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Metal−Sulfur Batteries: Overview and Research Methods Michael Salama,† Rosy,† Ran Attias, Reut Yemini, Yosef Gofer, Doron Aurbach, and Malachi Noked*

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Department of Chemistry, Bar-Ilan University, Ramat-Gan 5290002, Israel ABSTRACT: Rechargeable metal−sulfur batteries (RMSBs) represent one of the most attractive electrochemical systems in terms of energy density and cost. In most of the proposed systems, the anode side is metallic and the cathode side is elemental sulfur impregnated in a porous matrix. Despite the relatively low voltage of these systems, they attract a lot of attention and are considered to be very promising as nextgeneration batteries for the following reasons: (1) utilization of active metal anodes enables a leap in specific energy due to the high capacity of metal anodes in comparison to intercalation compounds, (2) sulfur as a cathode exhibits high theoretical specific capacity (1675 mAh/g), and (3) system components make RMSBs low-cost, less toxic batteries. Nevertheless, the high reactivity of metallic anodes (e.g., Li, Na, Mg, and Al) and the solubility of sulfur species in the electrolyte render these batteries unstable and hinder their practical realization. In this Perspective, we focus on rechargeable sulfur batteries with active metal anodes, present important studies conducted in this field, and summarize the reported methods and techniques that are mandatory for effective and practical studies of RMSB.

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metal−sulfur batteries (RMSBs) may be very promising in terms of energy density, abundance of elements, and costs. However, even after decades of research, they are still falling short in terms of cycle life, round trip efficiency, and reliability. In the current Perspective, we review the core challenges and reported strategies associated with the realization of practical RMSBs. We begin with metallic anodes, continue with sulfur cathodes, and finalize with full RMSB systems. We further conclude the discussion with a detailed perspective and clear suggestions regarding experimental conditions and needed standards for reports on RMSBs. We strongly believe that the batteries’ R&D community made great breakthroughs in the last decades; however, we should be very careful with publishing statements regarding RMSB capacity, reversibility, longevity, and cost in the context of real demands and practical use. Metallic Anodes. Motivation and Challenges for Their Utilization. The practical capacity limit of the commonly used graphite anodes in LIBs is around 300 mAh/g. This specific capacity is much lower than the practical specific capacity of sulfur cathodes, which can exceed 1000 mAh/g. Hence, coupling graphite anodes and sulfur cathodes in batteries means a pronounced limitation in energy density due to the

ithium-ion batteries (LIBs) are vital components in our daily life, powering portable and medical devices, energy storage units, and electric vehicles (EVs). However, despite the huge success of these electrochemical energy storage systems, there is a strong incentive to develop batteries with higher energy and power densities and lower cost in order to fulfill the current energy storage and conversion needs of our society.

Rechargeable metal−sulfur batteries (RMSBs) can prove to be very promising in terms of energy density, abundance of elements, and costs. Effective, commercially viable electrochemical systems that can practically compete with the currently used LIBs need to meet several core demands. They need to be cheaper than LIBs in terms of $/Wh, store and deliver a higher amount of energy at higher power, demonstrate comparable cycle life at virtually 100% Coulombic efficiency, and contain as much as possible nontoxic components. Another factor that should be considered is material abundance, meaning that, while some systems may show promising results in the lab, up-scaling these systems to widely used commercial products may be limited by the availability of the raw materials in the earth’s crust. Rechargeable © XXXX American Chemical Society

Received: November 15, 2018 Accepted: January 7, 2019 Published: January 7, 2019 436

DOI: 10.1021/acsenergylett.8b02212 ACS Energy Lett. 2019, 4, 436−446

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Cite This: ACS Energy Lett. 2019, 4, 436−446

