: asymmetry parameter. When Δσ is 0, η is to be N/A as the process involves division by 0. dCQ: quadrupolar coupling constant. eηQ: quadrupolar asymmetry parameter. When ηQ is N/A, it means electric field gradient around the nuleus is small or 0. a
disappearance of the peak after charging is likely due to peak broadening derived from the large structural asymmetry around S atoms in the amorphous or polysulfide, while the disappearance of the 25Mg NMR peak is simply caused by the release of Mg from the cathode. In addition, it should be noted that the observed peaks in the discharged cathodes are completely different from those for RS-MgS, indicating that a different structure is present. In order to understand these sharp peaks in more detail, we performed ab initio calculations for NMR spectrum simulations.25−27 Chemical shift parameters and quadrupolar coupling constants (CQ) for both 33S and 25Mg were calculated for three structures of MgS: RS-MgS having octahedral coordination with S around Mg, as well as ZB-MgS and WZMgS, where Mg shows the tetrahedral coordination with S. Crystal structures of RS-MgS and ZB-MgS are shown in Figure 5c,d, respectively. Here we used the observed distance of 2.5 Å for the tetrahedral M−S bonds in ZB-MgS and WZ-MgS, while 2.6 Å for the octahedral one in RS-MgS,21,26 whose results are
particles dissolve into the electrolyte with the reaction of Mg + MgS8 → 2MgS4 that would be completed by 400 mAh/g. Hence we have revealed electrochemical reversibility, as well as structural and morphological irreversibility in the reaction of a Mg-S battery, though reaction products are still ambiguous. Finally, we have used ex situ solid state NMR to investigate the nuclear state of reaction products for both 33S and 25Mg in cathodes.11,24 Observed 33S and 25Mg NMR spectra for fully discharged and charged cathodes are presented in Figure 5a,b, respectively, together with those of RS-MgS as references. In Figure 5, peaks are observed for discharged cathodes, namely, at −255 ppm for 33S NMR (Figure 5a), and 67 ppm for 25Mg NMR (Figure 5b). There is no peak for Mg electrolyte, though it is involved, as it is hard to observe it by itself. Observation of solid-state NMR peaks in cathodes suggests that reaction products are precipitated from the electrolyte in the fully discharged state, while they disappeared in charged ones, indicating that almost of all the discharging products are consumed by charging. Regarding 33S NMR spectra, the E
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ZB-MgS in bulk cathode materials. In addition, it has been indicated that the observed electrochemical reversibility of MgS batteries is essentially due to the structural instability of ZBMgS. These results suggest not only the possibility of a chargestart rechargeable system for Mg-S batteries like the present lithium ion batteries31 (using ZB-MgS as starting cathode materials) but also the development of novel, next-generation batteries using active materials with metastable structures.
summarized in Table 1, and calculated spectra are presented in Figure 5a,b. Simulated spectra for RS-MgS exhibited good agreement with measured spectra. More surprisingly, calculated spectra for ZB-MgS showed the peaks at chemical shifts almost identical to those observed for a fully discharged cathode in both 33S and 25Mg NMR spectra. Because the observed cathodes show the sharp peaks in the NMR spectra, quadrupolar interactions of discharged products must be small, which can be confirmed by CQ for ZB-MgS in Table 1. It should be noted that absolute values of CQ for WZ-MgS are relatively large (Table 1), indicating the quadrupolar peak broadening for this structure.27 Here we can conclude that discharging products in the cathode of the Mg-S battery with sulfone based electrolyte possess local ZB-MgS tetrahedral coordination around Mg. As the discharging products did not show any XRD peaks, the cluster size of ZB-MgS must be smaller than approximately ∼3−5 nm.28 Regarding the chemical stability of this reaction product, it has been confirmed by NMR measurements that the obtained ZB-MgS is stable irrespective of washing treatments (Figure S6). Although there is a report claiming that discharging products should be general among Mg-S batteries,11 actual electrolyte dependence is now under investigation. Consequently, the reaction mechanism of a Mg-S battery using sulfone-based electrolyte can be explained as follows. In the initial discharging, Mg reacts with S8 in a solid state, forming MgS8. In the second stage, when sulfur is reduced further to make MgS4, the MgS4 dissolves into the electrolyte. Dissolved polysulfides are further reduced in the electrolyte, and then in the final stage of discharging, amorphous products, whose microstructure is ZB-MgS, precipitate onto carbon in the cathode. On the other hand, in charging reaction, ZB-MgS is oxidized completely to form amorphous sulfur or polysulfide, whose electronic state is identical to that of initial S8. There seem to be a couple of reasons to find ZB-MgS in this study, which is entirely different from the previous observations. One is the development of 33S MAS NMR technologies including ab initio calculations, and the other one is the different electrolyte used in our Mg-S system. Though electrolyte dependence as well as cycle performance of these phenomena are under investigation, we think that the former is essential for this discovery. Moreover, this is the first demonstration that metastable ZBMgS can be synthesized by electrochemical processes as bulk cathode compounds, as opposed to epitaxial stabilization in thin films.29 As this is a direct wide band gap semiconductor,30 our study opens the door for a wide range of applications of ZB-MgS such as in electronics, optoelectronics, as well as in energy devices.
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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.8b02105. Experimental details, electrochemistry, discharge and charge profiles of Mg-S batteries, XRD of sulfur cathodes, TXM images, and NMR spectra (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yuri Nakayama: 0000-0003-1705-5078 Michael F. Toney: 0000-0002-7513-1166 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was originally supported by Sony Corporation, and is currently supported by Murata Manufacturing Co., Ltd. A part of this work was supported by NIMS microstructural characterization platform as a program of “Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The authors wish to thank Dr. Toshimi Fukui for his fruitful discussions, Dr. Mitsunori Nakamoto for his kind support on ab initio calculations, and Dr. Hideki Kumita for his helpful support on NMR analysis. We are also grateful to Dr. Yoshinori Kitajima for his cooperation on XANES experiments.
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REFERENCES
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CONCLUSIONS Reaction mechanisms of Mg-S batteries with sulfone-based electrolyte have been studied. Electrochemical reversibility together with the structural irreversibility during discharge and charge reactions has been observed in this system. It has been found that the discharging reaction products have the local structure of ZB-MgS in the amorphous phase, neither RS- nor WZ-MgS, which shows both chemical stability and electrochemical reversibility. It is well-known that ZB-MgS is a metastable phase that must be stabilized by epitaxial strain, while RS-MgS is the most stable phase for MgS.29 Hence, this is the first observation of F
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Chemistry of Materials
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DOI: 10.1021/acs.chemmater.8b02105 Chem. Mater. XXXX, XXX, XXX−XXX
