Toward Highly Reversible MagnesiumâSulfur Batteries with Efficient

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Towards Highly Reversible Magnesium-Sulfur Batteries with Efficient and Practical Mg[B(hfip)4]2 Electrolyte Zhirong Zhao-Karger, Runyu Liu, Wenxu Dai, Zhenyou Li, Thomas Diemant, B. P. Vinayan, Christian Bonatto Minella, Xingwen Yu, Arumugam Manthiram, R. Jürgen Behm, Mario Ruben, and Maximilian Fichtner ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01061 • Publication Date (Web): 25 Jul 2018 Downloaded from http://pubs.acs.org on July 25, 2018

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ACS Energy Letters

Towards Highly Reversible Magnesium-Sulfur Batteries with Efficient and Practical Mg[B(hfip)4]2 Electrolyte Zhirong Zhao-Karger,*a Runyu Liu,b Wenxu Dai,b Zhenyou Li,a Thomas Diemant,c B. P. Vinayana, Christian Bonatto Minella,a Xingwen Yu,d Arumugam Manthiram,d R. Jürgen Behm,a,c Mario Ruben,b,e Maximilian Fichtnera,b a

b

Helmholtz Institute Ulm (HIU) Electrochemical Energy Storage, Helmholtzstr. 11, D-89081 Ulm, Germany

Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), P.O. Box 3640, D-76021 Karlsruhe, Germany c

Institute of Surface Chemistry and Catalysis, Ulm University, Albert-Einstein-Allee 47, D-89081 Ulm, Germany

d

Materials Science & Engineering Program and Texas Materials Institute, The University of Texas at Austin, Austin, TX 78712, United States

e

Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), CNRS, Université de Strasbourg, 23 rue du Loess, BP 43, F-67034 Strasbourg Cedex 2, France

Abstract Rechargeable magnesium (Mg) battery has been considered as a promising candidate for future battery generations due to unique advantages of Mg metal anode. The combination of Mg with a sulfur cathode is one of the attractive electrochemical energy storage systems that use safe, low-cost and sustainable materials and could potentially provide a high energy density. To develop a suitable electrolyte remains the key challenge for realization of magnesium

sulfur

(Mg-S)

battery.

Herein,

we

demonstrate

that

magnesium

tetrakis(hexafluoroisopropyloxy) borate Mg[B(hfip)4]2 (hfip = OC(H)(CF3)2) satisfies a multitude of requirements for an efficient and practical electrolyte, including high anodic stability(> 4.5 V), high ionic conductivity (~ 11 mS cm-1) and excellent long-term Mg cycling stability with a low polarization. Insightful mechanistic studies verify the reversible redox

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processes of Mg-S chemistry by utilizing Mg[B(hfip)4]2 electroylte and also unveil the origin of the voltage hysteresis in Mg-S batteries.

Battery technology represents a promising solution for renewable energy storage and emission-free transportation. Lithium-ion batteries (LIBs) are the main choice to power today's portable electronic devices and electric vehicles. However, current LIBs are approaching their theoretical limits in terms of energy density.1,2,3 In addition, concerns have been raised about the safety in operation and the supply of the key materials for a global electro-mobility and stationary energy storage. In particular, the world´s cobalt (Co) resources and reserves cannot meet the growing demand for NMC materials in near future.4 For these reasons, increasing attention has been paid to new high-energy battery systems relying on sustainable and low-cost materials. Rechargeable Mg battery has been proposed as one promising candidate for next-generation battery technology owing to the potentially high volumetric capacity of 3837 mAh cm−3 of Mg,5 which is considerably higher than that of Li (2062 mAh cm−3) and sodium (Na, 1136 mAh cm−3), respectively. In view of mobile applications, the volumetric energy density of a battery is particularly important to control the size of battery packs. In contrast to Li and Na, Mg metal, with a reduction potential of −2.4 V (vs SHE), can be safely handled under ambient conditions and does not form dendrites in repeated stripping/plating processes.6,7 Its ready availability, high abundance and low cost render Mg based batteries an economic and sustainable option. In recent years, considerable effort has been devoted towards Mg electrolytes with wide electrochemical window, high 2 ACS Paragon Plus Environment

