Anode-Electrolyte Interfaces in Secondary Magnesium Batteries

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Review

Anode-Electrolyte Interfaces in Secondary Magnesium Batteries Ran Attias,1 Michael Salama,1 Baruch Hirsch,1 Yosef Goffer,1 and Doron Aurbach1,*

Secondary magnesium batteries are still in the research stage, after the first prototype of a magnesium-based battery was demonstrated almost two decades ago. Since this breakthrough, despite tremendous efforts by numerous research groups, we are not aware of any system that exhibits better performance in terms of Coulombic efficiency over prolonged cycling. The scientific community is now focusing on the basic phenomena that hinder development of practical magnesium-based rechargeable batteries. Today, we have a better understanding of the structure of electrolyte solutions relevant to rechargeable Mg batteries and its effect on the electrochemical performance. New electrolyte solutions that are not based on organometallic moieties currently surpass the performance of the first generations of complex solutions. There is even an attempt to test alternative anode materials for magnesium-based energy storage systems. In this review, we summarize recent studies conducted in the field, with a focus on the anode/electrolyte solutions side. Introduction Electrochemical energy storage and conversion systems, in particular batteries, have been the subject of intensive investigation due to increasing demand for high energy sources, which can power electric vehicles and most portable electronic devices for prolonged time, and due to increasing awareness of global warming caused by burning fossil fuels.1 A practical battery system for commercial use needs to contain low-cost environmentally friendly components that are easy to handle. In addition, it should possess high energy and power densities and long cycle life over a wide range of temperatures. The intrinsic physical and chemical properties of magnesium––low reduction potential ( 2.37 V versus normal hydrogen electrode), high volumetric capacity of 3,833 mA hr cm 3, abundance, and non-toxicity––make it the ideal anode material. Implanting Mg metal anode in practical systems is not trivial. Magnesium has a strong tendency to form passivating surface films in a wide variety of solvents, salts, and contaminants, which completely block any electrochemical reaction. However, unlike lithium metal systems, magnesium does not form dendrite upon cycling.2 It is believed that the difference in the dendrite formation between lithium and magnesium can be explained because magnesium possesses a low diffusion barrier; therefore, once deposited, the Mg atoms are mobile resulting in smooth surfaces. One of the main obstacles in utilizing magnesium secondary batteries as a commercial system is the high susceptibility of current ‘‘state-of-the-art’’ magnesium electrolytes to contaminants, which even in ppm levels form a blocking layer on the metallic magnesium anode. The tendency of magnesium to form surface films results from its low reduction potential: reactive compounds are reduced on the Mg surface to form

Context & Scale The importance of efficient, lowcost, safe, and easy to fabricate electrochemical devices was never so obvious. From portable electronics to electric vehicles, and even load-level applications, the main obstacle remains how to ‘‘harvest’’ and ‘‘carry’’ the necessary power. Although the current rechargeable battery technologies are very impressive in terms of energy density and durability, they are limited in a number of key aspects, such as abundance, cost, and safety issues. The use of magnesiumbased electrochemical systems represents a huge leap in terms of safety and cost. However, the utilization of the magnesium anode is limited by its tendency to form stable passivation films that block any electrochemical reaction from taking place. Despite its inherent limitations, the feasibility of magnesium electrochemical systems is proven. The main routes in which researchers are tackling the problems are: development of new non-organometallic electrolyte solutions, and the development of new cathode materials that will be compatible with magnesium anode/electrolyte systems. Another approach is to use alternative anode material in order to completely bypass the passivation phenomena.

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electronic, and, more importantly, ionic, insulating surface films. In lithium-based electrochemical systems, the reduction potential is also low and thereby Li metal similarly reduces all the components of polar aprotic electrolyte solution species, forming electronic insulating surface films. However, surface films formed on lithium metal anodes, which comprise ionic Li compounds, conduct Li ions under electrical field, thus behaving as a solid-electrolyte interphase (SEI) on the active electrode.3 The electronic insulating nature of the surface films that form naturally on active metals prevents further reduction of electrolyte components. Hence, formation of passivating surface films on metallic magnesium anodes needs to be prevented. The electrolyte solution must be readily stable with magnesium and not being reduced by it. Commonly used solvents for magnesium batteries’ electrolyte solutions are currently ethereal, such as glymes and tetrahydrofuran (THF),4–14 which are not reduced by metallic magnesium. However, contaminants, such as water traces, CO2, protic residues (from manufacturing), and oxygen, form passivating films on the Mg anode surface and block deposition/dissolution of magnesium.15 Thus, the magnesium salt used as the electrolyte is not less important than the solvent; it must be soluble and stable toward both reduction (anode side) and oxidation (on the cathode side). Therefore, salts such as magnesium perchlorate and magnesium triflate are not suitable. Polar aprotic solvents, which are commonly used in non-aqueous electrochemistry, such as carbonates, lactones, and nitriles, also cannot be coupled with metallic magnesium-based batteries. While they are relatively stable with Mg metal electrodes, ethereal electrolyte solutions suffer from low polarity, which hinders their ability to effectively disassociate magnesium salts.16 This situation tremendously limits the number of viable salts that can be used for magnesium-based electrolytes in ether solvents. Another important challenge in the development of rechargeable magnesium batteries is to find high-voltage/high-capacity cathode materials that fully work in solutions in which Mg metal anodes behave reversibly. Most of the cathode materials that can reversibly store Li ions failed to similarly store Mg ions; the high charge density of the Mg ions due to their divalent character and small radius leads to strong interactions with the host material, which reduces the diffusion rate in host lattices and results in poor electrochemical performance. In addition, high-voltage metal oxide cathode materials cannot be easily integrated with Mg anode-electrolyte solutions compatible systems due to the need for the presence of moieties such as chlorides in the electrolyte solutions, which hinder an easy transfer of Mg ions from the solution phase into the host’s lattice. This review summarizes scientific and practical aspects related to Mg anode-electrolyte solution interphases and presents alternative routes to overcome challenges on the way to develop rechargeable magnesium battery systems. Electrolyte Solutions for Secondary Mg Batteries Assessment Tools A most important parameter reported for reversible Mg deposition behavior in electrolyte solutions is Coulombic efficiency. However, its proper measurement requires several steps that need to be considered, especially when analyzing the reversibility of active metal deposition processes. To actually measure the real Coulombic efficiency of Mg deposition processes using cyclic voltammetry (CV) may be inadequate due to possible distortion of the response by the slow kinetics of electrochemical processes in non-aqueous media (IR drop). A more reliable technique for assessing

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1Department

of Chemistry, Institute of Nano-Technology and Advanced Materials (BINA), Bar-Ilan University, Ramat Gan 5290002, Israel *Correspondence: [email protected] https://doi.org/10.1016/j.joule.2018.10.028

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the cycling efficiency of Mg deposition processes via chrono-potentiomery, namely, constant current (galvanostatic) measurements in which a known amount of charge is exchanged in every cycle. The most appropriate way to measure the reversibility of active metal deposition processes is to deposit electrochemically a certain amount of active metal on inert substrates and then dissolve and re-deposit periodically a fraction of the pre-deposited active metal (e.g., 20%), following the potential profile of the periodic process. After a certain number of cycles the remaining active metal is calculated by dissolving it electrochemically: the difference between the initial and remaining mass. Divided by the number of cycles, the cycling (Coulombic) efficiency and the loss of active metal per cycle are easily calculated. It is important to note that to date only few researchers have tried to provide a viable model for magnesium deposition processes.17,18 Interestingly, it seems that the chloride anions play a crucial role in reversible magnesium electrodeposition.19 Furthermore the complexes formed in magnesium electrolyte solutions result in complicated deposition/dissolution processes. Therefore these types of studies are of great interest. Grignard-Based Electrolyte Solutions The first generation of electrolyte solutions for non-aqueous magnesium batteries were based on Lewis acid/base reaction products of organo-magnesium (R2Mg) with organic halo aluminum compounds (AlCl3-nRn) in ether solvents, mostly THF. If the Lewis base/Lewis acid ratio is 1:2, these reactions yields complex electrolytes with the formal stoichiometry Mg(AlCl4-nRn)2 (R = alkyl or aryl group).20 One of the main driving forces in using these electrolyte solutions in the firstly presented rechargeable Mg battery prototypes was to fulfill the need for magnesium electrolytes that are readily dissolved in ether and allow a reasonably wide electrochemical window, so that Mg insertion cathodes can be coupled. Furthermore, the highly reactive organometallic components of these solutions act as scavengers for contaminants in the solutions, thus preventing formation of passivating films on the Mg metal anode surface. The reactivity of these electrolyte solutions toward moieties, such as trace oxygen, water, and protic species, resembles that of Grignardbased electrolyte solutions. The actual structures of the solutions, properties such as conductivity, anodic stability, and facile Mg electrochemistry, depend on the Lewis acid/base ratio of the reagents, the reaction of which forms the electrolyte species. The best formulation was found to be 1 equivalent of Bu2Mg with 2 equivalents of EtAlCl2 in THF, termed DCC (dichloro-complex) electrolyte solution.20 This first-generation electrolyte solution exhibits excellent electrochemical performance in terms of Coulombic efficiency of Mg deposition/dissolution cycling, reaching 100% efficiency, relatively high ionic conductivity of 1.4 mS cm 1, and an electrochemical window of 2.4 V, as depicted in Figure 1A. Further investigation of the DCC electrolyte solutions revealed an interesting finding:4,22–25 chlorine turned out to be the only relevant inorganic ligand that could be used, as other ligands such as fluorine or CN were found either to shrink the electrochemical window, or to passivate the magnesium surface. The Lewis acid/base reaction involves a trans-metallation step in which chlorine bonds with magnesium and the organic ligand bonds with aluminum. Therefore, the molecular ratio of the Lewis acid/base reaction dictates the formation of different solution species at equilibrium. Moreover, the magnesium maintains a coordination number of 6, while the coordination number of aluminum is 4, which are apparently rigid properties of the core metallic ions, maintained by the ligands (chlorides and organic groups), and by the coordination of the solvent molecules (solvation interactions). Therefore, in THF-based electrolytes, the main Mg solution species are MgCl+, Mg2Cl3+, and MgCl2, the former two being stabilized further by five and six THF molecules,

