Mapping the Challenges of Magnesium Battery - American Chemical

Perspective pubs.acs.org/JPCL

Mapping the Challenges of Magnesium Battery Jaehee Song, Emily Sahadeo, Malachi Noked,* and Sang Bok Lee* University of Maryland, College Park, Maryland 20742, United States ABSTRACT: Rechargeable Mg battery has been considered a major candidate as a beyond lithium ion battery technology, which is apparent through the tremendous works done in the field over the past decades. The challenges for realization of Mg battery are complicated, multidisciplinary, and the tremendous work done to overcome these challenges is very hard to organize in a regular review paper. Additionally, we claim that organization of the huge amount of information accumulated by the great scientific progress achieved by various groups in the field will shed the light on the unexplored research domains and give clear perspectives and guidelines for next breakthrough to take place. In this Perspective, we provide a convenient map of Mg battery research in a form of radar chart of Mg electrolytes, which evaluates the electrolyte under the important components of Mg batteries. The presented radar charts visualize the accumulated knowledge on Mg battery and allow for navigation of not only the current research state but also future perspective of Mg battery at a glance. materials (777 mAhcm−3). Additionally, the low cost and high abundance of Mg compared to Li (300 times more abundant than Li) makes Mg battery even more attractive. Furthermore, unlike metallic Li anode which has tendency to develop a dendritic structure on its surface upon the cycling of battery, Mg metal was shown to be free from such a hazardous phenomenon. Due to these merits of Mg as an anode, rechargeable Mg battery has attracted considerable attention among researchers in the last few decades.

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ithium-ion batteries (LIB) are considered among the most impressive successes of modern electrochemistry. Powering most currently used portable devices, these batteries ushered electronics into a new era of mobile energy, directly supporting and influencing our daily lives. However, the everincreasing demand for higher energy devices for transportation (electric vehicles), grid storage (power leveling), and portable electronics (laptops, smartphones, etc.) is challenging the scientific community to develop new chemistries and morphologies of energy conversion and storage materials to move beyond current Li-ion technology toward electrochemical storage devices with superior energy and power performance. The quest for allocation of new materials, new chemistries, and new morphologies has drawn many “newcomers” into the field of applied electrochemistry from multiple disciplines of science. The battery system is comprised of at least three components (anode, cathode, and electrolyte) that undergo complicated redox reactions; hence, in many cases, there is a dire need to carefully put forth introductory guidelines and perspectives for these newcomers in order to give a general picture of what has been accomplished, what has been found to be useful (or not), where the challenges are, and what are the strategies most likely to facilitate the next breakthrough in the field. We claim that rechargeable Mg battery (RMB) is a system that requires a clear and visualized perspective, even for scientists in the field of LIB due to the unique intrinsic challenges of RMB and mechanistically different electrochemical behavior of the Mg2+ ions (compared to Li+ and Na+), which affects all three components of a battery. Rechargeable Magnesium Battery. RMB is considered as one of the candidates for the next generation battery technology due to the following factors: Mg, when employed as an anode, possesses a factor of 5 higher volumetric capacity (3832 mAhcm−3) than commercialized LIB graphite anode © 2016 American Chemical Society

A newcomer to the field of RMB must realize that the electrochemical behavior of Mg2+ is different from Li+ in regards to all battery compartments. However, the aforementioned advantages of Mg battery become possible only when appropriate electrolytes and cathodes are employed. A newcomer to the field of RMB must realize that the electrochemical behavior of Mg2+ is different from Li+ in regards to all battery compartments; for example, when coupled with conventional electrolytes that are commonly used in LIB, Mg anode quickly becomes electrochemically inactive due to the formation of a passivation layer on the metal surface as a result of electrolyte species reduction. Additionally, the divalency of Mg ion results in very slow insertion kinetics in most conventional LIB cathode materials. The greatest obstacle for realization of RMB is considered to be the development of electrolyte solutions that enable reversible deposition/dissolution of Mg metal. Indeed, the majority of Received: February 18, 2016 Accepted: April 18, 2016 Published: April 18, 2016 1736

DOI: 10.1021/acs.jpclett.6b00384 J. Phys. Chem. Lett. 2016, 7, 1736−1749

The Journal of Physical Chemistry Letters

Perspective

Finally, we have illustrated where we think the efforts in the field should be focused next (and where not) under the context of full RMB system using our designed chart and will conclude what are the new directions we propose for taking the field into the next breakthrough. Chart Design. The methodology we used to organize all the previously reported work is based on a radar chart as explained below: It is well established that the one of the crucial components for the rechargeable Mg battery is the electrolyte solution; hence, the electrolyte was chosen to be the leading component for our designed radar chart. Each radar chart ranks the electrolyte under six parameters that are either mandatory or reflects a desired breakthrough for a rechargeable Mg battery: (a) compatibility with reversible deposition and dissolution of Mg2+ on Mg metal anode, (b) anodic reaction stability of the electrolyte, (c) stability against current collector corrosion, and (d−f) compatibility with three groups of cathodes, (d) higher voltage class cathodes (e.g., metal oxides), (e) lower voltage class cathodes (e.g., Chevrel phase), and (f) high capacity conversion cathodes (e.g., sulfur). Figure 1a represents a blank radar chart where the metric of each component is quantified with scales of 1 to 5 that are given based on the criteria listed in Table 1. It is also important to note that a battery component with no designated rating, indicated by the center of the chart, is an area that is unexplored. Briefly, the degree of compatibility with Mg anode was evaluated by the reported Coulombic efficiency (CE) of Mg deposition/dissolution process. For anodic reaction stability and stability against current collector corrosion, ranges of voltage window were used as evaluation standards. Note that the ranges of voltage window were selected based on the demagnesiation voltages of common Mg-ion cathodes (vs Mg/ Mg2+) (e.g., 1.5 V for Chevrel phase, 2.7 V for sulfur, and 3.4 V for proposed higher voltage cathodes).7−11 The compatibility of Mg electrolyte with three groups of Mg cathode has been represented based on the reported cycling stability accompanied by a reversible Mg2+ insertion capacity of the cathodes in the selected Mg electrolytes. The standard has been set to mimic the projected requirements of cycling numbers of commercialized battery as close as possible, with the caveat that some of the reports are with flooded cells and far from real battery conditions. Additionally, for the ease of visualization there is a separation between 100 cycle and 30 cycles even though both are not good enough for battery (e.g., 100 cycles get the 3, though it is actually 2 or less). Figure 1b is an example radar chart generated

R&D has focused on synthesis and structural studies of new complicated organic electrolytes. It is fair to state, and important to acknowledge, that by carefully following the electrolyte synthesis procedures based on the pioneering work done by Gregori, Aurbach, Muldoon, Fichtner, and a few other research teams can help newcomers to the field of RMB to initiate their research work, even without substantial background in organic chemistry and even if their major focus is on a new electrode material or morphology. The extensive amount of Mg electrolyte work has been thoroughly discussed in the previous reviews.1−6 Therefore, in the current Perspective, we will only briefly introduce the previous work on Mg electrolyte for the ease of the discussion; instead, we will give more attention to the relatively new systems that have not been covered extensively in the previous reviews.