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efficiency and good compatibility with cathode materials.8,9,10 In fact, the quest for high energy cathode materials is still an on-going challenge.11,12 Due to the strong electrostatic forces between the doubly charged Mg2+ ions and the surrounding anions in the crystal matrix, Mg2+ insertion kinetics is intrinsically slow in most of the conventional intercalation oxide materials.13 The use of “softer” chalcogenides may facilitate a reversible insertion/desertion of Mg ions,13,14,15 however, it compromises the high voltage of intercalation materials, rendering the batteries unattractive in energy densities. A high capacity conversion-type cathode material could be an alternative option for Mg batteries with high energy density. Sulfur (S) is an attractive cathode material because of its high theoretical capacity of 1673 mAh g−1, low cost and nontoxicity. Lithium-sulfur (Li-S) battery has been suggested as a promising high-energy post-Li-ion system.16,17,18 However, safety problem associated with Li metal anode needs to be addressed for the commercialization of Li-S batteries. In contrast, with Mg as a safe anode, the two-electron conversion reaction of Mg2+ + S + 2e− ⇌ MgS yields a thermodynamic voltage of 1.77 V and a theoretical specific energy of 1722 Wh kg−1 and 3200 Wh l−1 for the Mg-S full cell. Moreover, the conversion reaction between Mg and S may circumvent the sluggish kinetics for Mg2+ intercalation. The initial challenge in developing Mg-S batteries has been to discover a sulfur compatible electrolyte with favorable electrochemical properities.10 The nucleophilic nature of the organomagnesium based electrolytes precludes their use for electrophilic sulfur cathodes. The use of non-nucleophilic Hauser base hexamethyldisilazide magnesium chloride (HMDSMgCl) for Mg electrolytes enabled the proof-of-the-concept of Mg-S battery.19 However, the Mg-S cells exhibited a low discharge voltage and only two cycles were demonstrated. We developed non-nucleophilic bisamide-based electrolytes through an one-step reaction of magnesium-bis(hexamethyldisilazide) (HMDS)2Mg and AlCl3 in glymes.20 This type of electrolytes exhibit favorable electrochemical characteristics and have been employed in 3 ACS Paragon Plus Environment


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different Mg battery systems.21,22,23,24,25,26,27,28 The Mg-S batteries with the bisamide-based electrolytes exhibited a flat discharge voltage plateau at 1.65 V, close to the theoretical emf value, and an initial discharge capacity of about 800 mAh g−1.21 However, a number of critical issues have been identified, including the large over-potential for charging, rapid capacity fading

and

poor

cycling

efficiency.

Gao

et

al.

introduced

lithium

bis(trifluoromethanesulfonyl)imide (LiTFSI) into the bisamide-based electrolyte to promote the reversibility of the Mg-S cells.22 The cell presented a discharge capacity of 1000 mAh g−1 with an upper discharge plateau at 1.75 V and good capacity retention for 30 cycles. It was demonstrated that the advanced concepts used for Li-S batteries can be transferred to the MgS systems with bisamide electrolytes. A sulfur nano-composite was fabricated with a functionalized reduced graphene oxide (rGO) to improve the Mg-S batteries.23 Yu et al. designed a Mg-S cell with the separators coated by activated carbon nanofibers (CNFs), which can serve as both a polysulfide scavenger and a current collector.24 In their study, the Mg-S cells with the bisamide-based electrolyte delivered a remarkable initial discharge capacity of 1200 mAh g−1 and lasted for 20 cycles without suffering a fast capacity drop upon cycling. Very recently, metal-organic-framework (MOF)-based high-rate Mg-S batteries were demonstrated using bisamide based electrolyte with addition of LiTFSI.28 Despite the considerable progress in improving the chemical compatibility, the electrolytes generated through Lewis base-acid reactions are generally composed of complex cation Mg2Cl3+ and various aluminate anions in chemical equilibria. Their electrochemical properties could be greatly altered by the dissolved cathode species and the choice of additive is also limited.29,30 In addition, the corrosion caused by Cl− ions restricts their practical utility with metallic current collectors.31,32 It has therefore been one of the goals to synthesize ion conductive “simple” Mg salts which have favorable electrolytic characteristics, are chemically compatible with the electrode 4 ACS Paragon Plus Environment