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Figure 1. Comparative Steady-State Cyclic Voltammograms Related to Several Mg-Based Electrolyte Solutions with THF as the Solvent Pt Working Electrodes, Mg Metal as Reference and Counter Electrodes (A) Comparison among aluminate, Grignard, and borate-based electrolyte solutions at various concentrations. Reprinted with permission from Aurbach et al., 4 copyright 2002. (B) APC electrolyte solutions at different concentrations, emphasizing the effect on anodic stability. The inset presents the entire voltammetric response of the solutions. Reproduced with permission from Pour et al., 21 copyright American Chemical Society.

respectively. Possible b-H elimination reactions limit the electrochemical window of this DCC electrolyte and in fact that of most organometallic electrolyte species in general. The ligand exchange reaction between Mg and Al also impacts the electrochemical window. The high tendency of chlorides to bond with the magnesium core and the preference of the organic ligands to bond with aluminum imply that the electrochemical window increases with increasing Cl:R ligand ratios due to the reduced electron density of Al, which makes the Al-R bond less susceptible to oxidation. Although using Grignard-like electrolyte solutions should be considered impractical, due to expected limited anodic stability, the above-described DCC electrolyte solutions circumvent serious obstacles related to the operation of reversible magnesium anodes. The reductive nature of the organometallic components in this family of electrolytes results in electrolyte solutions with almost no contaminants. The organometallic species act as scavengers and eliminate all the reactive atmospheric contaminants, both in the starting material and those formed during cells operation.21 As mentioned earlier, the b-H elimination has a severe impact on the electrochemical window. Therefore, by removing all b-located hydrogen, the electrochemical window of the system greatly improves. The first electrolyte with no b-located hydrogen was composed of the reaction product of PhMgCl and AlCl3. Ethereal solutions containing this electrolyte, termed APC (all phenyl complex), exhibit high anodic stability (>3.3 V), low overpotential for Mg deposition and dissolution, and, most importantly high Coulombic efficiency for repeated Mg deposition/dissolution cycling (reaching 100%), as seen in Figure 1B.21 The APC solutions can be considered as the best electrolyte solutions which contain organometallic species, for rechargeable Mg batteries applications. An open question, however, is to what extent they can allow the use of high-capacity/high-voltage cathodes for Mg batteries. Organoborate-Based Electrolyte Solutions Magnesium organoborate electrolyte solutions were explored by Gregory et al.;26 as the first non-Grignard magnesium salt electrolyte solutions in which Mg

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Figure 2. Electrochemical Performance of Boron-Based Electrolyte Materials (A) Typical steady-state voltammograms of Pt electrodes in different electrolytes: (A) 0.4 M Mes3BPhMgCl and (B) 0.5 M Mes3B-(PhMgCl)2. The inset is a zoom-in of the 1.5–4-V region. Reprinted with permission from Guo et al., 12 copyright 2012 Royal Society of Chemistry. (B) Selected cyclic voltammograms of 0.75 M MMC/G4 electrolyte on Pt electrode collected within the potential range 0.6–3.0 V versus Mg at 5 mV/s. Inset: cycling efficiencies of Mg deposition and dissolution. Reprinted with permission from Tutusaus et al., 29 copyright 2015, Wiley-VCH, Angew. Chem.

deposition/dissolution processes are reversible. The best solution reported by this team comprised 0.25 M Mg(BBu2Ph2)2 in THF and was deemed impractical, mainly due to a too narrow electrochemical window (<1.9 V versus Mg). Organomagnesium-borate electrolytes were further investigated. Guo et al.12 showed the first functional boron-based electrolyte––the reaction product of triarylborates (BR3) with Grignard reagents (RMgX, R = aryl, alkyl, etc., and X = Cl, Br). This electrolyte exhibits highly reversible magnesium deposition/dissolution (approaching reversibility of 100%) and an electrochemical window >3 V versus Mg in ethereal solutions. However, the solutions comprising these electrolytes unavoidably contain active Grignard reagents and are considered therefore as unpractical. Recently, Du et al. showed a new approach toward synthesis of boron containing electrolytes: tris(hexafluoroisopropyl)borate [B(HFP)3], MgCl2 and Mg powder were reacted in dimethoxyethane (DME).27 The resulting electrolyte solutions could exhibit a Coulombic efficiency around 98%, specific conductivity of 5.58 mS cm 1, and overpotential for Mg deposition around 0.1 V, as seen in Figure 2A. Furthermore, these electrolyte solutions could be coupled with sulfur cathodes. The structure of these solutions was determined. The major electrolyte components were found to be [Mg4Cl6(DME)6]2+ cations and [B(HFP)4]– anions. Interestingly, these cationic species are similar to those formed in MgTFSI2/MgCl2/DME solutions.28 Borohydride-Based Electrolyte Solutions Mg(BH4)2 is a commercially available salt. Due to its strong reductive nature, it acts as scavenger for reducible contaminants in electrolyte solutions, similar to Grignard reagents. Mohtadi et al.30 were the first to report on reversible magnesium deposition/dissolution with Mg(BH4)2/(DME or THF). This system, which was the first halidefree solution in which magnesium deposition is reversible, suffer from relatively low Coulombic efficiencies (<40% for THF, <70% for DME), and thereby were found to be impractical. However, this pioneering paved the way for further development and optimization of borohydride-based electrolyte systems. Later work conducted by Tutusaus et al.29 demonstrated a new approach. Mg(CB11H12)2 boron cluster (magnesium monocarborane, or MMC salt) was synthesized as an electrolyte, which ethereal solutions can exhibit reversible Mg deposition processes, as presented in Figure 2B: high Coulombic efficiency of more than 99% and low overpotential

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Figure 3. Cyclic Voltammetry Measurements of Nitrogen-Containing Electrolytes (A) Cyclic voltammograms and linear sweep voltammetry (inset) of 0.25 M MACC 2:1 solution in DME. The working electrode is Pt, while the counter and reference electrodes are Mg metal. Measurements were obtained at 25 mV s 1 and ambient conditions. Reprinted with permission from Doe et al., 35 copyright 2014 Royal Society of Chemistry. (B–D) Typical cyclic voltammograms with Pt electrodes of 0.5 M THF solutions are shown for different ratios of Mg(HMDS) 2 and MgCl 2 , respectively: (B) 2:1, (C) 1:2, and (D) 1:4. The scan rate was 100 mV/s with Pt working electrode and Mg ribbons as both reference and counter electrodes. Rreprinted with permission from Liao et al., 14 copyright 2012 Royal Society of Chemistry.