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SEIs formed on active metals, which changes their morphology upon cycling, crack under the mechanical strains developed at the electrode/electrolyte interface due to the huge volumetric changes during the repeated deposition/dissolution of the metal. Formation of cracks in the protective surface films generates new metallic/electrolyte interfaces, and hence, the electrolyte solution is further reduced to form new SEIs. This fracture and repair mechanism induces uneven metal deposition that eventually leads to the formation of dendrites and consumption of a large amount of solution components in side/parasitic reactions. Coating the metallic anode before being in contact with the electrolyte medium was demonstrated as an effective approach in suppressing dendrites formation and maintaining long-term stability of metal anodes. Kozen et al.22 and Kazyak et al.28 showed that an extremely thin layer deposited directly on metallic lithium anodes can suppress their tendency to form dendrites. Similar success was demonstrated later by Lou et al.4 with sodium metal anodes as well. Zheng et al.29 showed that application of thin films comprising carbon nanospheres directly on lithium metal anodes surface facilitates their long-term stability and suppresses dendrite formation. Archer and co-workers30 showed that coating by multifunctional artificial SEIs is extremely effective in extending the cycle life of Na and Li anodes. It became known and well demonstrated that producing protective layers that prevent reactions between active metal anodes and electrolyte solutions extend their durability remarkably. However, the effectiveness and practicality of this approach should be very carefully examined and should be tested under conditions that are relevant to practical batteries in terms of loading, the relative amount of electrolyte solutions, and rates. We further discuss these conditions in this Perspective regarding the experimental setup and Coulombic efficiency measurements. Additives. It was demonstrated that active additives in standard electrolyte solutions can be used to form stable/ modified SEI layers that suppress parasitic reactions on active metals.31 The main advantage of this approach relies on the fact that by using a high enough amount of additives in solutions we can develop self-healing properties of the protective surface films, unlike the previously mentioned coating strategy. This advantage, though useful to extend battery life in laboratory systems, becomes a bottleneck when practical batteries are assembled. Parasitic reactions, even with self-healing nature, might consume too many active ions from the metallic anode, forcing the use of excess electrode material that feeds these ongoing parasitic reactions. Two of the most used additives are lithium oxalyldifluoroborate (LiODFB), which promotes the formation of highly stable surface films containing LiF,32,33 and LiNO3 in 1,3-dioxolane solution, which promotes the formation of a stable interphase that suppresses undesirable surface reactions on the anode side.34,35 Zhang et al.33 utilized LiPF6 as an additive in LiTFSI-LiBOB electrolyte solutions and showed significant stability of lithium metal anodes. A comprehensive review of additives that facilitate operation of more stable metallic lithium anodes was recently reported by Dingchang Lin et al.2 Solid Electrolytes (SEs). SE has a huge advantage over the more traditionally used liquid electrolyte solutions in enabling operation of reversible active metal electrodes.36 Generally, welldesigned SEs may possess high chemical, thermal, and mechanical stability. Moreover, these types of electrolytes act as the separator in the cells. Therefore, the realization of such electrolytes is of great interest. Coupling active metal anodes

relatively low specific capacity of the anode side and the need to develop an effective prelithiation process for graphite anodes. Furthermore, use of a lithiated graphite anode will result in a voltage penalty that will reduce the energy density of the full cell configuration.1 These limitations boost the community to couple the sulfur cathodes with anodes possessing much higher specific capacity compared to that of the carbonaceous materials. Among a few possible candidates, metallic anodes are very attractive due to their high theoretical capacity [Li (3860 mAh/g),2 Mg (2205 mAh/g),3 Na (1166 mAh/g),4,5 Al (2979 mAh/g)6] and relatively low redox potentials. In the next section, we discuss relevant metallic electrodes and focus on the challenges associated with their realization as rechargeable anodes. We further describe strategies reported for every anode system, which facilitates their reversible electrochemical reactions. The context of this section is anodes for rechargeable M−S batteries; hence, we do not discuss here reports on rechargeable batteries based on metal anodes, which conjugate them with cathodes other than sulfur (e.g., intercalation compounds or oxygen). Excellent review articles on rechargeable batteries based on metal anodes coupled with cathodes other than sulfur are available in the literature.7−16 Additionally, this Perspective briefly discusses the Li anodes and Li−S systems because this field is crowded, widely explored, and covered by many review articles.17−21 Nevertheless, because Li electrodes are the most studied anodes in aprotic solutions with significant advancement and reported strategies for their practical utilization in rechargeable batteries, we mention some approaches and features of Li metal anodes that might become relevant for other metal anodes and can serve as guidelines for their use in RMSBs. Monovalent Anodes. Lithium and sodium metals are probably the most reported anodes for the next-generation RMSBs.2,4,22−24 The reasons behind their choice are (1) the availability of diverse optional cathode materials (e.g., reversible intercalation hosting cathodes and potentially promising conversion cathodes), (2) the nature of the surface films formed on their interface, which behave like a solid−electrolyte interphase (SEI): permeable to the relevant ion but blocking electron transport from the active metal to the electrolyte medium, and (3) the commercial success of LIBs promoted further intensive studies of many kinds of Li and Na battery models. Unfortunately, many challenges are aroused in the route for the realization of reversible metallic anodes in rechargeable batteries. Issues related to the high reactivity of the metal/ electrolyte medium interphase, the tendency of the metal to form dendritic morphology upon deposition, and the virtually infinite volumetric changes associated with periodic metal deposition/stripping, which may avoid any possible stabilization of metal anodes during prolonged cycling. In order to suppress the reactivity of Li and Na anodes toward the relevant electrolyte solutions, several strategies were adopted: (1) coating the active metal with protecting surface films before contact with the electrolyte medium,4,22,23,25,26 (2) the use of active additives in the electrolyte solutions that react predominantly to form a desirable SEI, and (3) the use of solid electrolytes (SEs).27 Coatings. Monovalent metals, namely, Na and Li, are highly reactive; their low redox potential ensures that most of the electrolyte species in contact with the metal should be reduced. These spontaneous reductions form surface layers that are ionconductive and electrical insulators (namely, SEIs). However, 437