The main goal of this paper is to organize RMB systems that have been reported so far into a convenient “map”, which visualizes the overflowing information in an intuitive way. The main goal of this paper is to organize RMB systems that have been reported so far into a convenient “map”, which visualizes the overflowing information in an intuitive way. Mapping the reported work, as we will demonstrate here, helps put things in perspective and instantly sheds the light on what should be accomplished in order to demonstrate a breakthrough in the field of RMB. We will not focus on every fundamental insight and specific detail of the previously published electrolytes system nor focus on the chemical pathways and molecular structure of the electrolyte species but rather give a clear and visualized perspective on the outcome of each electrolyte evaluated under the perspective of a full RMB system. We believe that the newcomers into the field can easily use our perspective and designed map to carefully choose the electrolyte solution that is appropriate for their research. Furthermore, any newly reported results can be easily organized into the presented map and evaluated under this Perspective. We believe the readers of this paper will get the full picture of where the challenges in Mg battery are and where the next breakthrough is needed and likely to happen.

Figure 1. (a) Blank radar chart with six electrolyte components. Each component is ranked 1 to 5 based on the criteria listed in Table 1. The center of the chart represents that the corresponding component has not been explored. (b) Example radar chart of DCC electrolyte that exhibits 100% CE for Mg deposition/dissolution, 2.2 V of anodic stability and stability against the current collector corrosion, over 500 stable cycles with low voltage cathodes, and incompatible with high voltage and sulfur cathode.12 1737



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organic solvents, such as carbonates, in addition to simpler Mg salts, such as Mg(ClO4)2, react with the Mg metal surface to create a blocking or passivating layer that hinders the transport of Mg2+ ions through the interphase leading to poor CE. The inherent incompatibility of conventional electrolytes in Mg-ion systems is a major impetus for the synthesis of new, complex Mg salt-based electrolytes that are capable of reversible Mg deposition/dissolution. Extensive research efforts for the past decades have resulted in development of many attractive Mg electrolytes demonstrating great compatibility with Mg anode and high CE, which will be discussed in this paper. Recently, studies have revealed that the CE of Mg deposition/dissolution process is governed not only by the components and purity of the electrolyte (e.g., trace water concentration) but also by the coordination of cationic speciation.13,14 Therefore, understanding the Mg2+ speciation in the electrolyte responsible for Mg deposition/dissolution is the core premise in achieving high CE with reduced surface fouling. Anodic Reaction Stability. The electrochemical window of an electrolyte is defined by the voltage domain in which neither cathodic reduction nor anodic oxidation of the electrolyte takes place. In other words, both salt and solvent of an electrolyte have to be stable against reduction and oxidation at the strongly reducing anode and strongly oxidizing cathode surfaces, respectively. The nonconventional types of electrolytes that are stable toward reduction at the metal surface (such as Grignardbased electrolytes) have been employed for Mg battery, in order to prevent the formation of Mg passivation layer on the anode interface. Unfortunately, a significant drawback of these electrolytes is their low anodic reaction stability. In many cases, the anodic reaction stability of the electrolyte will become the bottleneck for utilization of cathode materials whose charge/ discharge voltages are higher than the stability limit of the electrolyte. Stability against Current Collector Corrosion. Chloride has been one of the key “ingredients” for many Mg electrolytes; unfortunately, some of the chloride species cause serious corrosion of the common current collectors, such as stainless steel and aluminum.15 This means that even though such chloride-containing electrolytes have been reported to exhibit a high anodic stability and wide voltage window on an inert substrate such as Pt, the corrosion of less-noble substrates could occur at a lower voltage, making the “practical” voltage window narrower than the ideal case. Therefore, when considering the voltage window of an electrolyte, its stability against the current collector corrosion must be also taken into account. This implies that in many cases of chloride-containing electrolytes, the practical voltage window may be significantly narrower than the values reported with Pt current collector. This crucial aspect was extensively and systematically studied by few leading research groups in the field.4,16−19 Lower Voltage Class Cathodes. Chevrel phase (e.g., Mo6T8 where T = S, Se) exhibits the most reversible Mg-ion insertion/ extraction reaction with highest kinetics among many reported Mg cathodes. It has an average discharge voltage of less than 1.5 V vs Mg/Mg2+ which suits well within the stability limit of most Mg electrolytes. More importantly, it provides a superior Mg2+ insertion/extraction capability owing to its unique crystallographic structure that is beneficial for the divalent charge transfer upon the insertion of Mg2+.12 Other cathodes that can be suited into lower voltage class include MoS2, WSe2, and TiS2.20,21 Higher Voltage Class Cathodes. We have categorized cathodes whose average operating voltage lies above or close to the anodic

Table 1. Criteria for the Scales of (a) Mg Anode, (b) Anodic Stability, (c) Stability against the Current Collector Corrosion, and (d) Cathodes (a) scale

Mg anode: Coulombic efficiency

1 2 3 4 5

incompatible > 80% > 90% > 95% 100% (b)

scale

anodic stability: voltage range (vs Mg/Mg2+)

1 2 3 4 5

< 1.5 V 1.5 to 2.5 V 2.5 to 3.5 V 3.5 to 4.5 V 4.5 to 5 V (c) stability against current collector corrosion: voltage range (vs Mg/Mg2+)

scale 1 2 3 4 5

< 1.5 V 1.5 to 2.5 V 2.5 to 3.5 V 3.5 to 4.5 V 4.5 to 5 V (d) scale

cathodes: reversible cycling stability

1 2 3 4 5

incompatible < 30 reversible cycles < 100 reversible cycles < 300 reversible cycles < 500 reversible cycles

for dichloro-complex organohaloaluminate electrolyte (DCC) that has been shown to exhibit the following electrochemical performances: 100% CE for Mg deposition/dissolution, less than 2.2 V of anodic reaction stability and stability against current collector corrosion, over 500 stable cycles with low voltage cathodes, and incompatible with high voltage cathode and conversion sulfur cathode. In the first section of this manuscript, we will justify the components that we chose as evaluation parameters of a given Mg electrolyte in each radar chart and use the chart to rank the reported electrolytes. In the second section of this paper, we use the charts to demonstrate how the reported strategies have improved the functionality of certain electrolytes by overcoming the scientific challenges of Mg battery. For example, a few cathodes have shown to exhibit improved Mg2+ insertion kinetics by nanosizing the material; the improvement observed in each component of the chart was applied to the corresponding chart if the degree of the improvement has resulted in a change to the metric scale described Table 1. Finally, we will conclude by providing research perspectives and future point of view based on the visualized information on these radar charts. Choice of Parameters for Radar Chart Mg Anode. One of the most important properties of an Mg electrolyte is its compatibility with an Mg anode. This compatibility is demonstrated by a good CE, which is defined here as the ratio of the charges accumulated during the Mg deposition/dissolution. It is important to note that the trace amount of water and conventional electrolytes that contain 1738