(<250 mV) for Mg deposition. Furthermore, Mg(CB11H12)2 could be used as electrolyte in triglyme and tetraglyme solutions, which means a possibility to develop nonvolatile solutions for rechargeable Mg batteries.29–32 Electrolyte Solutions with Nitrogen-Containing Electrolytes Mg-hexamethyldisilazane (Mg-HMDS) could be potentially used as an interesting electrolyte component. Zhao-Karger et al. found that by reacting MgHMDS with aluminum trichloride in THF, a non-reactive Mg salt solution was obtained with an electrolyte structure of) Mg2Cl3$6THF)+ (HMDSAlCl3)-.33 This electrolyte could potentially be utilized for Mg-S batteries due to its intrinsic non-reactive nature.34 It exhibits oxidative stability up to 3.2 V and its ethereal solutions exhibit Coulombic efficiency of ca. 100%, as depicted in Figures 3B and 3C. Magnesium Aluminate Chloride Complex Solutions MgCl2 can interact as a Lewis base in ether solutions with AlCl3, which is considered as a strong Lewis acid. Reaction between MgCl2 and AlCl3 in a 2:1 ratio in THF forms practical electrolyte solutions that contain no organometallic compounds. Possible cations in such solutions are THF-stabilized MgCl+ and Mg2Cl3+ moieties coupled with AlCl4– anions. These electrolyte solutions were termed MACC solutions

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(magnesium aluminum chloride complex). As solutions which do not contain any active species that can scavenge atmospheric contaminants, the MACC solutions require a conditioning step which cleans them from reactive contaminants, the presence of which in solutions leads to unavoidable passivation and de-activation of Mg electrodes. An easy conditioning process includes galvanostatic cycling of these solutions in Mg cells during which high surface area Mg electrodes are formed and react with all active contaminants in the solution and scavenge them. After conditioning, these electrolyte solutions exhibit reversible Mg electrodeposition at nearly 100% reversibility and low overpotential (<250 mV), as demonstrated in Figure 3A. It was suggested that the conditioning process also helps to complete the MgCl2-AlCl3 reactions that form stable MgCl+/AlCl4– electrolyte.36 Electrolyte Solutions Based on Mg(TFSI)2 Mg(TFSI)2 (trifluoro-methane-sulfonimide [TFSI]) is the only commercial ether soluble magnesium salt. Recently, it was shown to disassociate to coordinated Mg+2 cations and uncoordinated TFSI anions in ether solvents. Ethereal Mg(TFSI)2 solutions exhibit poor electrochemical performance in all parameters except conductivity (>3 mS for 0.5 M in DME), as seen in Figure 4A.37,38 Mg(TFSI)2/DME is not a stand-alone electrolyte solution, as it does not enable reversible magnesium deposition. It was demonstrated that this electrolyte is highly susceptible to contamination, as it contains no organometallic components that act as scavengers for the native contaminants in the solution. Moreover, it was shown that even extremely clean MgTFSI2 solutions reach only ca. 70% Coulombic efficiency in repeated Mg deposition/dissolution cycling, for which high over-voltages are required. Although the pure MgTFSI2/glyme electrolyte solutions show poor electrochemical performance, their high anodic stability drove the scientific community to work intensively on these systems.19 The presence of Mg2+ species that are not coupled with anions in the MgTFSI2 solutions enables interference in the coordination of Mg cations in ethereal solvents, and soften their solvation interactions, thus increasing their activity in electro-reduction and deposition processes. It was found that adding MgCl2 to MgTFSI2/DME solutions improves their electrochemical properties, as seen in Figure 4B.38 MgCl2/MgTFSI2/DME solutions can reach columbic efficiency of up to 99%, low overpotential (<200 mV), and uniform and crystalline magnesium deposits (Figure 4D). These electrolyte solutions also need a preliminary conditioning step in order to clean them from impurities, what enables operation of passivation-free Mg electrodes, as shown in Figure 4C.38 A rigorous study of the structure of chlorides containing MgTFSI2 solutions revealed the existence of Mg2Cl22+ and Mg3Cl42+ as solution species, at least in DME-based solutions.28 It is clear that, in DME/MgTFSI2 solutions, the strong complexation between the DME molecules and the Mg cations forms Mg(DME)32+ moieties as major cationic species, the electrochemical reactivity of which is hindered by the strong solvation shell. The role of MgCl2 in these solutions in discussed in the next section. The Effect of Chloride Ions on the Electrochemical Behavior of Ethereal Magnesium Salt Solutions One of the most intriguing and central issues in the field pertains to the effects of chloride anions on solution structures, interfacial chemistry, and the electrochemistry of magnesium in non-aqueous solutions. The importance of this topic cannot be overstated. Chlorides, in the current context, pertain to both complex anions and cations, serving as ligands, and solvating or surface-adsorbed anions.4,25,28,39 As noted before, the first solutions that supported fully reversible magnesium electroplating were Grignard reagents in ethers.26 These RMgX reagents included

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Figure 4. MgTFSI2 -MgCl2 Electrochemical Conditioning Process (A) Typical cyclic voltammetry (CV) of Pt working electrodes (WE) in (A) MgTFSI2 0.5 M in DME solution, scanned at 25 mV/s. The inset shows a CV of a Pt electrode in MgTFSI 2 0.5 M solution in diglyme. (B) MgTFSI 2 0.25 M with MgCl2 0.5 M in DME, scanned at 25 mV/s. Mg foils served as reference and counter electrodes (RE and CE). Addition of MgCl 2 significantly reduces the overpotential for both deposition and dissolution processes. (C) Selected CV cycles during the conditioning procedure of MgTFSI 2 0.25 M with MgCl 2 0.5 M in DME solution, scanned at 1 mV/s. Pt served as WE and Mg as both RE and CE. (D) Typical CV of Pt WE in conditioned MgTFSI 2 0.25 M + MgCl2 0.5 M DME solution, scanned at 25 mV/s, with Mg foils as both RE and CE. Reprinted with permission from Shterenberg et al., 38 copyright 2015.

compounds where X = Cl, Br, and probably also I. To extend the electrochemical stability window of these solutions, as well as to greatly improve their ionic conductivity, Lewis acids, such as RxAlCly and RxBCly, were reacted with stoichiometric ratios of the Grignard reagents to yield a wide spectrum of complex anions and cations holding various proportions of Cl and organic ligands.25 In a nutshell, the ratio between the amounts of Cl ligands and organic ligands determines some of the most important characteristics of their solutions. In general, the higher the Cl/R ratio, the wider is the electrochemical stability window. However, in parallel, the magnesium deposition overpotential is higher, and the Mg deposition and dissolution kinetics is lower. Moreover, higher proportions of Cl/R lead to somewhat lower magnesium deposition cycling efficiency. Solutions were selected for battery testing in a way that optimizes these properties.

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Naturally, the Lewis acid core atom (B, Al, Fe, Sb, etc.) also has great impact on these properties, as well as on the nature of the organic ligands. Later, when solutions based on ‘‘simple’’ anions, such as TFSI and ClO4, showed preliminary attractive properties, it was found that addition of chlorides, mainly by adding MgCl2, greatly improves the electrochemical properties of the solutions. These findings boosted interest in the role of chlorides in solution properties and interfacial processes.40 Much earlier, in the beginning of research and development in rechargeable Mg batteries, chlorides were not considered essential for reversible magnesium electrodeposition. In fact, in the seminal paper by Gregory et al.,26 the solutions contained no chlorides whatsoever. Later, Aurbach et al. showed that addition of complexes containing chlorides was crucial for attaining electrolytic solutions with sufficiently wide electrochemical windows.20 These researches and others later showed that chloride-containing species are a double-edged sword in terms of the solution properties. On the one hand, they improve the oxidation stability of the solution.35,41–43 On the other hand, they may have unfavorable influence on the interfacial electrochemical magnesium deposition/dissolution process parameters. Despite their importance, the exact role of the chloride-containing species on the interfacial processes is still not fully understood. Rational hypotheses were presented, and some of them received important support from experimental and theoretical calculations. Better understanding was gained regarding the role of chlorides in the solution structures.21,28,37 Apart from the process of electrochemical deposition/dissolution of magnesium, one of the greatest concerns associated with chlorides is their corrosive nature, both toward ‘‘inert’’ components in the cell and to cathode materials, in particular metal oxides. In fact, even gold electrodes were found to be heavily corroded under some circumstances. It is beyond the scope of the current review to go in depth into the proved and hypothesized roles of chlorides in the electrochemistry of magnesium rechargeable cells. Nonetheless, no review on the subject will be complete without a brief account on this issue. Hence, in the following bulleted paragraphs we list a concise commentary on this important subject. The list is divided into two sections: the first relates to organometallic-based complex salt solutions, and the second relates to ‘‘simple’’ salt solutions. Organometallic-Based Complex Salt Solutions In complex salt solutions based on organometallic compounds, the bonds of the chloride ligand to the Lewis acid core anion leads to increased stability of the anion, and hence also of the solution, toward oxidation. As a rule of thumb, the higher the Cl:R ratio (R, organic ligand), the wider the electrochemical stability window.4 In the limiting cases of complexes that contain only Cl ligands, e.g., AlCl4, the electrochemical stability window is the widest, limited by the Cl2 evolution reaction, at 3.2 V versus Mg. This is very close to the potential stability window of the solvents, namely, of ethers. As a result, it may occur that both solvent and anion oxidation will contribute to the observed electrochemical stability limit. This aspect is probably strongly dependent on the electrode material, and to a lesser degree also depends on the identity of the ether.4,22 The ratio of Mg to X, the Lewis acid core atom (e.g., Al or B), determines the nature of the anions and cations formed in the solution. In cases where the Cl:R and Mg:X ratios were formulated to yield Cl-ligated Mg cations only