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materials and non-corrosive to the battery components. Gao et al. recently studied the thermodynamics of the Mg-S cell with solutions of Mg(TFSI)2 in DME, where the cells were cycled at a low current rate.33 In a new approach, the battery performance was significantly enhanced using a concentrated electrolyte consisting of Mg(TFSI)2-2MgCl2 in DME, where a discharge capacity above 600 mAh g−1 after 100 cycles was achieved.34 Zhang et al. prepared a boron-centered anion-based magnesium (BCM) electrolyte and examined its suitability for Mg-S batteries.35 Due to the low solubility of MgF2 in the ethereal solvents, the electrolyte contained a very low concentration of 0.05 M. The detection of the new anion species tetra(hexafluoroisopropyl)borate [B(hfip)4]− and the increase in Mg ion concentration implies an unanticipated side reaction on the Mg anode. In new attempts, the same group prepared the electrolyte through the reaction of tris(hexafluoroisopropyl)borate [B(HFP)3] with MgO or MgCl2, forming new electrochemical active complex Mg cations.36,37 With all these B(HFP)3 based electrolytes, the Mg-S cells displayed a flat discharge voltage plateau at about 1.1 V and a lower charge over-potential compared to other reports. In addition, a beneficial effect of the Cu current collector on the Mg-S cell performance was observed, but the mechanistic reason behind it was not clarified. Nevertheless, the corrosion issue induced by Cl− ions in these electrolytes restricts their use with the conventional metal current collectors. Recent progress made in highly efficient non-corrosive Mg ion-conductive salts provides new prospective for the realization of Mg battery technology. TOYOTA research group reported that magnesium monocarborane (MMC) Mg(CB11H12)2 in glyme exhibits a high anodic stability (~3.8 V) and a high Mg cycling efficiency.38 Arnold et al. demonstrated that magnesium hexafluoroisopropylaluminate Mg[Al(hfip)4]2 in ethereal solvents can reversibly deposit magnesium with high oxidative stability (> 3.5 V vs Mg/Mg2+) and conductivity (> 6 mS cm−1).39 These electrolytes have not been employed for Mg-S batteries so far. We recently established