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conventional electrolyte systems will react on the surface of magnesium to form a real passivation layer which blocks the movement of Mg2+ completely. It is for that reason that RMB R&D has been mainly focused and relied on the development of electrolyte that will not form this ionically insulating interphase. It was demonstrated almost a century ago that magnesium can be deposited from ethereal Gringard reagent solutions without the formation of such interphase.39 However, the electrolyte solutions based on Gringard reagents, which are strong reducing agents, possess very low anodic stability. Therefore, their utilization in a full battery system will be limited by their oxidation reaction on the cathode. Additionally, the ionic conductivity of these electrolyte is too low for a battery system which can be mainly attributed to the low dissociation level of the Grignard species in the ether solvent (RMgX, where R = alkyl or aryl group and X= halide). The first coherent research on electrolyte for RMB was conducted by Gregory et al.40 They reported a reversible deposition/dissolution of Mg from THF solutions of organoborate (i.e., Mg(BBu2Ph2)2) and organohaloaluminates. These studies encouraged the community to farther pursue ether-based electrolyte solution for RMB, with focus on achieving higher anodic stability and optimized ionic conductivity compared to the electrolyte reported by Gregory et al. As a following work, Aurbach and co-workers discovered that reacting optimized ratios of Lewis acids and RR′Mg (e.g., AlCl3 and Bu2Mg, respectively) form a complex solution with increased anodic stability (>2.1 V vs Mg) and 100% reversible Mg deposition and dissolution in practical reaction kinetics. It is this work, combined with the systematic screening of various Lewis acids (based on B, Al, P, and Sb), that enabled the first demonstration of RMB prototype with dichlorocomplex organohaloaluminate electrolyte (DCC). Their study showed that Al-based Lewis acids are outperforming the other candidates and optimized the organic ligands identity and concentration of the complex solutions. Besides 100% reversible Mg deposition/dissolution, the DCC solutions exhibit high ionic conductivity, in the range of 1 to 1.4 mS cm−1 and anodic stability window of up to 2.2 V vs Mg. Unfortunately, the encouraging demonstration of first prototype RMG, though it is scientifically exciting, is not enough to compete with LIB due to the relatively low operating voltage and the low capacity of the Chevrel phase cathode used in this battery system (more discussion of the Chevrel phase cathode can be found later section of this paper). In order to increase the energy density, one obvious approach would be to extend the electrochemical voltage of the electrolyte and hence the voltage of the cell (provided that an appropriate cathode is also found). On that account, Aurbach group discovered the oxidation mechanism of DCC (elimination of hydrogen on carbon β to the Al−C bond) and developed a second generation of electrolyte, namely all phenyl complex (APC) electrolyte, which is a stabilized version of the DCC electrolyte by substitution of the alkyl group in the complex with aromatic groups.41,42 This electrolyte was synthesized by in situ reaction of AlCl3 with PhMgCl (equivalent ratio of 1:2).43 The APC maintained the excellent 100% reversibility of the anode reaction, ionic conductivity of 2 mS cm−1 and demonstrated significant enhancement of the anodic stability from 2.2 to 3.2 V, all of which could potentially enable the utilization of high voltage cathode materials (e.g., V2O5). APC is a good example for emphasizing the difference between conventional electrolytes used for LIB and some of the

reaction stability limit of Mg electrolytes as higher voltage class cathodes. Compared to the Chevrel phase cathodes, most of higher voltage class cathodes had very little success in terms of their achieved capacity and cycling stability for the following reasons. First, many higher voltage class cathodes, mainly metal oxides, suffer from very slow Mg2+ insertion reaction kinetics that results in little or no Mg2+ insertion. This phenomenon has been attributed to the strong electrostatic interaction between the doubly charged Mg2+ ions and surrounding ionic environments (i.e., electrolytes and cathode ionic matrix) that hinder the insertion of Mg2+. Second, the stable voltage window of most Mg electrolytes is not adequate to accommodate the operating voltage of these cathodes which is typically greater than 3 V vs Mg/Mg2+. In addition, Mg electrolytes, many of which contain chloride species and Grignard reagent type of compounds, can be often very corrosive to oxide materials. Several metal oxides have been explored as Mg-ion cathodes, including V2O5,9,22−26 MnO2,7,8,27−29 and MgCo2O4.10,30 Conversion Cathodes. Due to the sluggish Mg2+ insertion and limited anodic stability of the Mg electrolytes, there are only limited choices of high voltage Mg cathodes. As a result, the reported energy of Mg battery falls far behind from that of Li-ion batteries. For example, the theoretical energy density of the first prototype Mg battery utilizing Mo3S4 as a cathode is less than half of the commercialized Li-ion batteries and only comparable to Ni−Cd battery system.12 An alternative way to improve the energy density of a battery system is to utilize a cathode with high capacity. In this regards, sulfur, which exhibits a very high specific capacity (>1600 mAhg−1), has attracted great interest in Li/S,31 Mg/S,32 Al/S,33 and Na/S34 battery systems. When employed in a Mg battery system, sulfur provides a theoretical capacity of 957 mAhg−1 with an average operating voltage of 1.77 V.35 Therefore, the theoretical specific energy density of Mg/S battery is more than four times of that of a commercial LiCoO2/ graphite cell despite the relatively low operating voltage of Mg/S system. Furthermore, the low cost of both Mg and sulfur makes the Mg/S a great candidate for the next generation of battery. Another interesting aspect of sulfur as a Mg cathode is that the reduction of sulfur takes place through the conversion of sulfur to sulfide, instead of the interstitial diffusion of Mg2+. This unique conversion pathway of sulfur redox process may provide an advantage over many oxide-based cathodes, which often suffer from the problematic slow Mg2+ insertion kinetics. However, the electrophilic sulfur spontaneously reacts with many of the in situ generated Mg electrolytes due to nucleophilic moieties exists in them. Additionally, even when nonnucleophilic electrolyte is used, it has been reported that the MgS formed upon the discharge process is irreversible leading to fast capacity decay.32 Brief Review on Reported Electrolytes. In order to ease the discussion and establish common language, we will briefly introduce the main electrolyte solutions reported so far for RMB in this section. For more thorough discussion of electrolytes with detailed description of the electrolyte molecular structure and chemical synthesis and reactions, the reader is encouraged to read the comprehensive reviews written by Aurbach group and Muldoon research team.1,3,4,36−38 The electrochemical potential of Mg metal is less negative than the lithium; however, it is negative enough to reduce most of the conventional electrolyte solutions that are commonly used in LIBs. Both solvents (electrophilic as nitrile or alkyl carbonates) and anions (perchlorate, PF6−, etc.) of the 1739