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multivalent metal anodes in high-capacity rechargeable batteries should be relatively easy vs either intercalation or conversion reaction cathodes. In fact, full rechargeable Mg battery prototypes based on Mg metal anodes were reported almost 20 years ago,45 and their cycle life was comparable to that of LIBs. The remaining issue is to find cathodes with redox activity within the electrochemical window of the relevant electrolyte solutions and with an attractive specific capacity that will enable elaborate high-energy-density batteries. Sulfur cathodes possess high capacity with reversible electrochemical activity in low enough voltage regimes; therefore, their conjugation with magnesium anodes is particularly attractive and interesting. Aluminum. Aluminum has a higher volumetric capacity than any of the other proposed anode materials (8040 mAh/cm3). Therefore, it represents a very attractive choice as an anode for secondary batteries in general. The main challenge in utilizing aluminum as an anode material comes from its tendency to form highly stable passivation layers that may lead to an electrochemical inertness. Recent studies revealed that by using various types of ionic liquids aluminum can be reversibly electrodeposited.55−57 Nevertheless, the high charge density of Al3+ ions practically prevents the availability of any known Al ion intercalation cathode material. It seems therefore that only conversion reaction-type cathodes such as sulfur represent a promising approach. Sulf ur Cathodes. Motivation for Utilizing Sulfur as a Cathode Material in Rechargeable Batteries. Sulfur can serve as a conversion cathode material, meaning that elemental sulfur can be reduced electrochemically to produce metal sulfide with the lowest oxidation state of sulfur (namely, with high specific capacity).58,59 Unlike the intercalation compounds, sulfur does not have active intercalation sites; all of the bulk elemental sulfur may react electrochemically during discharge.60,61 With this reduction mechanism, despite the low electrochemical potential (0.407 V vs SHE), sulfur is a very promising electrode material for energy storage and conversion devices because it exhibits extremely high theoretical specific capacity (1675 mAh/g).62−64 Furthermore, the conversion mechanism makes sulfur a universal cathode material that can be conjugated with any metal anode, at least in theory. Indeed, it was found that the formation of metal sulfide is not cation-specific;62 therefore, a wide range of metals can function as the anode material in sulfurbased batteries.65−67 It is important to note that the resulting metal sulfide density drastically changes during cycling (ca. 79% upon Li uptake, for example);68 these volumetric changes cause the system to lose contact with the current collector and effectively remove some of the cathode material from the cell. Furthermore, volumetric changes represent a serious safety concern for the commercial production of sulfur-based batteries. The main solvents used in sulfur-based electrochemical systems are ethers.69,70 The rationale behind this choice is that ethers can act as relatively good solvents for metal polysulfide species, which are formed by sulfur reduction as intermediate products. Thus, high utilization of sulfur cathodes is possible in ethereal electrolyte solutions. However, the high solubility of polysulfides in the electrolyte solutions is a major problem in sulfur-based batteries. The dissolved polysulfides diffuse to the anode side and are continuously reduced therein. This situation, the so-called “shuttle mechanism”, causes capacity loss on the cathode side because a continuous electron transfer from active metal anodes in these systems to the polysulfide moieties in solution avoids their reoxidation to elemental sulfur in the charging process. In addition, reactions of polysulfide moieties at