robust

synthetic

routes

for

Mg

fluorinated


alkoxyborates

and

Mg


alkoxyaluminates in pure solid form as new class of electrolyte salts.40 In particular, magnesium tetrakis(hexafluoroisopropyloxy) borate Mg[B(hfip)4]2 can be straightforwardly synthesized by the dehydrogenation reaction of Mg(BH4)2 with hexafluoro-2-propanol (hfip-H) in DME with high yield. The DME solution of Mg[B(hfip)4]2 is capable of reversible Mg plating/stripping with a coulombic efficiency close to 100% and possesses an extremely high anodic stability (> 4.3 V). Moreover, Mg[B(hfip)4]2·3DME is thermally stable up to 150 °C and also air and hydrolysis stable, which makes it safe in storage and convenient in handling for practical applications. Mg[B(hfip)4]2 is basically compatible with any type of cathode and anode materials and can serve as an universal electrolyte for Mg batteries. The applicability of the Mg[B(hfip)4]2 electrolyte for Mg-S battery was preliminary verified in our previous work.40 Despite the improved cyclibility and Coulombic efficiency, it was also indicated that the Mg-S system still encounters critical problems such as the large voltage hysteresis and capacity fade upon cycling. In order to further evaluate the practicality and performance reliability of the Mg[B(hfip)4]2 electrolytes, we continued to optimize the composition and extensively investigated its bulk properties including ionic conductivity, polarization behavior and long-term cycling ability. The interfacial phenomena occurring between the Mg[B(hfip)4]2 electrolytes and Mg anode was examined as well. With this knowledge, we aim at a comprehensive insight into the electrolyte properties in view of application perspectives and their impacts on the full cell performance. At the same time, we attempted to identify the origin of the voltage hysteresis in Mg-S batteries in order to develop solutions and thus enhance the battery performance. In this work, we further optimized the synthesis of Mg[B(hfip)4]23DME (further denoted as MgBhfip) salt in a facile and scalable way to investigate the practical use of the electrolytes. By refluxing the Mg(BH4)2 and HOC(H)(CF3)2 in DME for 2 h, the conductive salts can be prepared in good yield and high purity. A detailed description of the synthesis procedures can 6 ACS Paragon Plus Environment

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be found in Supporting Information. It is worth noting that the purity of the electrolyte constituents is crucial and trace impurities could be detrimental to the electrochemical performance. In Li-S batteries, DME has shown good performance as electrolyte solvent owing to its low viscosity, limited solubility of sulfur and good chemical stability against reactive metals. So, DME was also chosen for the electrolyte preparation in this work. To optimize the electrolyte composition, MgBhfip solutions with different concentrations were prepared. It should be noted that the electrolyte concentration in our previous report was calculated with the amount of Mg(BH4)2 initially used for the reaction and the added solvent.40 In this work, the molar concentration of the electrolyte is based on the molar mass of Mg[B(hfip)4]23DME, i.e. the isolated solid Mg[B(hfip)4]23DME was dissolved in a volumetric flask with proper amount of DME for the desired concentration.

Concentration dependent properties of Mg[B(hfip)4]2 electrolyte Mg plating/stripping behaviors in the electrolytes at a concentration from 0.1 to 0.4 M have been examined on a Pt electrode. The cyclic voltammograms (CVs) shown in Figure 1a indicate that all the solutions allow reversible Mg deposition/dissolution and the current densities increase with the increase of the concentration, which reflects the tendency of the ionic conductivity in correlation with the concentration of the electrolyte. It was also noticed that the over-potential for Mg deposition decreases as the electrolyte concentration increases. For instance, the onset potential for deposition gradually shifts from −0.43 to −0.32 V as the electrolyte concentration increases from 0.1 to 0.4 M. In contrast, Mg stripping process seems energetically favored at all concentrations, commencing at about the equilibrium potential of 0 V.



(a)

(b)

(c)

(d)

Figure 1. (a) CVs in MgBhfip/DME solutions on Pt electrodes at a scan rate of 50 mV s−1; (b) CV in 0.3 M MgBhfip/DME solution on different metal electrodes at a scan rate of 50 mV s−1; (c) LSVs in 0.3 M MgBhfip/DME solution on different metal electrodes at a scan rate of 1 mV s−1; (d) conductivities of MgBhfip/DME solutions at 23 °C. Further, it was observed that the electrochemical Mg deposition is influenced by the nature of the working electrode, most likely due to the altered desolvation energy of the [Mg(DME)3]2+ ions and the Mg nucleation energetics on the surface of different electrodes. Various electrodes including Pt, stainless steel (SS), copper (Cu), aluminum (Al) and carbon coated aluminum(C-Al) have been examined (Figure1b). The lowest plating potential of −0.25V was observed on a Cu electrode. Independent to these interesting features for Mg deposition on different substrates, the Mg plating/stripping behavior in a symmetric cell with Mg electrodes can directly indicate the reduction potential of Mg2+ ion at the Mg anode in a full cell, which will be discussed later. 8 ACS Paragon Plus Environment