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process in order to achieve the desired electrochemical performance in MACC electrolyte.51,52 The conditioning process results in free Cl− anions in the electrolyte, where Cl− depassivates the electrode surface thereby allowing a reversible Mg deposition/dissolution.14 Atomistic studies done by Canepa et al. elucidated the structure of the electrolyte and the important role of charged (MgCl)+ complexes.89,90 However, the high chloride content significantly lowers the anodic reaction stability down to 1.8 V vs Mg/Mg2+ toward stainless steel. The prototype Mg battery employing MACC electrolyte and Chevrel phase cathode has shown electrochemical performance with 100% CE and stable cell capacity after 90 charge/discharge cycles.50 Borohydride to Boron-Cluster. Mohtadi et al. have introduced the use of magnesium borohydride Mg(BH4)2 for reversible Mg deposition/dissolution inspired by the strong reducing power of Mg(BH4)2.53 This inorganic and halide-free electrolyte provides 94% of CE in the presence of LiBH4, by which the ionic pair of Mg(BH4)2 species is reduced. A study by Shao et al. has further improved the CE of Mg(BH4)2 electrolyte by replacing the solvent to diglyme from monoglyme.54 The critical weakness of borohydride-based electrolyte is in its relatively poor anodic stability (2.4 V vs Mg on glassy carbon). In an effort to improve the anodic stability of borohyride, Carter et al. studied other borohydride compound, MgB12H12, which is known for excellent anodic stability of its anion (4 V vs Mg).55 However, due to the low solubility of MgB12H12 in ethers, they have intentionally reacted a carborane cluster with Grignard reagent, after which the synthesized Mg salt becomes soluble in THF. The reaction product of this carborane with Grignard reagent resulted in 1-(1,7-carboranyl MgCl). It was reported that the boron-cluster electrolyte provides the maximum 94.4% CE and the anodic stability of 3.2 V vs Mg with non-noble current collectors. A follow-on work was conducted by the same research group where a halogen-free (Grignard-free) electrolyte was demonstrated based on the reaction products of AgCB11H12 and MgBr2 in THF. The produced simple salt with composition of Mg(CB11H12)2 showed an impressive stability toward most current collectors with an anodic stability of ∼3.8 V vs Mg (Figure 2a).56 The great compatibility toward Mg anode, high anodic stability, and noncorrosive nature of boron-cluster electrolyte make it great candidate for an electrolyte system to be coupled with higher voltage cathodes. Indeed, Tutusaus et al. managed to test the first coincell with high voltage cathode in full RMB, without limitation on charging by corrosion of the stainless steel for 10 stable charge/discharge cycles with α-MnO2 cathodes (Figure 2b).56 It should be noted that the low cycling stability with K-αMnO2 cathode was found to be linked with the nature of the electrochemical reaction in this specific cathode.57 Unlike Li that can be inserted reversibly into K-αMnO2, discharge in the presence of Mg induced conversion reaction with formation of two wide band gap insulators, MgO and MnO on the cathode interface that blocked the reversibility and limited cycle life of the battery. Thus, regardless of the relatively low cycling performance demonstrated with K-αMnO2, the presented electrochemistry of boron-cluster electrolyte is an impressive achievement as a full cell system consists of a higher voltage class cathode and fully Mg compatible electrolyte, providing a promising outlook for high voltage Mg battery. Bis(trifluoromethylsulfonyl)imide (TFSI)/Glyme-Based. There have been increasing amount of interests toward magnesium

electrolytes used for RMB and will help to illustrate the novelty and importance of the methodology recently developed by Muldoon and co-workers.44 In LIB, Li salt is dissolved in the solvent in a controllable concentrations and composition of the salt into the hosting organic medium, the identity of the solvent and salt can hence be tuned to the desirable battery system. On the other hand, APC electrolyte is synthesized by in situ reaction between PhxMgCl2−x and PhyAlCl3−y, for example: 9 PhMgCl + 4.5 AlCl3 → Ph4Al− + 2 Ph2AlCl2− + PhAlCl3− + 0.5 AlCl4− + 4.5 Mg2Cl3+, this kind of reaction, form various species coexist under dynamic equilibrium in the ether solvent, not like the simple case of electrolyte for LIB. One disadvantage of this mixture of species is that some of them are undesirable, in that they may corrode current collectors or react with electrophilic cathodes (e.g., sulfur). To overcome that, Muldoon and co-workers have developed multiple systems by various approaches to decrease the negative effect of corrosive species and to facilitate non-nucleophilic electrolytes for sulfur cathodes. The new strategies will be discussed in more details in the designated section in this perspective. In general, by reacting Hauser-based compound HMDSMgCl with Lewis acid AlCl3, Kim et al.44 were able to demonstrate a new nonnucleophilic electrolyte with superior properties compared to the HMDSMgCl system reported by Liebenow.45 Furthermore, they clearly showed that crystallization of the electrochemically active species can enhance the potential stability (3.2 V vs Mg) and CE. Fichtner and co-workers have recently demonstrated the synthesis of this electrolyte by reacting HMDS2Mg with AlCl3, and they demonstrated that their approach enables choice of ethers other than THF and they utilized this electrolyte in rechargeable Mg/S battery.32,46 In order to overcome the corrosion of current collectors by Mg electrolyte, Muldoon et al. have identified the corroding species in the chloride containing electrolyte and synthesized magnesium organoborate electrolyte with enhanced anodic stability and current collector corrosion resistivity (up to 4 V vs Mg on stainless still) via ion exchange strategy.15 We believe this work can become extremely important if the reactivity of the high voltage magnesium organoborate with magnesium metal is properly addressed. Another important development, with respect to boron-based electrolyte, was achieved by Guo et al. through reaction of tri(3,5-dimethylphenyl)borane and PhMgCl in THF.47,48 This electrolyte showed high anodic reaction stability (3.5 V vs Mg) and close to 100% CE. As stated previously, more extensive discussion of these electrolyte solutions can be found in recent review papers;1−5 thus, only brief description was covered herein for ease of discussion and clarity. Instead, more focus has been given onto the few systems that are less extensively covered and relatively new as will be discussed in the following. Magnesium Aluminum Chloride Complex (MACC). It should be noted that the majority of electrolytes for Mg battery contain heavy organometallic magnesium complex. These types of electrolytes, however, could be highly hazardous and hardly dissociate especially at the oxide surfaces. In this regard, Aurbach and Liu groups have demonstrated simple yet stable inorganic electrolyte which is composed of reaction products of MgCl2 and AlCl3 in THF, namely magnesium aluminum chloride complex (MACC).49,50 This inorganic Mg electrolyte is shown to have extended voltage window (3.4 V vs Mg with Pt and glass carbon) by eliminating Grignard organometallic species, which of many are easily oxidized. In recent studies by Barile et al., it was discovered that there needs to be an electrolytic precycling 1740