with SEs may circumvent many of the challenges that plague their practical use. SE is expected to block any unwanted contaminants from reaching the anode side due to the fact that the transference number of the SE is expected to be close to 1 for the active metal cations. Two types of SEs are currently examined: polymer electrolytes based on PEO,37 PEGDME,38 and PVDF-HFP (gel type SE)38 and inorganic SEs such as sulfide glass Li2S−P2S5,39 β-alumina,40 and NASICON.41 Utilization of SEs in M−S batteries is challenging due to a different mechanism of sulfur reduction, compared to the reduction of sulfur in organic solvents. This point is discussed in more detail later in this article. In addition to Li and Na−S batteries, recent studies tested the feasibility of K−S battery systems.42,43 Potassium has a low redox potential (−2.93 V), is highly abundant in the earth’s crust, and has a reasonable theoretical capacity (685 mAh/g); however, the high reactivity of the metal practically reduces chances for realization of metal-based K−S secondary batteries. Multivalent Anodes. The sluggish kinetics of multivalent ions in SEIs and SEs comprising any polymers or ionic compounds renders the solutions and strategies proposed for lithium and sodium (metal) batteries irrelevant for multivalent metal anodes. It was shown that SEs on the surface of multivalent metallic anodes form real passivation layers that block ionic transport.44,45 In any working electrochemical device based on multivalent metal anodes, the surface of the anode should be passivation-free. Even small amounts of active contaminants can produce a compact passivation film that renders the anode useless due to its insulating properties (both ionic and electric) and consequently results in huge impedance. Fortunately,

Fortunately, careful design of appropriate electrolyte solutions can enable reversible operation of multivalent metal anodes in passivation-free conditions. careful design of appropriate electrolyte solutions can enable reversible operation of multivalent metal anodes in passivationfree conditions. Additionally, it was recently reported by Canepa et al.46 that, at least in theory, a special SE based on ternary spinel chalcogenides should have high enough Mg ion mobility and might serve as a SE. Magnesium. There are a number of review articles that cover the success in R&D of electrolyte solutions for rechargeable Mg batteries.47−49 In general, all of the reported systems are etherbased electrolyte solutions with complex structure.44,50,51 There are three main types of electrolyte solutions: those containing organometallic complexes, solutions containing a conventional Mg salt, and solutions containing boron-based electrolytes.52−54 All of these electrolyte solutions enable reversible magnesium deposition/dissolution processes in them with low overpotentials and high Coulombic efficiencies. All of the electrolyte solutions mentioned above rely on a passivation-free situation thanks to the use of ethereal solvents, which are intrinsically stable in contact with Mg metal. Unlike the cases of monovalent metal anodes, magnesium deposition is nondendritic. This fact together with the availability of several types of solutions in which Mg anodes are passivation-free (thus behaving reversibly) forced the community to believe that the implementation of 438

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Figure 1. (a) Discharge and charge of Mg−S at 50 and 25 μA, respectively. (b) XPS spectra of S 2p spectra of the cathode as-prepared (I), after first discharge (II), and after first charge (III) are compared with standard samples of S powder (top) and MgS powder (bottom).101 Charge− discharge curve of an S/CMK400PEG composite with (c) PVDF binder and (d) CMC binder in the electrolyte in diglyme (gray), tetraglyme (red), diglyme/PP14TFSI (blue), and tetraglyme/PP14TFSI(green).66 (e,f) Working mechanism of the Mg−S battery with LiTFSI additive.102 Copied with permission from Nature101 and with permission from Wiley.66

Rechargeable Metal−Sulf ur Battery (RMSB). Challenges and Mechanism. In the previous sections, we briefly summarized the advantage of metallic anodes and sulfur cathodes for batteries. The high specific capacity of both types of electrodes motivated the community to realize full battery prototypes based on them. In order to utilize the full potential of sulfur-based batteries, the anode side must be metallic. So far, studies have focused on the most viable options for the anode side, namely, Li,64 Na,65 Mg,81 and Al.82 Monovalent Systems. The most studied sulfur−metal systems are Li−S batteries.61,64,68−71 The theoretical energy density of a Li−S rechargeable battery is 2600 Wh/kg. Several methodologies were employed in order to reduce or nullify the polysulfide’s dissolution problem and to address the extremely low conductivity of sulfur.83 The basic concept is to produce a barrier that should retard the polysulfide dissolution while retaining the permeability of the electrolyte solution (or at least the Li ions), thus maintaining the lithium−sulfur electrochemical reaction. Several interesting attempts to demonstrate Li−S cells with coated sulfur cathodes were presented. The coatings included polymeric species,66,84 ceramic membranes,40,65,85 modified graphene oxide sheets,60 and other carbonaceous materials.86 One of the most important approaches to minimize the negative shuttle effect is to