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(no R ligands around the Mg cations), various species were identified, such as MgCl+, MgCl2, and Mg2Cl3+ (all strongly complexed also by the ethereal solvent molecules).25 However, the interfacial magnesium electrodeposition was shown to be strongly dependent on the R:Cl ratio, which practically dictates the solution species: the higher the Cl to R ratio, the lower the exchange current density for magnesium deposition and the deposition overpotential increases. Two factors were suggested to be relevant for these findings: the specific adsorption of Cl-containing species onto the electrodes that decreases the exchange current density (i0) and increases the deposition or dissolution overpotential (Vov), and the easier deligation and/or desolvation of Mg-R are with respect to Mg-Cl species.4,24,25 However, electrochemical studies have shown that Cl-containing solutions are corrosive to common construction components of batteries, such as stainless steel and aluminum. Typically, the corrosion was demonstrated as electrochemically driven at high voltages, rather than due only to contact. Cl-containing species have also been shown to be detrimental to transition metal oxide cathodes. Different chlorine species may have different corrosive properties. It is speculated that anions possessing chlorides, such as AlCl4, R2AlCl2 and the like, will be more corrosive than chloride-containing cations, such as Mg2Cl2 and MgCl+.22 Simple Salts-Based Complex Solutions The introduction of simple salts such as TFSI, PF6, and HDMS, and strongly Mg-complexing solvents such as DME, was considered an important breakthrough. It potentially enables production of Mg-containing electrolyte solutions without highly reactive organometallic components. It was recognized that addition of chlorides in these solutions is critical in order to reach sufficiently high magnesium deposition Faradaic reversibility, as well as to achieve reasonable deposition and dissolution overpotentials.38 Yet, there have been few reports demonstrating chloride-free, simple salt solutions that support reversible magnesium deposition. The data presented by Ha et al. raise some questions about the validity of the results.44 Also, considerable weight should be given to the water content of any system characterized for magnesium deposition efficiency, which was not done in these studies. The chlorides are introduced by adding MgCl2, LiCl, or TBACl, in various proportions. It is important to note that these ‘‘additions’’ or ‘‘mixing’’ are far from being similar to addition of inert or supporting electrolytes. The dissolution of MgCl2 in these solutions is the outcome of chemical reactions between the salts. The solvents play a critical role in these reactions that yield a new range of complex Mg cationic species. Cations such as the solvate Mg2+3DME, Mg2Cl22+, Mg3Cl5, and others have been identified.12,21,28,37 The impact of chlorides on the magnesium deposition process in the simple salt-based solutions is critical. Solutions based on Mg(TFSI)2, Mg(PF6)2 in THF, DME, and glymes require a substantial proportion of chlorides to support magnesium deposition at high efficiency and reasonable overpotentials.35,38,45 There are various proposed hypotheses for the role of chlorides in these solutions. Some have stronger empirical support, while others remain as mere hypotheses or speculations. Some interesting propositions came from computational studies, which have both strengths and limitations. TFSI is suspected to react (and thus be incompatible) with pure, surface film free, magnesium anodes, such as those that form during electrochemical deposition. Based on electrochemical measurements (CV and cycling efficiency) it appears that the reaction is rather slow, yielding mostly soluble products or precipitates that do not lead to magnesium passivation.46 Other anions such as PF6–, ClO4–, and BF4– are all expected to be even more reactive with pure magnesium. These anions contain core atoms at very high oxidation states (P5+, Cl7+, and B3+), and their

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reduction, forming for example MgF2 or MgO, is associated with very large, negative DG values. Even AlCl4 is expected to be thermodynamically incompatible with magnesium. The addition of chlorides in these solutions has shown to dramatically increase the magnesium deposition Faradaic reversibility. Cycling efficiencies reaching 100% have been reported. It is hypothesized that the chlorine-containing species reduce the accessibility of reactive anions such as PF6 and TFSI to the anode surface, thereby protecting it from reactions with the reducible anions. Theoretical calculations and in situ surface analyses have shown that specific adsorption of chlorides and/or chloride-containing species on the bare magnesium anode are favorable. These notions support the hypothesis that chloride species play a decisive role in protecting the reactive Mg surface from fast detrimental reactions with the anions, thereby acting as a ‘‘soft’’ protective layer. This layer is markedly different from SEI layers that govern the behavior of Li and LiC6 anodes. In the magnesium system, these ‘‘films’’ do not accumulate, as a result of reduction (corrosion) reactions and solid interphase precipitation. These protective layers are held by weaker adsorption forces, are extremely thin, and give way immediately under dynamic Faradaic current passage. These protective layers cannot be defined as SEI layers, and magnesium ions do not pass through them in the same manner as lithium ions migrate through the SEI. Free magnesium ions, in chloride-free solutions, such as MgTFSI2 in DME, are very strongly chelated by multidentate solvents such as glymes. Very stable solvates have been identified.37 It is hypothesized that such strong interactions with the solvent cast a high energetic penalty on the magnesium electrodeposition process (as well as on intercalation). During magnesium deposition or intercalation, an overpotential is necessary to overpower the solvation energy, as well as for solvation of the caged Mg ion by free solvent molecules. The addition of chlorides, as noted above, yields a variety of MgxCly complexes, which diminish the high charge density associated with free Mg++ ions and reduce the solvate energy, as well as the solvation forces. Thus, it is hypothesized that these, Mg-Cl complexes lower the energy penalty associated with dislodging Mg from its solvates. In turn, this reduces the overpotential for magnesium deposition and supports higher current densities, thereby improving the Faradaic efficiency.28,38 In complex salt solutions that contain Cl ligands, it is hypothesized that the nature of the electroactive Mg-Cl complex has a strong influence, not only on the metal deposition/dissolution at the anode, but also on the cathode intercalation charge-transfer step. It was speculated, for instance, that MgCl+ would be easier to intercalate into a host as a molecular cation compared with the ‘‘naked’’ magnesium ion. As a monovalent ion, MgCl+, which possesses much smaller charge density, may show higher solid-state diffusion rates. However, resorting to such intercalation mechanisms is associated with several detrimental drawbacks. The principal one is abandonment of the core benefit of magnesium as doublecharged cation.21 Cl species in the electrolyte solutions were also shown to effectively mitigate the difficulty in initial utilization of natural-oxide passivated Mg anodes, for both simple salt and organometallic-based solutions. It was proposed that chlorides facilitate the dissolution or breakdown of these thin tenacious passivating films, thus conditioning the anode surfaces.46 It is simple to acknowledge that the existence of chlorides in solution has multiple functions for magnesium-based cells, some with clear benefits, and others