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Figure 2. (a) Linear sweep voltammograms of 0.75 M Mg(CB11H12)2/tetraglyme electrolyte on different electrode materials with a scan rate of 5 mVs−1. (b) Initial discharge−charge profiles of a rechargeable Mg battery with 0.75 M Mg(CB11H12)2/tetraglyme (black line) and 0.2 M APC (red line) as the electrolyte, a Mg anode, and a α-MnO2 cathode under a constant current density of 0.2 mAcm−2.56 Reprinted with permission from ref 56. Copyright 2015, Wiley-VCH.

Figure 3. (a) Selected CV cycles during the conditioning procedure of MgTFSI2 0.25 M with MgCl2 0.5 M in DME solution. Pt served as the WE, and Mg as the RE and CE, scan rate 1 mV/s. (b) Repeated cyclic voltammograms of composite Chevrel phase electrodes (85% Mo6S8, 10% carbon black, 5% PVDF), which undergo repeated intercalation/deintercalation processes with Mg2+ ions in MgTFSI2 0.25 M + MgCl2 0.5 M/DME solution, 0.1 mV/s.59 Reprinted from ref 59, available under Creative Commons Attribution license. Copyright 2015, Sheternberg et al.

A recent study by Shterenberg et al. provided very interesting results for Mg(TFSI)2-based electrolyte where it was demonstrated that the addition of MgCl2 to Mg(TFSI)2/glyme (DME) solution yields superior electrochemical behavior (98% CE and 3.1 V of voltage window after conditioning process) compared to that of the Mg(TFSI)2/glyme solution alone (60% CE).59 The significantly improved electrochemical performance of MgCl2 + Mg(TFSI)2/DME has been attributed to the presence of chloride species that not only stabilize the Mg2+ cation but also preferentially adsorb onto the surface of Mg, thereby reducing harmful reactions between the Mg surface and organic species. It was also discussed in the study that the electrochemical performance of the MgCl2 + Mg(TFSI)2 electrolyte strongly depends on the purity of the electrolyte solution supported by the observed improvement of CE and reduced overpotential upon the conditioning process (electrochemically or chemically), during which impurities are eliminated (Figure 3a). Shterenberg et al. provided a preliminary result of a full cell test utilizing Chevrel phase cathode which further proved the excellent compatibility of the conditioned MgCl2 + Mg(TFSI)2 electrolyte

bis(trifluoromethylsulfonyl)imide, Mg(TFSI)2, both in liquid electrolyte58−60 and ionic liquid61−63 systems as a promising Mg electrolyte candidate. Such TFSI−-anion-based electrolytes exhibits high anodic reaction stability, possesses excellent ionic conductivity, and reduces current collector corrosion. As Mg electrolyte, Mg(TFSI)2 is one of the very few Mg salts that possesses reasonable solubility in ethereal solvents, which is the only type of solvents that was demonstrated as free from parasitic reaction toward Mg anode.59 Note that we have categorized TFSI/glyme-based electrolyte separately from the conventional type electrolyte as we believe that the reported electrochemical performance of these electrolytes is highly significant and deserves in-depth discussion. Motivated by favorable properties of Mg(TFSI)2 as an Mg electrolyte, several studies investigated the Mg deposition/ dissolution behavior and speciation chemistries of Mg(TFSI)2/ glyme.58,61,63,64 However, the formation of passivation layers on Mg anode surface could not be completely avoided in Mg(TFSI)2/glyme solutions as can be implied by the reported poor reversibility and low stability. 1741



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upon crystallization. Therefore, we believe the crystallization process of the electrolytes, although it adds another step to the already complicated electrolyte synthesis process, could play a significant role in achieving the breakthrough of technology for RMB either by extending the voltage or enabling reversible Mg/S battery system. Nanostructured Electrode. Investigating nanostructured electrodes to improve the electrode kinetics has been one of the most devoted research areas in the field of rechargeable batteries. Potentially, nanostructured electrodes provide unique benefits for a battery system including superior ion diffusion and electron transport as well as large surface area compared to the bulk electrodes. These advantages of nanostructured electrodes especially become more useful for many oxide-based Mg cathodes as a strategy to overcome the serious kinetic barrier of Mg2+ insertion process. For example, nanosizing the electrode has shown to effectively improve the Mg2+ capacity of a few high voltage cathodes either via insertion or conversion, such as manganese oxide (conversion) with APC65 and other oxides in conventional electrolytes.7,22 Dual-Ion Approach. The lack of high capacity and fast kinetic Mg cathode has resulted in increased attention toward a new concept of Mg battery, namely hybrid magnesium−lithium ion battery or dual-ion Mg battery. This dual-ion approach takes advantage of dendrite free Mg anode and high capacity Li-ion cathode where both Li+ and Mg2+ ions are present in the electrolyte. On the anode side, only Mg is reversibly deposited and dissolved because the redox potential for Li deposition/ dissolution is 0.67 V lower than that of Mg deposition/ dissolution.66 On the cathode side, Li+ insertion takes place which results in high capacity and good cycling stability. This dual-ion approach has been applied to many recent studies coupled with APC-,67 ((HMDS)2Mg + AlCl3)-,35 and (MgCl2 + Mg(TFSI)2)59-based electrolytes where improved cathode capacity and cycling stability were achieved compared to the Mg battery system. For example, the dual-ion approach was recently demonstrated in (HMDS)2Mg + AlCl3 electrolyte with sulfur cathode system where Li-ion plays role as a mediator for the enhanced reversibility of Mg/S battery (Figure 4).35 Indeed, the presence of Li-ion increases the solubility of the discharged Mg sulfide species and thus electrochemical reaction reversibility, which has been attributed to the possible participation of the Li-ion in the electrochemical reaction on the cathode side.