the anode side corrode the active metal and form dead sites. These situations lead to low Coulombic efficiency of metal− sulfur cells. The most attractive ethereal solvents for M−S batteries are 1,2-dimethoxyethane (DME), and tetra(ethylene glycol)dimethyl ether (TEGDME) due to their ability to dissolve polysulfides and due to their relatively high cathodic stability. Rigorous studies were conducted in order to mitigate the shuttle effect while maintaining the benefits of using ethereal solvents.71,72 A main course of action is to encapsulate the elemental sulfur in conductive carbonaceous material, thus keeping the dissolved polysulfide as a semiliquid cathode material, locked in a position that prevents the cathode from dissolving into the solution itself. Especially, the reports on quasi-solid-state behavior of sulfur encapsulated in carbon by SEIs are interesting.73,74 There are also reports on the use of sticky surfaces for polysulfide species,75 selective separators,61,76 and organosulfur polymers77−80 for M−S battery prototypes. Despite major progress demonstrated in recent years, the complexity of sulfur-based electrochemical power sources remains a challenge regardless of the anode materials. We review below several interesting and important attempts to conjugate sulfur cathodes with metallic anodes to form full rechargeable RMSBs. 439

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Figure 2. (a) CV cycles of conditioned 0.25 M MgTFSI2 with 0.5 M MgCl2 in DME solution. Pt served as the working electrode and Mg as the reference electrode and counter electrode at a scan rate of 25 mV/s. (b) CV cycles of conditioned 0.25 M MgTFSI2 with 0.5 M MgCl2 in DME solution after discharging a sulfur cathode at the same cell and solution. Pt served as the working electrode and Mg as the reference electrode and counter electrode at a scan rate of 25 mV/s. (c) Efficiencies of repeated CV cycles of conditioned 0.25 M MgTFSI2 with 0.5 M MgCl2 in DME solution. (d) Efficiencies of CV cycles of conditioned 0.25 M MgTFSI2 with 0.5 M MgCl2 in DME solution after discharging a sulfur cathode at the same cell and solution.44 (e) Potential of the Mg anode during cycling in a three-electrode cell with a sulfur cathode, with Mg as the reference and Mg foil as the anode.103 Copied with permission from Wiley.103

areal loading of sulfur yet the minimal shuttle effect and reasonable cycle life.88−91 The S−C encapsulation concept was later adapted to Na−S systems.40 All of the above-described approaches demonstrated improvement in sulfur utilization and in the stability of the sulfur cathodes. However, most of them only partially eliminated the problem of polysulfide species dissolution and their negative effect on the anode side. In turn, the work of Markevich et al.74 with quasi-solid-state sulfur/C cathodes demonstrated full elimination of these problems but

encapsulate sulfur into porous carbon matrices, as was demonstrated first by the pioneering work of Nazar and coworkers in 2009.87 This idea was further developed by Elazari et al., who demonstrated monolithic C−S cathodes.86 However, the main problem with cathodes based on sulfur encapsulated into carbon is the too high amount of carbon in the composite cathodes, which limits the total specific capacity of these cathodes. Consequently, other concepts were developed and realized. Recently, some important work emphasized the high 440

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Figure 3. Galvanostatic discharge curves of an Al−S battery (a) at a current density of 30 mA/g with various molar ratios of EMICl:AlCl3 ionic liquid electrolytes and (b) at various current densities with a 1:1.5 EMICl:AlCl3 molar ratio; (c) discharge−charge curves at 30 mA/g with a 1:1.5 molar ratio EMICl:AlCl3 ionic liquid; (d) respective discharge and charge capacities for galvanostatic cycling.55 Copied with permission from Elsevier.