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detrimental. It is not so simple, however, to elucidate the exact roles of these species in a given solution and their respective magnitude. Other halogens, such as Br and I, cannot substitute for Cl, as their lower oxidation potentials would narrow dramatically the electrochemical stability window. Fluorine is not conceivable as a substitute, owing to the greater stability of MgF2 and its insolubility. To date, no other anions/ligands have been found to replace the problematic chloride. Unpublished experiments with cyanide did not show promise. Coming up with a feasible alternative for chlorides, as well as to the use of ethers as solvents, may therefore be one of the challenges facing the community. One of the approaches, already mentioned above, is the use of chloride-free solutions that showed excellent performance in half and full cells. MgTFSI2 is currently the only readily available ether-soluble simple salt relevant for reversible Mg systems, therefore the development of other salts with improved electrochemical properties is of great interest. Qu et al. preformed a computational study in order to examine the feasibility of various ‘‘modified TFSI’’ type electrolytes.47 This approach presents a new and intriguing route to develop new suitable salts for magnesium secondary batteries. Artificial SEI The lack of native or tailored ion-conducting surface films is by far the most important reason for the complexity in developing metallic magnesium-anodebased batteries. In a recent paper,48 a new approach to resolve the problem was presented. The concept is to use appropriate solvents, salts, and/or additives that will develop stable magnesium ion-conducting surface films. The authors termed this approach as ‘‘artificial SEI’’ deposition. Such surface films may enable integration of Mg metal as an anode material with conventional electrolyte solutions in rechargeable magnesium systems. In simple terms, this approach is designed to make the magnesium anode function in the same way as lithium cells. Recently, Ban et al. published the first report on Mg2+-conducting and electronicinsulating artificial interphase for Mg metal anodes.48 The Mg anode coated with the artificial interphase was synthesized by mixing Mg powder, carbon black, polyacrylonitrile (PAN), and Mg(CF3SO3)2, followed by thermal cyclization of the PAN units((C3H3N)n) at 300 C for 1 hr under Ar atmosphere. The polymeric Mg2+-conducting interphase thus formed was reported to be composed of a matrix of pyridine-based aromatic rings, hybridized with a network of multi-coordinated Mg(CF3SO3 ) units, the products of the PAN cyclization process. In addition to the structural analysis, HRTEM images revealed that the polymeric surface film had a thickness of 100 nm. The coated Mg anodes were characterized electrochemically with two reductionvulnerable electrolyte solutions of 0.5 M Mg(TFSI)2 in ACN and PC. These salts and solvents are known to react on the Mg metal anode to form an impermeable passivation layer. As a benchmarking system, they also cycled the coated anodes in APC electrolyte solution. Bare and coated magnesium electrodes were galvanostatically cycled in each of the electrolyte solutions. The coated and bare Mg anodes showed nearly the same electrochemical performance in APC electrolyte solution. However, due to lower interfacial ionic conductivity, the coated anodes showed slightly higher plating overpotential of around 0.1 V (Figure 5A).

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Figure 5. Impact of Artificial SEI on the Overall Electrochemical Performance (A and B) Voltage responses of symmetric Mg cells under repeated polarization with and without artificial interphase in different electrolyte systems at a current density of 0.01 mA cm 2 showing reversible Mg deposition/stripping in (A) APC electrolyte, where each deposition/stripping cycle lasts for 1 hr, and (B) 0.5 M Mg(TFSI) 2 /PC electrolyte, where each deposition/stripping cycle lasts 30 min. The cell made with pristine Mg electrodes shows huge overpotential at the beginning and fails after 135 cycles, whereas the cell made with the Mg 2+-conducting interphase-protected Mg electrodes performs prolonged cycles in carbonate-based electrolytes. The reversibility is conspicuous in the latter, as proven over 1,000 cycles. (C) Voltage hysteresis versus cycle numbers for symmetric Mg electrodes with 0.5 M Mg(TFSI) 2 /PC electrolyte. Lower voltage hysteresis is observed for interphase-protected Mg electrode, where the artificial interphase prevents reductive decomposition of PC. Reprinted with permission from Son et al., 48 copyright 2018 Nature.

More importantly, the coated Mg anodes exhibited apparently good stripping and deposition performances, with good reversibility over 1,000 hr of galvanostatic cycling in the PC-based electrolyte solution. In contrast, the uncoated Mg anode showed poor electrochemical deposition/stripping behavior due to PC reduction and formation of an impermeable passivation film (Figures 5B and 5C). The coated Mg anode was also coupled with a V2O5 cathode in 0.5 M Mg(TFSI)2/PC electrolyte solution with and without water to form a full cell setup, containing the coated anode. The full setup showed better electrochemical activity than the full cell with an uncoated anode. The Mg-coated/V2O5 full cell exhibited a reversible capacity of 47 mAh/g during 40 cycles, while the uncoated Mg anode containing the cell exhibited sharp capacity fading, down to zero, after 40 cycles. Moreover, addition of 3 M water to the electrolyte solution resulted in improved electrochemical performances of the Mg-coated/V2O5 full cell. This water-containing cell showed five cycles with a reversible capacity of 140 mAh/g.48 However, there is a question about what really cycles in this configuration. As demonstrated by Verrelli et al. the electrochemical process that occurs in V2O5 cathodes in the presence of Mg cations in dry and wet alkyl-carbonate-based electrolyte solutions is related to protons insertion rather than to Mg ions insertion.49

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In addition, despite these apparently impressive results, this approach is still far from being realized. The work presented in that review consists of several experimental procedures that might hinder the true electrochemical properties of the electrodes. For instance, the depth of discharge (DOD) selected for the galvanostatic cycling efficiency experiments, as well as the total amount of magnesium retained, are not known. Thus, it is not easy to estimate the true cycling efficiency values. Controlled experiments, with well-defined initial magnesium amounts and DODs of substantial amounts of charge, carried out over, say, 100 cycles, with complete magnesium stripping in the end, are crucial in order to substantiate the presented results. Also, to correctly investigate the full cell performance or the anode performance during full cell operation, the anode and cathode charge density (mass load) must be correctly balanced. Alternative Anode Materials for Rechargeable Mg Batteries The intrinsic properties of Mg metal, in terms of low reduction potential, high volumetric capacity, safety, and low cost, makes it a natural choice of anode material for rechargeable, high-energy density, cost-effective, and ecologically benign batteries. However, the implementation of Mg metal as the anode in commercially viable systems is hampered by the following main unsolved challenges. Anode-Electrolyte Solution Compatibility As outlined in the previous section, Mg metal reacts rapidly when it comes in contact with nearly all polar aprotic solvents (carbonates, nitriles, lactones, esters, etc.). To date, the only family of polar, aprotic organic solvents that showed thermodynamically compatibility with magnesium metal are ethers. Many of the conventional electrolyte anions (ClO4 , BF4 , SO3CF3 , PF6 , AsF6 , and others) are also chemically incompatible with the strong reducing capacity of the metal. Ubiquitous ambient contaminants, such as water, CO2 and O2, as well as industrial production of byproducts such as alcohols, also pose great complications due to their reactions with the anode.50 In all of the above cases, the core problem is that the surface reactions of metallic magnesium anodes with any of these compounds yield electronic and ionic insulating surface films. In all but a few cases, thin stable deposits precipitate on the anode surface and form a tenacious passivation layer. In stark contrast to lithium anodes, these interphases entirely block the electrochemical activity of the anode, rendering it completely isolated and inactive.16,20,51,52 The high reactivity of the Mg metal therefore acts as a double-edged sword. On the one hand, it imparts the metal with the desired virtue of high (negative) voltage, but on the other hand it casts enormous restrictions on the selection of the medium with which it may act electrochemically in a favorable manner. Theoretically, it is not impossible that novel advancements will lead to the formation of novel SEI layers that mimic lithium in their protecting, yet electrochemically active virtues. However, at the current state of affairs, a crucial factor in reversible magnesium anode solution chemistry is that the electrode remains free from any solid passivation layer in order to support reversible stripping and deposition. Thus, only electrolytic solutions that are completely chemically stable with magnesium metal may preserve the necessary condition of a passivation-free anode in direct contact with the electrolyte solution. Even then, many compatible electrolyte solutions cannot be used in practical systems, mainly due to issues of safety, electrochemical stability window, and cost. Mg Metal Anode/Electrolyte/Cathode Compatibility One of the challenges on the road to develop a commercial rechargeable magnesium battery is to integrate high-voltage cathode materials with viable compatible