with Mg anode (Figure 3b). However, the stability of this electrolyte against the corrosion of common current collectors might be an issue due to the presence of chloride, although the exact value has not been reported. Conventional. Due to the lack of Mg electrolyte that has wide enough voltage window for higher voltage Mg-ion cathodes, many studies involving high voltage cathodes thus far have often employed conventional electrolytes, such as Mg(ClO4)2 in acetonitrile or propylene carbonate, that are highly stable for high voltage cathodes (above 4 V vs Mg/ Mg2+).7−11,22,29 Unfortunately, because the conventional electrolytes are not compatible with Mg anode, no functional full-cell electrochemistry was able to be demonstrated thus far. Instead, most studies have focused on studying the cathode system alone by utilizing the conventional electrolytes either by performing electrochemical testing in a three-electrode-cell configuration or using a very slow current so that the passivation layer formation could be slow enough to demonstrate the first few charge/ discharge cycles. Strategies to Overcome the Limiting Factors. Crystallization of Mg Electrolyte. As discussed in the introduction, majority of demonstrated RMB systems are utilizing in situ generated electrolyte. In many cases, the in situ generated electrolytes contain undesired species (e.g., corrosive or nucleophilic components), which make the utilization of this electrolyte impossible when conventional current collector (e.g., aluminum, stainless still) or electrophilic (e.g., sulfur with APC) cathode is used. Electrolytes which are less corrosive, and nonnucleophilic were demonstrated by Liebenow et al.45 however, their anodic stability was too low. In efforts to avoid this complicated electrolyte “soup”,3 a new approach has been introduced by Muldoon and co-workers research team.44 They demonstrated that crystallization process of the in situ generated electrolytes could improve the overall electrochemical performances, in terms of anodic stability and Coulombic efficiency, and also remove the nucleophilic species in the electrolyte. For example, the authors discovered that crystallization of their (HMDSMgCl + AlCl3)based electrolyte extended the electrochemical window from 2.5 to 3.2 V vs Mg. This enabled the utilization of this nonnucleophilic electrolyte in a full Mg/S system. Furthermore, the authors proposed that the crystallization approach can also facilitate non-nucleophilic organohaloaluminate electrolyte, due to removal of PhMgCl and Ph2Mg nucleophilic components

Figure 4. (a) Cyclic voltammograms of Mg deposition/dissolution in a three-electrode cell at a scan rate of 100 mV/s at room temperature. Inset: CE of Mg deposition/dissolution and electrochemical stability of the electrolyte. The onset of electrolyte decomposing can be identified through the intersection of x axis with the tangent of current rise at the end of anodic scan (dashed line). (b) Cycling stability of the Mg/S battery in electrolyte with and without LiTFSI.35 Reprinted from ref 35. 1742



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of water molecules after substantial electrochemical reaction in the water-containing electrolyte; MnO2 undergoes structural reorientation upon cycling in the water-containing electrolyte by which improved Mg2+ insertion was continuously observed in a dry electrolyte system. A similar phenomenon was reported by Nam et al. where it was shown that the crystal water molecules in MnO2 play a significant role in the Mg2+ insertion.8,72 This study has shown that the cycling in aqueous Mg electrolyte induces a phase transformation from a spinel-Mn3O4 to a crystal water-containing birnissite-MnO2, after which an improved Mg2+ insertion capacity was observed in an aqueous electrolyte. Unfortunately, it should be noted that the presence of water molecules will be fatal to Mg anode; hence, water cannot be used as ultimate solution. Therefore, one could consider developing an alternative strategy that would reduce the effective interaction between Mg2+ and host lattice. For example, Liang et al. have shown that the interaction between Mg2+ and host lattice of MoS2 is significantly weakened by expanding the interlayer distance of the host compound.73 In the study, a controlled amount of poly(ethylene oxide) was inserted between the layered MoS2 prior to the Mg2+ intercalation. Both first-principles simulation and experimental results have shown that the diffusivity of Mg2+ in MoS2, which is not active for Mg2+ intercalation in its pure form, is drastically improved upon the interlayer expansion. This result suggests a promising outlook that the interlayer expansion approach could potentially be applied to other layered-type and higher voltage host compounds. Strategies for Studying High Voltage Cathode Reaction. In searching for promising Mg-ion cathode candidates and evaluating their prospect in Mg battery application, computational studies provide invaluable information where the firstprinciple calculation determines the essential parameters of a cathode compound including operating voltage, phase stability, and Mg-ion mobility. For example, recent computational studies by Ceder group investigate the feasibility of several compounds as multivalent-ion battery cathodes, from which information such as structural topology of multivalent-ion cathodes that offers high

It needs to be stated that though the dual ion strategy possess an inherent challenge, due the potential solubility limit of one of the cation in the electrolyte, it can be optimized when proper composition is chosen as demonstrated by Yoo et al.; hence employing Li salts with greater solubility may increase the energy density of dual-ion system.66 Although the role of Li+ in most dual-ion studies have been accredited to the fast solid-state kinetics of Li+ complementing the limited diffusion of Mg2+, more careful attention needs to be given to anode side due to potential alloying of Mg−Li (Mg-rich) on the anode side68 Reducing the Mg2+ And Host Lattice Interaction. Because the limited diffusion of Mg2+ in most host compound is caused by the strong interaction between Mg2+ and host lattice that impede Mg2+ mobility in the host, one obvious strategies to improve the Mg2+ ion kinetics would be to weaken this interaction. It has been well established that utilizing aqueous electrolyte or adding strong dipole molecules in the Mg-ion cathode system significantly improves the Mg insertion capacity.7,8,27,69,70 For example, Novak et al. first demonstrated that the trace amount of water molecules in the organic Mg electrolyte enhances Mg2+ insertion/extraction reaction in layered V2O5, where the water solvation shell mitigates the electrostatic force during the insertion/extraction process.70 Recent study by Gautam et al., further investigated the role of water molecules during the Mg2+ insertion in Xerogel-V2O5 by DFT calculation and found that the presence of water molecules affects the phase change behavior during the Mg2+ insertion as well as the overall Mg2+ insertion voltage.71 Recently, we have reported a similar approach for MnO2, where water molecules were added into the organic electrolytes (i.e., Mg(ClO4)2/propylene carbonate) in which MnO2 nanowire electrodes were tested.7 It was found that the water molecules indeed enhances Mg2+ insertion capacity as well as reversibility, where the degree of improvement was found to be dependent on the ratio between Mg2+ and water. In addition, it was demonstrated that the MnO2 electrode continues to show improved Mg2+ insertion capacity even in the absence

Figure 5. (a) V2O5 structure of α and δ polymorphs on the a−b plane with the yellow spheres indicating the intercalant sites. As indicated by the dashed blue regions, both the polymorphs differ by a change in the stacking of the V2O5 layers. (b) Activation barriers for the diffusion of the different intercalating ions in the α and δ polymorphs are plotted. The solid lines correspond to the empty lattice limit (charged state), whereas the hollow lines correspond to the full lattice limit (discharged state).25 Reprinted from ref 25, available under Creative Commons Attribution license. Copyright 2015, Gautam et al. 1743