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The linear sweep voltammograms (LSVs) display that the MgBhfip electrolyte is stable up to > 4.5 V on Pt (Figure 1c), demonstrating the highest oxidative stability so far. Moreover, it is also compatible with the standard stainless steel cells and the metals that are commonly used as current collectors such as Al and C-Al foils. It is worth to mention that the anodic stability of the Mg[B(hfip)4]2 on Pt in fact exceeds that of the DME solvent. A similar behavior has been observed in some studies on Cl-free Mg electrolytes,38 which might be attributed to adsorption of some species in the electrolyte on the electrode. A chemical analysis of the surface of the electrodes would be helpful to clarify this phenomenon. Nevertheless, the LSV method with inert electrode such as Pt and glassy carbon is in general accepted to evaluate the anodic stability of an electrolyte.41 The outstanding stability of the electrolyte originates from the high oxidation resistance of the highly fluorinated anions due to the strong electronwithdrawing effect of the fluorine atoms. In addition, the weakly coordinating nature of the [B(hfip)4]− anion enables the easy dissociation of the salt, facilitating favorable Mg deposition/dissolution in the electrolyte. These results suggest that the MgBhfip electrolyte could be employed for high-voltage cathode materials. Ion conductivity is a key parameter to evaluate an efficient and practical electrolyte, as it quantifies the ion mobility within the bulk electrolyte and in part determines the rate capability of batteries. A sufficiently high ion conductivity of electrolyte solution lowers the internal resistance and minimizes the ohmic drop, thus ensuring the power output of cell. The ion conductivities of the electrolytes are presented in Figure 1d. When the concentration increases from 0.1 to 0.3 M, the conductivity increases nearly linearly and then decreases at higher concentrations, most likely due to a reduced ion mobility and formation of ion pairs. In fact, the 0.5 M electrolyte appears to be an oversaturated solution. The 0.3 M Mg[B(hfip)4]2 electrolyte has a high conductivity of approximately 11 mS cm−1 at 23 °C, which is in the range of the Li ion conductivities in state-of-the-art LIBs.42 9 ACS Paragon Plus Environment


Polarization and long-term cyclability of Mg[B(hfip)4]2 electrolyte The polarization behavior and long-term durability are important factors to assess electrolyte’s reliability and practicality. Swagelok-type symmetric Mg cells were assembled and the galvanostatic cycling experiments were carried out by discharging and charging the cell at a constant current for 0.5 h, respectively. Typically, a rest time of 0.5 h was taken before the cycling. Figure 2a shows that the Mg|Mg cells containing 0.1 M and 0.3 M electrolytes exhibit an excellent cycling stability with a low polarization potential of < 0.09 V at a current density of 0.1 mA cm−2 for more than 1000 h while the cell with 0.4 M electrolyte shows gradually increasing over-potential after about 260 cycles. The polarization behaviors at increased current densities have been examined with all the electrolyte solutions. The results for the 0.1 M and 0.2 M electrolytes can be found in Supporting Information. When a current density from 0.1 to 1 mA cm−2 was applied successively, the cells containing 0.3 M electrolyte could persevere a low polarization of about 89 mV at a high current of 0.5 mA cm−2 and the 0.4 M electrolyte could hold a polarization of < 0.1 V at a current of < 0.4 mA cm−2 (Figure 2b). The potential vs time profiles presented in the insets of Figure 2a and 2b generally exhibit flat potential curves for all the cells during galvanostatic cycling, implying the smooth Mg plating process. However, regardless of the electrolyte concentration, a noticeably larger polarization about ±0.3 V for Mg plating and stripping was recorded in the initial a few cycles. To gain insights into these phenomena, the potentiostatic electrochemical impedance (EIS) was monitored representatively with the cell containing 0.3 M electrolyte during the rest periods at the OCV and after galvanostatic cycles. Figure 2c displays the Nyquist plots measured under various conditions. The impedance was measured to be 24 KΩ cm2 after keeping the cell at the OCV for 0.5 h, which should account for the high polarization in the initial galvanostatic cycles. The cell impedance was observed to increase incrementally with the rest time and became stable at approximately 587 kΩ cm2 after 50 h at