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anode-electrolyte systems. Most of the studies in this field have focused on anodeelectrolyte compatibility issues, and considerably less attention have been given to the cathode side. At the beginning, most of the electrolyte solutions that enabled reversible deposition of magnesium exhibited a relatively low anodic stability window of around 2–2.4 V vs Mg/Mg2+. This relatively narrow voltage limitation disables the use of high-voltage metal oxide cathodes. In recent years, several systems of non-Grignard-based electrolyte solutions have been developed that exhibit much wider anodic stability (up to 3.3 V). However, most of these electrolytic solutions contain chlorides, which may be corrosive to metal oxide cathode materials. In addition, electrolyte solutions based on chloride and aluminum chloride are corrosive for battery construction components, such as current collectors that contain nickel and stainless steel. Moreover, recent research found that DME, which is a very important solvent in non-aqueous Mg electrochemistry, has a negative impact on the intercalation of Mg ions into a V2O5 cathode.53 Clearly, integration of high-voltage cathode material with a magnesium metal anode in the appropriate electrolyte solution, to form a high-energy magnesium battery, is no trivial undertaking. Mitigating both issues of anode- and cathode-compatible electrolytic media can possibly be achieved through alternative paths. For instance, several groups suggested use of alternative anode materials that operate well in conventional electrolyte solutions. The main concept underlying these propositions is as follows: magnesium alloys, if thermodynamically favorable, should reduce to some extent the reduction capacity of the anode. Simply put, such materials will exhibit lower voltage (less negative redox potential), which if selected appropriately may lead to anodes that are chemically compatible with solvents, salts, and possibly even some contaminants, allowing utilization of a wider variety of viable electrolytic solutions that support fully reversible magnesium alloying-dealloying anode reactions. For such a material to be of interest, it must simultaneously satisfy several strict conditions: 1. The material must be cheap, ubiquitous, and ecologically benign. The electrochemical magnesiation-demagnesiation reaction must exhibit a high rate, without compromising on reversibility. 2. The alloying-dealloying process must be highly reversible. 3. The magnesium diffusion (or phase propagation) within the base metal and the alloy must be sufficiently fast. 4. The voltage penalty must be sufficient for anode compatibility with the projected electrolytic solution, yet not excessive to avoid reducing the energy density of the system beyond the feasibility limit. 5. The difference in voltage between the magnesiation and demagnesiation processes should be as small as possible. 6. Any potential alternative anode material cannot be exotic, expensive, or hazardous. 7. The specific capacity achievable with the anode at adequate rates of electrochemical process must be sufficient in order to be non-prohibitive. 8. Any need for inert components, such as conducting additives, binders, and current collectors, must be evaluated with careful consideration of the feasibility of using the proposed materials. In light of the challenges associated with metallic magnesium anodes, and considering the complex requirements listed above, intercalation or alloy compounds

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are gaining more interest as alternative anode candidates for rechargeable magnesium batteries. Bismuth-Based Anodes Bismuth and bismuth-based compounds are among the most studied materials as alternative anodes for rechargeable magnesium batteries over the past decade.54–60 Bismuth is considered to be one of the most interesting candidates as magnesium anode material due to its rhombohedral crystalline structure. This structure facilitates formation of alloys that exhibit high volumetric capacity.59 Bi exhibits high theoretical gravimetric capacity with Mg, 385 mAh/g, assuming transfer of six electrons and insertion of three Mg atoms to form the alloy:60 2Bi + 3Mg2+ + 6e / Mg3Bi2 The main driving force that focused the attention of researchers to Bi is the superionic conductivity of Mg in Zintl, b-Mg3Bi2, as reported previously.61 Matsui and coworkers studied the electrochemical behavior of this alloy and some derivatives, and evaluated the feasibility of Bi, Sb, and Bi1-xSbx alloys at different stoichiometry as anode materials for rechargeable non-aqueous magnesium systems.59 The electrochemical studies were conducted in DCC-type complex solutions. The electrodes were made by electrodeposition of the base metal from aqueous solution. While antimony showed very poor cycling performance of about 16 mAh/g after 50 cycles, Bi and Bi0.88Sb0.12 showed impressive results at a current density corresponding to 1C. The Bi-based anode exhibited a maximum specific capacity of 257 mAh/g at the 22th cycle and 222 mAh/g at the 100th cycle, while Bi0.88Sb0.12 exhibited maximum specific capacity of 298 mAh/g, which decreased to 215 mAh/g after 100 cycles. The capacity fading during cycling was suggested to occur due to losses of electrical contact caused by continual expansion/contraction of the anode. Moreover, a feasibility test of the Bi anode with conventional electrolyte solution of Mg(N(SO2CF3)2)2/acetonitrile solution was also conducted. The result indicates that Bi can undergo Mg alloying from acetonitrile-based solution. However, only one cycle was demonstrated, and cycling stability is critical for feasibility assessment.59 It is important to note that the superionic mobility of Mg is reported specifically for the Zintl phase b-Mg3Bi2. This phase is stable only above 703 C; below this temperature, the alloy adopts the a phase, which does not possess superionic conductivity. Also, some of the experiments, while demonstrating proof of concept, were carried out with alloy layers so thin that they only have distant correspondence with real-life systems. Shao et al.58 evaluated the electrochemical activity of Bi nanotubes (Bi-NTs) that were synthesized by hydrothermal reaction. The motivation was to reduce the Mg diffusion lengths and thus to mitigate kinetic hurdles with the alternative anode material. The Bi nano and micro particles were studied in Mg(BH4)2, LiBH4/diglyme solution (Figure 6). The nanostructured Bi anode showed superior performance compared with the micro-Bi anode, especially at high C rates (216 mAh/g for the Bi-NTs compared with 51 mAh/g for the Bi-micro at 6C). In addition, the Bi-NTs showed narrower peaks and lower overpotential than the Bi-micro (Figure 6A). These results showed a faster response for the nanostructured Bi. However, at low C rates (C/20 up to C/2) the performances of the micrometric material were in line with the

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Figure 6. Electrochemical Performance for Magnesium-Bismuth Alloy as Anode Materials (A) Cyclic voltammograms of Mg insertion/deinsertion in bismuth. (B) Discharge/charge profile of an Mg-Bicell. (C) Rate performance of an Mg-Bicell. (D) Cycling stability and Coulombic efficiency of bismuth electrode for reversible Mg insertion/ deinsertion. Cell configuration: Mg/0.1 M Mg(BH 4 ) 2 -1.5 M LiBH 4 -diglyme/Bi. Reproduced with permission from Shao et al., 58 copyright American Chemical Society.

nanostructured Bi-NTs. In addition, the Bi-NTs anode was tested in a full cell setup, which was comprised of Mo6Se8 as cathode and 0.4 M MgTFSI2 in diglyme as conventional electrolyte solution. These full cells showed a mid-point discharge voltage of around 0.75 V and a specific capacity of 90 mAh/g. The full cell also exhibited very good cycling stability, with 92.3% capacity retention after 200 cycles. X-ray diffraction (XRD) measurements confirmed that the Bi anode went through reversible Mg2+ insertion.58 An obvious weakness of these materials that is clearly observed is a large voltage hysteresis, and thus energy penalty, between the magnesium alloying and dealloying. Unfortunately, some essential experimental details, such as the anode mass load, are not reported (it is only mentioned that, for the full cells, cathode limiting couple was used). This precludes the ability to assess the possible gap between the demonstrated results and the real-life, practical expected performance. Furthermore, despite the impressive electrochemical behavior exhibited with the nanostructured Bi-NTs, this type of material might be difficult to implement in commercial systems due to its exotic nature and manufacturing costs. The electrochemical behavior and alloying mechanism of Mg with Bi were studied also by Murgia et al. using electrochemical measurements coupled with XRD.55 The study was conducted using two electrode cells, which contained Mg as counterreference electrode and an organometallic-based electrolyte solution. Surprisingly, the results reveal a biphasic process between Bi and Mg3Bi2 without any intermediate amorphization, which is typical for alloy-type electrodes. In addition, micrometric Bi and Mg3Bi2 prepared by ball-milling were electrochemically characterized. The

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Figure 7. Galvanostatic Curve Obtained at 2C, after Initial Activation Sweeps Inset: evolution of discharge and charge capacities and Coulombic efficiency. Reprinted with permission from Murgia et al., 55 copyright 2015 Royal Society of Chemistry.