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Mg2+ ion mobility can be obtained.25,74−76 Figure 5 shows the calculated energy paths diagram for the diffusion of different intercalants in α and δ polymorphs of V2O5 demonstrated by Gautam et al.25 Although both α and δ polymorphs of V2O5 consist of layers of VO5 pyramids (Figure 5a), they differ structurally due to a shift in the layer stacking, accompanied by different interlayer distance and change of the anion coordination environment. The authors presented the computed migration energies of the intercalating ions (Li+, Mg2+, Ca2+, Zn2+, and Al3+) and demonstrated that the ionic diffusivity, which can be estimated from the maximum energy difference along the diffusion path, strongly depends on the coordination of anions within V2O5 indicated by the two very different diffusion topologies observed in α-V2O5 and δ-V2O5 (Figure 5b). MnO2 is another strong candidate for Mg battery cathode that has been received much attention in the field. Until very recently, it had been commonly assumed that the charge storage mechanism of MnO2 with Mg2+ follows similar insertion path as that of Li+. Although it is most likely the case in aqueous or water-containing electrolyte systems that Mg2+ undergoes a reversible insertion/deinsertion process in MnO2, a few recent studies suggest that Mg2+ takes different electrochemical mechanism than Li+ in organic electrolyte system. The consecutive studies by Mizuno research team suggest that MnO2 undergoes a conversion process rather than insertion of Mg2+ based on comprehensive experimental and DFT calculations.57,65,77 It was found in the study that an amorphous layer or shell composed of mixed (Mn, Mg)O is formed on the surface of MnO2 upon the first discharge, whereas the core remains as pristine MnO2. The authors concluded that the origin of this amorphous layer is the product of MnO2 conversion reaction upon the discharge, which predominantly occurs at the surface of MnO2. Further DFT calculation study revealed that the conversion reaction is more favored than the insertion of Mg2+ in MnO2 due to the high thermodynamic stability of MgO. On the basis of this conclusion, the authors suggested that the formation of amorphous layer could be the one of the possible causes for the observed poor cycling stability of MnO2 with Mg2+, along with possible dissolution of Mn and overcharging behavior (i.e., electrolyte oxidation). Although conversion mechanism was investigated specifically for MnO2, the general thermodynamic stability of MgO implies that the conversion reaction can be generalized to other metal-oxide-based cathodes in Mg2+ system. Furthermore, based upon the proposed surface conversion mechanism of MnO2 in organic electrolyte systems one could optimize the morphology and dimensionality of the material such that the surface capacity can be fully utilized while minimizing the unreacted portion of the material. The radar charts summarizing and illustrating the work that has been demonstrated are found in Figure 6. The references used to generate the charts can be found in the figure caption. Outlook for High Energy RMB. The presented radar charts, for which we have designed in order to illustrate the work that have been done in the field of Mg battery, provide useful information for researchers such as the choice of proper electrolyte for new cathode structures, unexplored directions for specific electrolytes, challenges that are yet to be overcome, and challenges that are less important to focus on. We have performed a careful analysis and predicted the potential significance and merit for further investigation, which will be discussed in this section. The proposed outlook is represented

in the radar charts by providing a few promising research directions and strategies (Figure 7). Outlook for High Voltage Cathode in RMB Electrolyte. In order to demonstrate a high-voltage Mg battery and be able to provide an energy density that is comparable to that of Li-ion battery, there needs to be (1) a high voltage cathode with good Mg2+ mobility and (2) an electrolyte which possesses a stable voltage window that can accommodate the cell operating voltage. However, the presented radar charts suggest that there is still a serious limitation in the realized Mg2+ insertion capacity with high voltage cathodes even with the electrolytes whose voltage window is sufficient for higher voltage cathodes (i.e., ≥ 3.2 V vs Mg/Mg2+). Understanding the interfacial reactions and interactions between the anode, cathode, and electrolyte solution species in a battery is critical to its development. Many of the reactions at these interfaces can hinder battery performance, such as the case of the ion-blocking anode passivation layer in Mg battery systems. Similarly, it is well known that the SEI layer that forms on both anodes and cathodes in lithium battery systems greatly impacts performance. With regard to cathodes for RMB, thin protection layers with desired functionality could help improve the reversible cyclability and lifetime of cathodes. Some important properties for protection layers are good conductivity of the divalent Mg2+ ion, electrochemical stability, and robust structure (i.e., good mechanical stability). Recent studies on protected lithium cathodes have shown that application of the solid electrolyte lithium phosphorus oxynitride (LiPON) as a protection layer may improve cathode performances.78 We suggest that utilization of the aforementioned computational studies (done by Ceder group) can help find appropriate surface modification materials for high voltage Mg-ion cathodes, similar to what has been done with Li-ion cathodes,79,80 which could help facilitating Mg-ion insertion by mediating the interfacial electrochemistry and preventing undesired surface reactions between the oxide and chloride species from the electrolyte. This discovery could help open the choice of viable cathodes, especially at higher voltages where there is increased possibility for undesirable interfacial oxidation reactions, in addition to improving performance by mediating the electrochemical reaction at the electrode interface. We also claim that, as the radar charts indicate, some of the electrolytes with reasonable anodic stability have not been explored with some of the possible high voltage cathodes in nanostructured cathodes, dual ion electrolyte, or both. Further studies are encouraged to explore these directions (Figure 7). For example, studying the compatibility of high voltage class cathodes with organoborate and MACC electrolytes, both of which have shown to exhibit high anodic stabilities, could bring to interesting insight in moving toward the realization of the high voltage Mg battery. Therefore, investigating these unexplored fields to answer the existing scientific questions possess potential significance that could lead to a next breakthrough in RMB. Outlook for High Capacity Cathode in RMB Electrolyte. The electrolyte crystallization strategy demonstrated by Muldoon research team for (HMDS + AlCl3)-based electrolyte may open a new direction for utilization of APC electrolyte in Mg/sulfur system with possibly slightly different electrochemical behavior of the polysulfide. MACC electrolyte was also proposed by Liu et al. as non-nucleophilic electrolyte with some preliminary evidence of chemical compatibility of MgCl2−AlCl3 and 1744



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Figure 6. Radar charts for Mg electrolytes illustrating reported performances. The dotted area represent accomplished improvements by utilizing the indicated strategies: (a) APC,41−43,65 (b) HMDS-based + AlCl3,4,44,17,32,35,46 (c) Mg organoborate,15,47,48 (d) MACC,14,49−51 (e) boron-cluster,55,56 (f) Mg(TFSI)2 + MgCl2,59 and (g) conventional.7−10,22 *Experimental data was not provided but expected to provide at least the shown performance.