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the OCV. However, a substantial reduction of the interfacial resistance was recorded after cycling the cell for several times. As shown in Figure 2d, the impedance was found to be approximately 236 Ω cm2 for the cell after 10 cycles at 0.5 mA cm−2, which is more than 3 orders of magnitude lower than its initial value. An even lower resistance of about 118 Ω cm2 was measured after 100 cycles. Accordingly, the polarization of Mg plating and stripping decreases with extended cycling, which is consistent with the results obtained with the symmetric cells as mentioned before. The corresponding galvanostatic cycling profiles are presented in Figure S5. It is worth noting that a similar trend of the cell impedance change was observed with Mg(CB11H12)2 electrolyte as well.43

(a)

(b)

(c)

(d)

Figure 2. (a) Long-term Mg stripping/plating in MgBhfip/DME electrolytes at a current density of 0.1 mA cm−2; (b) cycling performance of the electrolyte at various current densities; (c) Nyquist plots of the Mg|Mg cell with 0.3 M MgBhfip/DME electrolyte with different rest



time at OCV; (d) Nyquist plots of the Mg|Mg cell with 0.3 M MgBhfip/DME electrolyte after cycling. The high impedance of the Mg|Mg cells with the MgBhfip/DME electrolyte after the rest periods might hint at the formation of an interfacial layer on the Mg surface. The surface of the Mg foil before and after soaking in the 0.3 M electrolyte for a week showed almost no morphological change in scanning electron microscope (SEM) images and no detection of other elements by energy dispersive X-ray spectroscopy (EDX) (Figure S6). This might indicate that the surface film could result from the adsorption of the solvent or/and salt anions rather than reaction of Mg and with the electrolyte constituents. Interestingly, a dark grey colored film on the Mg electrode was observed after disassembling the symmetric cells after cycling. We assume that a kind of solid electrolyte interphase (SEI) might be formed during the electrochemical cycling, which could facilitate the excellent stability of the cells over long-term operation. A thorough analysis of the SEI is currently underway in a separate study.

Mg-S batteries with Mg[B(hfip)4]2 electrolyte The binder-free activated carbon cloth (ACC) based sulfur composites have been used in several studies on metal-sulfur batteries.34,44 Here we chose ACC for the fabrication of a model sulfur cathode for its convenience for the analysis of the sulfur chemistry in Mg based systems. The sulfur cathode denoted as ACCS was prepared using a modified melt-diffusion method with sulfur loading approximately 1 mg cm−2 (Details on the preparation and characterization can be found in Supporting Information). As the dissolution of polysulfide was reported to be a major reason for the capacity fade of the Mg-S batteries over cycling,21 we adopted the strategy by using concentrated electrolyte to suppress the solubility and diffusion of the polysulfide.34,45 It is supposed that the solubility of polysulfides could be reduced by increasing the Mg solute in the electrolyte according to the common ion effect.