micrometric Bi anode exhibited a reversible specific capacity of 300 mAh/g at a discharge rate of 2C, with Coulombic efficiency of 98.5% after 50 cycles (Figure 7). In addition, the Mg3Bi2 was coupled with Mo6S8 in a conventional electrolyte solution of 0.5 M Mg(TFSI)2 in diglyme. The full cell shows an electrochemical signature, which is typical to this system with a discharge voltage of around 0.6 V. Moreover, the intercalation process on the cathode side and the de-alloying on the anode side during discharge were confirmed by ex situ XRD measurements. However, they also found that complete demagnesiation of Mg3Bi2 could not be achieved.55 Another study that focused on understanding the mechanism and diffusion pathway for Mg in the Bi anode was conducted using 25Mg NMR spectroscopy. This study also demonstrated two-phase alloying reactions of Bi and Mg. In addition,25Mg variable temperature NMR measurements revealed fast exchange between the two Mg sites in the Mg3Bi2 material.62 Synthesis and electrochemical analysis of a Bi/carbon NTs composite has been shown by DiLeo et al. The composite material was synthesized via electrochemical deposition of Bi on CNTs from aqueous solution of Bi nitrate. Monitoring the deposition process was made by the electrochemical quartz crystal microbalance technique. The electrochemical investigation was made in acetonitrile-based solution containing 0.5 M magnesium perchlorate (Mg(ClO4)2) and 0.5 M dipropylene glycol dimethyl ether. The Bi/CNTs composite exhibited specific capacity of 180 mAh/g measured by CV at a scan rate of 0.5 mV/s. However, a sharp decrease in capacity was observed in the second and third cycles (80 and 49 mAh/g).60 Such sharp capacity fading, in addition to the relatively complicated synthesis, makes this material irrelevant for commercial utilization. In addition to the empirical studies mentioned above, density functional theory (DFT) computations also demonstrated the great potential of Bi to serve as anode material for rechargeable magnesium ion batteries. A computational study conducted by Jin et al. shows that the diffusion barrier for an isolated Mg ion in Bi is 0.67 eV. Moreover, the diffusion barrier does not change with the alloying level. The relatively small calculated diffusion barrier suggests the feasibility of fast charge/discharge

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Figure 8. Comparative Analysis of Magnesium-Tin and Magnesium-Bismuth Anode Materials First cycle galvanostatic magnesiation/de-magnesiation curves for Sn/Mg (black) and Bi/Mg (red) half-cells (using organohaloaluminate electrolyte) highlighting their achievable theoretical capacities. Inset: XRD spectra for (1) as-fabricated Sn, (2) magnesiated Sn (or Mg 2 Sn––peak positions marked with arrows), and (3) de-magnesiated Mg 2 Sn. Reprinted with permission from Singh et al., 63 copyright 2013 Royal Society of Chemistry.

rates for Mg-Bi alloys and their potential as anode materials for secondary magnesium ion batteries.62 Tin-Based Anodes (Pure Metal, Alloys, Solid Solutions, and Intermetallics) Another well-studied alternative anode material for rechargeable magnesium batteries is tin (Sn). Tin-based anodes possess great advantages over Bi anodes, in terms of lower insertion potential and higher theoretical specific capacity. Moreover, the molecular weight of tin is considerably lower than that of Bi, and its abundance is approximately two orders of magnitude greater. While each Bi anode can exchange 1.5 electrons per Bi atom, each Sn atom can exchange four electrons, as depicted in the following formula: Singh et al. examined the feasibility of using Sn powder films as an insertion anode for rechargeable magnesium batteries.63 Galvanostatic charge-discharge curves at low current density of 0.002C in organo-haloaluminate electrolyte, showed that the Sn anode potentially shows better electrochemical performance than the Bi anode. The experimental results shows that the insertion potential into the Sn anode is 0.15 V vs Mg/Mg2+, while the insertion potential into Bi anode is around 0.23 V vs Mg/Mg2+. On the one hand, this translates to higher cell voltages, but, on the other hand, this voltage difference may be insufficient for suppression of electrolyte reduction and passivation. Moreover, the hysteresis between insertion and de-insertion was found to be much lower for the Sn anode than for the Bi anode (50 and 90 mV for Sn and Bi, respectively). In addition, the Sn anode exhibited a specific capacity of 903 mAh/g. The discharge process, however, as seen in the galvanostatic curves, seems to be highly irreversible. This leads to diminished reversible capacity (Figure 8A). Sn + 2Mg2+ + 4e / Mg2Sn In addition, rate capability measurements show that, at rates above 0.002C, the specific capacity of the Sn anode is dramatically decreased. So, on the one hand, it

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appears that Sn suffers from inhibitive poor insertion kinetics. On the other hand, however, the Coulombic efficiency was shown to increase with the charge-discharge rate. Furthermore, evaluation of Sn as the Mg-absorbing anode in 0.5 M Mg(N(SO2CF3)2)2/DME and with organohaloaluminate electrolyte solutions in full cells (Mo6S8 cathode) was also carried out. The results showed that both systems display very similar electrochemical performances, with 82 mAh/g at the first cycle, followed by a sharp decrease in capacity to a stable value of less than 50 mAh/g.63 Sn-based compounds have also been shown to be interesting candidates to serve as anodes for rechargeable magnesium batteries. Cheng et al. studied the electrochemical properties of an SnSb alloy as an anode material for secondary Mg batteries.64 They studied its potential charge storage mechanism using a combined computational and experimental approach. Their first work revealed that, during the initial charge-discharge process, the SnSb particles irreversibly transformed into a porous structure composed of Sb-rich and pure Sn sub-structures. After this process, which was also termed as conditioning, the nanosized Sn particles were found to exhibit highly reversible Mg alloying, while the Sb-rich zones showed poor Coulombic efficiency due to trapping. Although the Sb-rich substructure lowers the material’s specific capacity, they were found to be vital for the formation of stable Sn nanoparticles.64 A following study, which focused on the charge storage mechanism and the electrochemical properties of SnSb after activation (conditioning), also revealed superior electrochemical behavior for the activated SnSb alloy over pure Sn. SnSb exhibits lower overpotentials for magnesiation/demagnesiation, which points to improved kinetics for the processes compared with pure Sn. In addition, the SnSb alloy possesses high specific capacity of 420 mAh/g versus less than 300 mAh/g for the pure Sn at a current density of 50 mA/g (Figure 9B). Moreover, SnSb showed incredible rate capability with 70% capacity retention (equivalent to 300 mAh/g), even at current densities as high as 1,000 mA/g (Figure 9C). The SnSb anodes also possess good cycleability, with 270 mAh/g after 200 cycles at current density of 500 mA/g (Figure 9D).65 Wang et al.66 investigated the Mg cation diffusion properties in both b- and a-Sn via DFT calculations. The diffusion barriers for isolated Mg atoms were found to be 0.395 and 0.435 eV in the a and b forms of Sn, respectively. In addition, the diffusion barriers were found to decrease in the case of a-Sn with increasing Mg concentration, while they are expected to increase with the b form. These results suggest that the a phase may be more suitable to serve as anode material for rechargeable magnesium batteries (RMB) than the b phase.66 Another material of this kind, also proposed as an anode material for RMB, is indium (In). Murgia et al. studied the electrochemical behavior of micrometric In powder with Mg-organohaloaluminate electrolyte solution.67 The In-based composite anode exhibited a high discharge specific capacity of 425 mAh/g at a rate of C/100 and a low alloying potential of 0.09 V vs Mg/Mg2+ (Figure 10). Unfortunately, the micrometric In-based electrode showed poor electrochemical performance at higher, more reasonable rates of C/50 (C/20 and C/10), with fast voltage fading during the discharge process. After electrochemical grinding, these anodes showed improved electrochemical performances, with specific capacities of 280 and 300 mAh/g at C/10 and C/20, respectively. The poor rate capability of the micrometric indium points to some kinetic limitations that the authors suggest may

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Figure 9. Comparison of Electrochemical Behavior of SnSb and Sn for Mg Magnesiation-Demagnesiation (A and B) CV acquired at 0.05 mV/s (A) and charge-discharge profiles acquired at 50 mA/g (B). (C and D) Specific capacity for SnSb at different current densities (in mA/g) as noted (C) and cycling stability of SnSb, at 500 mA/g (D). Reprinted with permission from Cheng et al., 65 copyright 2015, Wiley-VCH Adv. Mater.

be alleviated by going to nanometric particles.67 At any rate, indium is not a cheap metal, and, apart from substantiating the understanding of the electrochemistry of magnesium intermetallic and alloy anodes, it has no practical significance. The feasibility of Pb to serve as an anode material for Mg ion batteries was also examined. Obrovac et al. studied the electrochemical behavior of sputtered Pb (0.24 mm) and Pb powder-based composite electrodes.68 Both the sputtered and the Pb powder-based composite electrodes were found to alloy electrochemically with Mg. The electrochemical study was conducted with a Grignard-based electrolyte solution of 0.5 M ethylmagnesium chloride with or without 0.25 M AlCl3 in THF at 60 C. The sputtered Pb anode showed impressive reversible specific capacity of 450 mAh/g, in addition to a low discharge voltage of 0.125 V vs Mg. The Pb-based composite electrode showed also a similar discharge voltage, but delivered a much lower specific capacity of 275 mAh/g.