MgCl2−AlEtCl2 electrolyte with sulfur via NMR studies.50 The authors strongly suggested the possible application of MACC electrolyte for Mg/S battery system upon further optimization. Overall, sulfur cathode if realized as a reversible cathode in full RMB may become a breakthrough in terms of energy density of RMB. Outlook for artificial solid electrolyte interphase on Mg anode. On the other side of the battery, protection layers on the Mg metal anode, also called “artificial SEI layers”, could have positive effects on battery performance as well. Despite its challenges, this task has been suggested by a few researchers as a possible next step in the Mg battery research.8,81 Ideally, the protection layer should be a Mg-ion conductor with high transference number, allow reversible magnesium deposition/ dissolution, enable a high CE, and protect the magnesium metal from parasitic reactions. Although improvements in other areas, such as the electrolyte, have been the focus for making a RMB with a metal anode, successful metal anode protection could have many benefits. One issue anode protection could solve is the use of conventional electrolyte for Mg battery systems. Currently, using conventional electrolytes is almost impossible unless a different type of anode replaces the Mg anode.

Ideally, the protection layer should be a Mg-ion conductor with high transference number, allow reversible magnesium deposition/ dissolution, enable a high CE, and protect the magnesium metal from parasitic reactions. Although improvements in other areas, such as the electrolyte, have been the focus for making a RMB with a metal anode, successful metal anode protection could have many benefits. Surface modification strategies have been demonstrated in Li battery where the surface of Li metal was effectively protected from undesired reactions (i.e., dendrite formation).82−85 Similar to the cathode side, solid electrolytes such as LISICON85 and 1745



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Figure 7. Radar charts for Mg electrolytes with proposed outlook and promising research directions: (a) APC,41−43,65 (b) HMDS-based + AlCl3,4,44,17,32,35,46 (c) Mg organoborate,15,47,48 (d) MACC,14,49−51 (e) boron-cluster,55,56 (f) Mg(TFSI)2 + MgCl2,59 and (g) conventional.7−10,22 *Experimental data was not provided but expected to provide at least the shown performance.

LiPON have been used to protect Li metal anodes. These studies on Li metal anode protection highlight the importance of solid-state materials that are Mg-ion conductive and demonstrate how anode protection could improve interfacial interactions and overall performance. Although solid electrolytes have been the subject of some research work in Mg battery, these materials, including inorganic solid and polymer/gel electrolytes, are primarily investigated to help solve problems associated with liquid electrolytes such as incompatibility with magnesium metal, corrosion of current collectors, and volatility.81,86−88 However, it could still be more cost-effective and commercially seamless if conventional electrolytes with known wide voltage window could be utilized for Mg batteries. Therefore, if we can synthesize solid electrolytes that can be used as a protective layer for Mg surface, it may be possible to use the scientific discoveries learned from Mg solid state electrolyte toward the utilization of the conventional electrolytes and ultimately achieve a high voltage Mg battery. Discovering new Mg solid state electrolyte with good Mg2+ mobility and stability will certainly be a significant breakthrough in Mg battery technology. However, the slow solid state kinetics of Mg-ion will make this task extremely challenging.

Discovering new Mg solid state electrolyte with good Mg2+ mobility and stability will certainly be a significant breakthrough in Mg battery technology. However, the slow solid state kinetics of Mg-ion will make this task extremely challenging. Nonetheless, we believe that the initial studies on the Mg solid state electrolytes combined with the recent computational studies provide guidance for material design; these materials are certainly promising not only toward applications of Mg solid state battery, but also toward surface protection of Mg anode. Conclusion. In this Perspective, we have provided convenient radar charts for reported Mg electrolytes that illustrate the electrolyte’s performances evaluated under the six important components. The extensive research efforts in development of 1746


The Journal of Physical Chemistry Letters Mg electrolytes have resulted in significant accomplishments as can be seen from the numbers of recently reported Mg electrolytes that not only exhibit excellent compatibility with Mg anode but also possess high stability. This continuing advancement in Mg electrolytes provides an optimiztic prospect in moving toward a high energy density Mg battery. On the other hand, we learn from the radar charts that one of the critical remaining challenges is the lack of cathode materials that can provide both good Mg2+ insertion capability and high operating voltage. Therefore, we believe that it is worthwhile to devote research efforts on employing the discussed strategies including nanostructuring the cathode, surface modification of the cathode, and dual-ion approach in order to optimize the capacity of the high voltage cathode. Alternatively, another promising approach to obtain high energy density in Mg battery system would be to utilize sulfur cathode, whose capacity is high enough to compensate its relatively low operating voltage. Finally, we also suggest finding a scientifically informed protection layer for the anode side for possible utilization of Mg metal anode in conventional electrolyte that will enable conjugation with high voltage cathodes, which have demonstrated promising Mg-ion insertion behaviors (e.g., V2O5, MnO2).

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ACKNOWLEDGMENTS

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REFERENCES

Perspective

We would like to thank Dr. Junkai Hu for his help designing and creating the cover art for this Perspective. This work was supported as part of the Nanostructures for Electrical Energy Storage (NEES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award Number DESC0001160.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Jaehee Song is currently a Ph.D. candidate in Chemistry at the University of Maryland, College Park, MD, U.S.A. She received her B.S. in Chemistry from the University of Maryland, College Park, MD, in 2010. Her research interests include synthesis and analysis of nanostructured materials for electric energy storage devices. Emily Sahadeo is a Ph.D. student in Chemistry at the University of Maryland, College Park, MD, U.S.A. She received her B.S. in Chemistry from Washington College in Chestertown, MD, in 2014. Her current research focuses on synthesizing both nanostructures and thin films for electrode materials in electrochemical energy storage systems. Malachi Noked is a postdoctoral fellow in the Department of Chemistry and Biochemistry and the Department of Materials Science and Engineering, University of Maryland (UMD), College Park, MD, U.S.A. He received his B.S. in Biophysics and his M.S. and Ph.D. (2013) in Chemistry from Bar-Ilan University, Israel, before arriving to UMD as a Fulbright Ilan-Ramon fellow. His research interest covers the synthesis of thin films, nanostructures, and porous carbon materials and their electrochemical behavior in electrochemical energy storage and as electrodes for water desalination. https://www. researchgate.net/profile/Malachi_Noked. Sang Bok Lee is a Professor in the Department of Chemistry and Biochemistry and the Deputy Director of the Nanostructures for Electrical Energy Storage (NEES) DOE-EFRC (Energy Frontier Research Center), University of Maryland (UMD), College Park, MD, U.S.A., and also holds an Invited Professorship at KAIST in Korea. He received his B.S., M.S., and Ph.D. (1997) in Chemistry from Seoul National University, Korea. His research focuses on the synthesis and control of heterogeneous nanostructures and the electrochemistry of nanomaterials from high power energy storage to electronics to nanomedicine. http://www.chem.umd.edu/sang-bok-lee/. 1747



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