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For comparison, the Mg-S cells with 0.3M electrolytes were examined, which in fact underperformed those with 0.4 M electrolyte (Figure S7). Therefore, a 0.4 M electrolyte was mainly employed for the Mg-S batteries in this work. Swagelok-type cells comprised of 0.4 M MgBhfip/DME electrolyte, ACCS cathode, and Mg foil anode were charged and discharged at a current rate of 0.1C (167 mA cm−2) at 25 °C. The galvanostatic discharge/charge profiles for the initial cycles display a relatively flat discharge voltage plateau at about 1.5 V, followed by a slope region until the cut-off voltage of 0.5 V (Figure 3a), indicating a stepwise reaction pathway. In addition, the discharge potential plateau slightly increases after the first cycle and the voltage gap between the charge and discharge narrowed. This is probably caused by the refreshment and activation of the Mg anode surface over cycling, which is in agreement with the results of the reduced impedance and polarization after the initial cycles in the symmetric cells as discussed above. A discharge capacity of about 930 mAh g−1 was delivered while the charge capacity slightly exceeded the discharge capacity. The Coulombic efficiency (calculated by dividing the charge capacity by the discharge capacity) of the first cycle was about 110%. The gradual capacity decline is attributed to the dissolution of magnesium polysulfide (MgSx) and the continuous loss of active material, which was verified by the appearance of an orange-reddish color on the separator. Owing to the relatively higher discharge voltage (~1.5 V in this work vs ~1.1 V in ref.37), the energy density of the Mg-S batteries with 0.4 M Mg[B(hfip)4]2/DME electrolyte was measured to be approximately 1248 Wh kg−1, which is competitive to the reported state-of-the-art Mg-S systems using the Cl containing electrolytes.34,37 Attempted to trap and reactivate the dissolved MgSx, the ACCS cathode was combined with a CNF-coated separator fabricated as reported in ref. 24. These cells exhibited charge/discharge profiles with mitigated voltage hysteresis and delivered a first discharge capacity of about 1000 mAh g−1 and a slow drop in the capacity over cycling (Figure S8). A reversible 13 ACS Paragon Plus Environment


discharge capacity of about 660 mAh g−1 with a Coulombic efficiency close to 100% was retained after 20 cycles by incorporation of the CNF-separator, whereas the standard cell setup with a uncoated separator yielded a reversible capacity of approximately 460 mAh g−1 after 20 cycles and 320 mAh g−1 after 60 cycles, respectively (Figure 3c). It is worth to mention that the extended cycling of the Mg-S cells with CNF-coated separator was obtained by using larger amount of the electrolyte with the Swagelok-type cell (Figure S9). However, the overloaded electrolyte may directly flow to the anode side, which leads to the loss of the beneficial effects of the CNF-coated separator and consequently resulted in relatively low capacity retention over cycling. An optimization of the cell setup and cathode material for improved cycling performance is separately under investigation. Overall, the charge/discharge performance and cycling stability of the Mg-S cells was improved by using the new MgBhfip/DME electrolyte in comparison with the previous studies with bisamide-based electrolyte.20,40 Nevertheless, further innovative strategies are required to achieve stable and high reversible capacity.

(a)

(b)


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(c)

(d)

(e)

Figure 3. (a) Charge/discharge profiles, (b) cycling performance of the ACCS-Mg cell with 0.4 M electrolytes;(c) CVs of ACCS with a three-electrode setup at a scan rate of 0.1 mV s−1; (d) potential change of MgCE and current wave of ACCS versus scan time; (e) Mg stripping/plating in MgBhfip/DME electrolytes with and without MgSx, while the inset shows the photos of the neat 0.4 M electrolyte (1) and the electrolyte with polysulfide solution (2). Furthermore, it is notable that the discharge/charge of the Mg-S cells is associated with a voltage hysteresis of about 0.63 V (Figure 3a), which will result in a relatively lower energy efficiency in potential application. To study its origin, we measured the CV with a threeelectrode cell (PAT-Cell from EL-CELL®) using ACCS as working electrode (WE) and Mg foil as counter and reference electrodes (MgCE and MgRE), respectively. Typical CVs for the first two cycles are presented in Figure 3c (The CVs with extended scans can be found in Figure S10). During the first anodic scan, the main reduction peak of sulfur appeared at 1.22 V, and the oxidation peaks at 1.83 V in the reverse scan. In the subsequent CV cycles, both of 15 ACS Paragon Plus Environment