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Figure 10. Galvanostatic Cycling of In-Based Formulated Electrode at C/50 and Corresponding Capacity Evolution at C/50 Inset: corresponding charge and discharge capacities evolution. Reprinted with permission from Murgia et al. 67

Although both materials can store magnesium at relatively high specific capacity, they suffer from extremely poor Coulombic efficiency. The authors contend that the low reversibility for magnesiation is probably due to electrolyte decomposition on the catalytic surface of the Pb electrodes. It is important to note that the experiments were carried out at 60 C, making it hard to compare the specific capacities, rates, and charge retention to room temperature systems. Unfortunately, the authors did not perform experiments with lead in simple salt solutions, which should possess lower susceptibility for degradation. Obviously, at this stage, this anode material seems of less interest for commercial systems, even if the ecological aspects of the material (lead) are ignored.68 Lithium Titanate-Based Anodes Li4Ti5O12 (LTO) possesses several excellent properties, which makes it an interesting candidate as an anode material for RMB. It was shown to reversibly store Mg ions with intrinsic ‘‘zero strain.’’ This characteristic results in low volume changes during Mg ion intercalation-deintercalation, leading to low-stress reactions and high cycle life. Wu et al.69 evaluated the electrochemical behavior of Li4Ti5O12 nanoparticles as an anode material in non-aqueous magnesium systems. The spinel LTO anode material was characterized in a Mg(AlCl2BuEt2)2/THF electrolyte solution. The anode material exhibited a specific capacity of 175 and 55 mAh/g at current densities of 15 and 300 mA/g, respectively (Figure 11A). In addition, LTO exhibits outstanding cycling stability with 100% Coulombic efficiency and capacity retention of 99.9% after 500 cycles at current density of 300 mAh/g (Figure 11B).69 The effect of the LTO particle size on its electrochemical behavior was also studied. The results revealed that particle size has a great impact on the electrochemical activity of the LTO as an intercalation compound. Intercalation of Mg cations into the LTO was obtained only when the particle size was below 40 nm. High specific capacity of 175 mAh/g and excellent cycling stability over 500 cycles with 95% capacity retention were obtained when the particles size was below 10 nm.70 Several important issues are reflected in this study. First, the very good cycling efficiency, particularly with nanosized oxide material, is not a trivial matter. Oxides are thought to be susceptible to detrimental surface reactions and corrosion in chloridecontaining media. The electrolytic solutions used in this study were of the DCC family. These solutions contain not only organometallics, but also chloride-containing aluminum Lewis acid species. Nonetheless, it appears that these had no detrimental effect on the material’s electrochemical performance. That having been said, there are also some clear drawbacks for these materials. The sloping discharge and charge curves exhibit quite a large voltage gap, leading to energy inefficiency. In the CV, for

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Figure 11. Cycling Performance of LTO (A) Comparison of the rate capabilities of the cell cycled at different current densities. (B) Cycling performance of the cells using LTO electrodes cycled at a current density of 300 mA/g. Reprinted with permission from Wu et al.,69 copyright 2014 Nature.

instance (Figure 3), one can see that, while the charging potentials are scanned down to 0.0 V vs Mg, the initial discharge currents do not commence before ca. 0.5 V vs Mg, and they reach as high as ca. 1.8 V. In addition, in contrast to a pure magnesium anode, LiTiOx must be utilized as a composite anode, adding inert materials that reduce its specific capacity. Although the majority of the publications in this field, dealing with alternative anodes for RMB, were focused on Sn- and Bi-based materials, several other materials were shown to possess interesting potential characteristics. Chen et al. investigated the electrochemical behavior of layered Na2Ti3O7/ MgNaTi3O7/Mg0.5NaTi3O7 (NTO) nanoribbons as an intercalation anode material for Mg ions.71 The NTO exhibited a reversible Mg2+ intercalation after an activation process during the first cycle. This ‘‘activation’’ process includes irreversible deintercalation of Na+ and intercalation of Mg2+. The theoretical intercalation capacity of the material is 0.5 Mg2+ cations per NTO unit, corresponding to a specific capacity of 88 mAh/g. However, in practice the NTO was found to deliver a maximum of 78 mAh/g in APC solution. Most interestingly, the NTO was tested as an anode material in full cells with simple salt solution of Mg(ClO4)2 in diglyme. V2O5 was used as a cathode. The full cells delivered a reversible specific capacity of 75 mAh/g (based on the anode mass), corresponding to 53 Wh/Kg. The specific capacity of the full cell decreased to 64 mAh/g after 30 cycles.71 Obviously, the high discharge voltage of the anode and the relatively low specific capacity of the NTO render this system as a proof of concept, rather than a practical one. To conclude this section, up to now several materials have been shown to reversibly store magnesium at low potential. These materials constitute potential alternative anodes for RMB. The driving force to invest resources for research in this direction stems mainly from the desire to resolve the passivation problem associated with metallic magnesium anodes in contact with various electrolytic solutions, particularly in simple salt solutions. The criteria for selecting practical anode materials are summarized above, in the first section. As shown, some materials, such as LTO, possess excellent mechanical stability but relatively low specific capacity. Other materials, such as Bi and Si, exhibit high specific capacity but poor cycling performance. Thus, further research is needed to develop an alternative electrode material that possesses all of these desirable properties.

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In addition, the penalty for using alloy/intercalation-based anodes comes in the form of low specific capacity contrary to the metal-based anode materials. The main driving force for utilizing this type of anode is the ability to couple these materials with electrolytes that are known to be non-feasible electrolyte solutions (Mg(ClO4)2 in PC, MgPF6 in various solvents, etc.). So, to justify any research into a new alternative anode material for rechargeable magnesium systems, feasibility tests with conventional electrolyte solutions must be taken into account. An important note for researchers in the field is that benchmarking the electrochemical properties of alternative anode materials for RMB with well-studied solutions that support high magnesium deposition reversibility is definitely a first step. However, thorough experimenting with simple ion salt solutions is very important, at least as a proof of concept. Of even more interest is the use of solvents other than ethers, such as carbonates and nitriles, as they possess the appropriate electrochemical oxidation stability. Conclusions and Perspective Until now, there has been no magnesium-based electrochemical device that can be considered practical for energy storage and conversion. The anode side suffers from passivation phenomena that can be affected by very small level of atmospheric, protic, and acidic contaminants. Any formation of stable surface films on Mg metal electrodes blocks it. This unique sensitivity of Mg anodes to surface reactions makes the selection of salts and solvents extremely limited. Furthermore, pairing high-capacity/high-voltage cathodes to any electrolyte solution in which magnesium anodes behave fully reversibly remains a major challenge in this field. It appears that, to elaborate non-reactive electrolyte solutions that do not contain species with organo-metallic bonds, in which Mg anodes behave reversibly, they may need to contain chloride ions. However, their presence in solutions may avoid a possible use of transition metal oxide cathode materials and promote corrosion of many conventional current collectors used in batteries, such as aluminum foils. Mitigating limitations of both anode and cathode sides in rechargeable Mg ion batteries may be achieved through implantation of alternative anode materials, which can operate in conventional electrolyte solutions. Using solutions comprising solvents such as alkyl carbonates and conventional Mg salts opens the door for using oxides such as V2O5, MoO3, and MnO2 as high-capacity/high-voltage cathode materials for rechargeable Mg batteries. In turn, giving up the use of Mg metal as an anode material in Mg batteries means losing the advantages of low redox potential and high specific volumetric and gravimetric capacity of Mg metal foil electrodes. However, since the cathodes are usually the limiting factor in batteries, giving up even 80% of specific capacity by moving from Mg foil anodes to Mg alloy anodes, as discussed above, may still be advantageous in terms of energy density if oxide cathodes can be used. This statement is obviously valid, by understanding that the major advantage of practical rechargeable Mg batteries will never be high energy density (discussing this point is beyond the scope of this paper). Although several materials, in particular Bi and Sn, possess low reduction potentials and high theoretical specific capacity in their electrochemical reactions with Mg ions, their poor electrochemical stability during prolonged cycling hampers their use in practical systems. Nevertheless this direction deserves further efforts and thereby was discussed herein in detail. Other directions that can promote development of practical rechargeable Mg batteries may include the development of chlorides-free electrolyte solutions possessing wide electrochemical windows, in which Mg electrodes can behave reversibly. There are also options for developing new cathodes for Mg batteries that are not so sensitive to the composition of the electrolyte solutions. Discussing these directions is

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beyond the scope of this paper; however, work in these directions is being carried out by a number of prominent research groups.

AUTHOR CONTRIBUTIONS Conceptualization, Y.G. and D.A.; Writing – Original Draft, R.A. and M.S.; Writing – Review & Editing, Y.G. and D.A. R.A. and M.S. contributed equally to the paper.

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