(∼300 °C) are their great disadvantage. To date, no commercial room-temperature Na−S batteries exist. It is becoming clear that the challenges for developing room-temperature Na−S batteries are quite similar to those for the Li−S systems, with even further complications due to the higher reactivity of sodium metal anodes. Multivalent Systems. Rechargeable magnesium batteries (RMBs) are the subject of rigorous studies by many prominent research groups, driven by the high abundance, low cost, low toxicity, and relatively good safety features of Mg metal and its compounds.50,51,95−97 In fact, magnesium−sulfur systems are among the most cost-effective batteries in terms of theoretical Wh/$. The theoretical volumetric energy density of Mg−S batteries is 3200 Wh/L.98 Therefore, vast resources are invested in the realization of magnesium−sulfur batteries.66,99,100 It is clear that development of rechargeable magnesium−sulfur batteries should be much harder compared to that of Li−S systems because, in contrast to the SEI model through which Li metal anodes function, magnesium metal anodes are expected to be fully passivated by reduction of polysulfide species on their surface. The first magnesium−sulfur system was demonstrated by Kim et al. in 2011;101 this pioneering work showed that magnesium anodes could be successfully coupled with sulfur

needs to demonstrate similar results with practical loading. Recent studies showed that organosulfur polymers exhibit high sulfur loading and increased anodic stability.92 The basic concept is to fix the sulfur to a conductive framework in order to inhibit polysulfide dissolution while mitigating sulfur’s low conductivity.77−80 Furthermore, 3D electrode architectures have demonstrated increased cycle life and high loading.88,93 The basic concept is that, by providing a conductive framework in which the electrochemical process occurs, we limit the possibility of parasitic reaction and at the same time circumvent the issues arising from the insulating nature of sulfur. On top of all of the work and achievements reported so far, the limits associated with the active metal anodes (Li, Na), namely, their high reactivity and their limited Coulombic efficiency need to be mitigated. To date, the only mass-produced practical sulfur-based batteries are sodium−sulfur batteries. Commercial Na−S batteries are used as large stationary batteries, mainly for load leveling, peak shaving, and energy harvesting.40,85,94 These systems, working only at high temperatures, include molten sodium metal anode, molten sulfur cathode, and a β-Al2O3 ceramic SE that serves also as the separator. Their discharge voltage is around 1.7 V, and their energy density is around 760 Wh/kg. The high operating temperatures of these systems 441

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moieties in a selected solution and then with a sulfur electrode that releases polysulfide species during its discharge process (sulfur reduction). For example, an excellent sulfur cathode that cannot be coupled with electrolyte solutions in which the active metal behaves reversibly is irrelevant for a practical battery system based on that metal. Hence, when working on these complex anode/electrolyte solution/cathode systems, separate compatibility tests for each pair of components should always be carried out before measuring full cells. The electrochemical performance of anodes should be determined by their Coulombic efficiency, overpotential, dendrite formation, and rate capability. Providing an accurate performance report on metallic anodes requires special attention to parameters that could be easily overlooked. Coulombic efficiency represents the sum of all reactions taking place in the system. Therefore, providing an accurate value for the ratio between charges involved in the metal deposition and in its dissolution at each individual cycle is of utmost importance. Cyclic voltammetry (CV) might be an inadequate measurement technique for such systems due to slow kinetics of some electrochemical processes that can distort the voltammetric response due to the effect of the so-called IR drop. Galvanostatic measurements are therefore more reliable for properly determining cycling efficiency than the use of repeated CV. In practice, by cycling a known amount of charge in a galvanostatic mode, the assessment of electrode stability by comparing their periodic voltage profiles is much more accurate than that by measuring them in voltammetric mode. Proper experiments that aim to test reversibility of metal deposition processes should be conducted by depositing a known amount of material on an inert electrode (which does not alloy with the deposited active metal) and then dissolve/deposit repeatedly a fraction of the initially deposited active metal (e.g., 10−30% of it), as depicted in Figure 4.44 After a certain number of cycles, the remaining deposited active metal should be fully dissolved electrochemically. The