the cathodic and anodic events occurred at higher potentials, which is consistent with the discharge/charge profiles with slightly increased voltages after the first cycle. The small shoulder signals in the CVs indicate a multi-step reaction in the Mg-S cells. The redox signals at 1.34 and 1.90 V (vs MgRE) in the second cycle corresponds to a voltage hysteresis of about 0.56 V between the reduction and oxidation of sulfur in the Mg-ACCS system. At the same time, the potentials vs MgCE were recorded. As shown in Figure 3d, the representative CV curve for the second cycle indicates that the reduction peak for sulfur is located at a slightly lower potential of 1.27 V compared with the peak value vs MgRE. However, the oxidative signal substantially shifted up to 2.43 V, which implies that the recharge of the sulfur cathode was restricted by the half-reaction at the MgCE. In addition, with a three-electrode PAT-Cell, the potential change of MgCE (vs MgRE) was simultaneously tracked while the oxidizing and reducing potentials were successively applied in the CV measurements. Figure 3d representatively illustrates the second CV cycle, in which the voltage of the MgCE is found to be about 0.1 V when the reductive potential was applied. This is in the range of the polarization values of the pristine electrolyte as discussed before. However, a dramatic potential drop by approximately −0.59 V for the MgCE was observed during the cathodic scan. This phenomenon might imply that a large energy is required to reduce the Mg2+ ions to Mg0 metal at the anode in the Mg-S system, which could be caused by the dissolved MgSx species in the electrolyte. To examine this assumption, we mimicked the electrolyte conditions in the Mg-S system by adding the MgSx diglyme solution to the 0.4 M electrolyte at a volumetric ratio of 1:4. (The synthesis of MgSx is described in Supporting Information). The Mg plating/stripping behavior of the MgSx containing electrolyte was checked at a current density of 0.1 mA cm−2 with the same three-electrode PAT-Cell (Figure 3e). For comparison, the neat 0.4 M electrolyte solution was measured under the same conditions, which resembles the polarization behavior as described above. The potentials for Mg stripping in both the MgSx


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ACS Energy Letters

containing and neat electrolyte were present at nearly 0.1 V in the initial cycles. But the Mg plating process in MgSx containing electrolyte was obviously impeded, suffering from a high potential barrier of > 0.5 V. The voltage gap diminished in the subsequent cycles. However, the negative oxidative potential values deviate from the reasonable Mg stripping potential of > 0 V and the voltage spikes during stripping actually hint at a deterioration of the electrolyte upon extended cycling and/or the occurrence of parasitic reactions of MgSx on the Mg electrodes. In fact, sulfur species were detected in the post-mortem analysis of the Mg electrodes by EDX (Figure S13). These results indicate that the over-potential for the reoxidation of sulfur cathode and the voltage hysteresis during cycling the full Mg-S cell mainly stems from the dissolved MgSx species in the electrolyte, which interfere with the Mg plating. Interestingly, despite polysulfide dissolution, the well-defined charge/discharge voltage plateaus in galvanostatic cycling and the characteristic redox signals in CVs verify the good reversibility of Mg-S battery chemistry in MgBhfip electrolyte. It is known that the sulfur cathode chemistry can be influenced by the employed electrolyte. Hence, the cathode reactions of the Mg-S system with Mg[B(hfip)4]2 electrolyte were investigated by analyzing the cathodes at different battery states by means of X-ray photoelectron spectroscopy (XPS). The samples were prepared by discharging and charging the ACCS cathodes to the respective voltages according to the discharge/charge profiles presented above. The S 2p spectra for the as-prepared ACCS composite display the characteristic spin-orbit-splitting doublet for elemental sulfur with the S 2p3/2 and S 2p1/2 peaks at 164.0 and 165.2 eV, respectively (Figure 4). With the cathode discharged to 1.0 V, the S 2p signal became broader and could be deconvoluted into three sets of peaks. The S 2p3/2 peak at 161.4 eV is assigned to MgS, that at 162.7 eV to S-S bonds of MgSx (2≤x

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