cathodes. The core message derived from this study was that the magnesium salt solution must be “non-nucleophilic” (a definition by the authors, Figure 1a,b). Fichtner et al.66 further studied the electrolyte solution effect on the overall electrochemical performance of this system; they found that reacting (HMDS)2Mg with AlCl3 and then further reacting the resulting electrolyte with MgCl2 yielded an electrolyte solution suitable for Mg−S batteries (Figure 1c,d). Gao et al.102 showed that certain detrimental processes on the cathode side can be suppressed by adding LiTFSI as an additive (Figure 1e,f). Recently, the Fichtner group published more advanced M−S systems, with great progress on the cathode side.66,81 Unfortunately, although a magnesiated sulfur cathode could be demonstrated, the electrochemical response of the Mg metal anode side in these systems remained mostly unexplored. In summary, several studies showed that magnesium polysulfide species are soluble in ethereal solvents.62 Therefore, sulfur as a cathode can show reversible behavior in some ethereal Mg salt solutions. Nevertheless, the importance and feasibility of Mg−S batteries is still a subject of debate. Some studies claim that the solutions in some sulfur−magnesium cells react to form SEIs on the Mg metal through which magnesium ions can migrate freely in and out of the anode,103 as seen in Figure 2e, while others claim that it is highly unlikely for magnesium ions to migrate through any surface film that can be formed on Mg metal surfaces (Figure 2a−d).44 Aluminum−sulfur electrochemical rechargeable batteries are still in a very early R&D stage; to date, only a few studies have been conducted in this field.55,82,104,67 The current aluminum− sulfur electrochemical systems comprise aluminum metal anodes, ionic liquid electrolyte solution, and sulfur composite cathodes. This system usually operates at less than 1.2 V upon discharge, as depicted in Figure 3.55 However, the potentially high capacity of these systems, if they will be proven to work, is supposed to compensate well for their low discharge voltage. Measurement Techniques for Metal Anodes for Rechargeable Batteries. Appealing and promising active metal−sulfur battery systems have pulled many research groups from multidisciplinary fields into the batteries R&D community. A major advancement was achieved due to these joint research efforts. New skills in materials synthesis, electrolyte additives, and analytical techniques contributed a lot to our current understanding of these complicated systems. RMSBs could potentially be the dominating electrochemical system in the future; however, the scientific community needs to bear in mind that every application will require addressing different challenges. However, current industry goals for batteries R&D are 350 Wh/ kg, 750 Wh/L, and 1000 cycles that could be translated into 10 calendar years. At the current point of time, there is a need to formulate guidelines for experimental conditions that will be commonly used throughout the community to explore and develop rechargeable batteries based on active metal anodes in order to keep all reports under similar experimental conditions and under the right perspective and context of practical rechargeable battery systems. In this section, we suggest some guidelines, which need to be addressed/reported whenever new solutions are offered for RMSBs. We believe that the electrochemical performance of any suggested RMSB should be assessed first for each cell component individually, prior to the assembly of full cells, thus testing the performance of multiparameter complicated electrochemical systems. Special attention needs to be given to the compatibility of the active metal anode with polysulfide

Figure 4. Macroreversibility measurements for a conditioned 0.25 M MgTFSI2 + 0.5 M MgCl2/DME solution. Magnesium corresponding to 0.5 Coulombs/cm2 was deposited on a clean Pt electrode. This Mg (on Pt) electrode was cycled galvanostatically 50 times at 10% depth of discharge at 1 mA/cm2. Mg foils served as a counter electrode and a reference electrode. The last process was galvanostatic oxidation (dissolution of the remaining active metal), at the same current density, with a cutoff voltage of 2.0 V, corresponding to a Mg-metal-free Pt electrode. 442

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reversibility of the system should be calculated by dividing the difference between the initial charge (of the first deposition step) and remaining charge (measured by the final deposition step) by the number of cycles. The results are in fact the average loss of charge per cycle (due to irreversible reactions in the system). The average cycling efficiency is thus determined by comparing the charge per cycle to the average loss of charge per cycle. The proposed experimental and measuring protocol for analyzing a metallic anode can be summarized in four simple steps:

Guidelines for experimental conditions to establish reproducible and comparable protocols for evaluating the reversibility of metallic anodes in RMSBs are proposed. side similar to what was demonstrated by Markevich et al. for lithium−sulfur systems).

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• Deposition of a known amount of metal • Cycling 10−30% of the predeposited metal • Electrochemically oxidizing the remaining predeposited metal • Reversibility of the system calculated by dividing the difference between the initial charge (used for deposition) and remaining charge (used for dissolution) This approach should be used once without the presence of any polysulfide to provide a solid baseline and then compared to a polysulfide-containing system. A recent work by Adams et al. provides a detailed experimental procedure for measuring Li metal deposition/dissolution reversibility.105 When working with any sulfur cathode, polysulfide moiety dissolution provides a special challenge. Testing the effect of these polysulfide species in solution on the cycling efficiency of the active metal anode should indicate the level of practicality of the system under study. Therefore, the above-mentioned procedure for a cycling efficiency test should be reported in both the presence and absence of polysulfide species in the selected electrolyte solution. The stability of the electrolyte solution in the presence of sulfur and metal polysulfide species is an important aspect when dealing with multivalent systems, where the complex structure of the electrolyte solution may play a key role in reversibility of the active metal in solution and in the intrinsic solution’s parameters, such as ionic conductivity. Hence, close attention should be given to the long-term stability of the electrolyte solution in the presence of sulfur species. Summary and Future Outlook. In this Perspective, we briefly reviewed the challenges associated with the realization of RMSBs and major advances and strategies to overcome these challenges. Despite massive research efforts and significant progress related to the reversibility of sulfur cathodes, there are still major obstacles to overcome before the realization of practical room-temperature RMSBs. Reports on full RMSB systems need to take into account the stability and reversibility of every cell component (cathode, electrolyte, and anode) under the electrochemical constraints/conditions in the full cell. Careful attention should be given to the reversibility of the metallic anodes in the presence of dissolved polysulfide solutions, especially when multivalent metal is used as the anode. We propose in this Perspective guidelines for experimental conditions that will help the community establish reproducible and comparable protocols for evaluating the reversibility of metallic anodes in RMSBs. Future research efforts in the field of RMSBs with multivalent metal anodes need to find pathways to overcome the undesirable formation of passivation by surface films on the anodes. These directions might relay on surface protection with SEs similar to that reported by Canepa et al. or by finding ways to completely inhibit the dissolution of sulfur species to RMSB electrolyte solutions (e.g., by quasi-solid-state reduction on the cathode

AUTHOR INFORMATION

ORCID

Michael Salama: 0000-0002-3495-2822 Rosy: 0000-0001-5985-8587 Ran Attias: 0000-0003-0528-7664 Doron Aurbach: 0000-0001-8047-9020 Malachi Noked: 0000-0001-8995-0632 Author Contributions †

M.S. and Rosy contributed equally.

Notes

The authors declare no competing financial interest. Biographies Michael Salama is a Ph.D. student at Bar-Ilan University (BIU). He received his M.Sc. degree in chemistry in 2015 in the field of magnesium electrochemistry and Mg rechargeable batteries. The focus of his research is the electrolyte/anode interface in magnesium batteries. Rosy is a Post-Doctoral researcher in the Noked group under the PBC fellowship program for outstanding postdoctoral researchers from India and China. She received her Ph.D. in electrochemistry in 2017 from Indian Institute of Technology, Roorkee. Her research includes alkali metal batteries, electrode protection, and online electrochemical mass spectrometry studies. Ran Attias is a Ph.D. student at BIU. He received his B.Sc. degree in biotechnology engineering in 2015 from Ben-Gurion University and his M.Sc. degree in chemistry in 2017 from BIU. The focus of his research is solid state and nonaqueous magnesium electrochemistry. Reut Yemini is a Ph.D. candidate in the Noked lab. Her main research area is Li−S batteries, where she is trying to overcome polysulfide shuttling in the electrolyte using atomic layer deposition of various coating layers to block sulfur in porous carbon. Yosef Gofer is a senior researcher and surface analysis expert at BIU, leading the rechargeable magnesium research project and managing the X-ray photoelectron spectroscopy facility. His research interest revolves around the electrochemistry of lithium and magnesium in polar aprotic media and development of spectroscopic methods (ex situ) for sensitive electrochemical systems. Doron Aurbach is a full professor in the chemistry department, BIU, Israel. He leads the electrochemistry group, developing rechargeable high-energy-density batteries for electromobility, electrochemical devices for large energy storage, water desalination technologies, electrocoloring processes, carbon engineering, and electronically conducting polymers. Malachi Noked is currently senior lecturer (Assistant Professor) Alon Fellow in the department of chemistry at Bar Ilan University in Israel. He earned his Ph.D. in chemistry in 2013 and was a Fulbright IlanRamon postdoctoral fellow and research associate at the University of Maryland from 2013 to 2016. His research interest focuses on electrochemistry, energy storage, and functional thin films. 443

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ACKNOWLEDGMENTS Rosy is thankful to the Planning and Budgeting Committee of the Council of High Education for awarding a postdoctoral research fellowship. The authors would like to acknowledge the support of the Planning & Budgeting Committee/ISRAEL Council for Higher Education (CHE) and Fuel Choice Initiative (Prime Minister Office of ISRAEL), within the framework of the “Israel National Research Center for Electrochemical Propulsion (INREP)”.

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DOI: 10.1021/acsenergylett.8b02212 ACS Energy Lett. 2019, 4, 436−446