Magnesium Aluminum Chloride Complex Research Articles

Unlocking Four-electron Conversion in Tellurium Cathodes for Advanced Magnesium-based Dual-ion Batteries.

Magnesium (Mg) batteries hold promise as a large-scale energy storage solution, but their progress has been hindered by the lack of high-performance cathodes. Here, we address this challenge by unlocking the reversible four-electron Te0/Te4+ conversion in elemental Te, enabling the demonstration of superior Mg//Te dual-ion batteries. Specifically, the classic magnesium aluminum chloride complex (MACC) electrolyte is tailored by introducing Mg bis(trifluoromethanesulfonyl)imide (Mg(TFSI)2), which initiates the Te0/Te4+ conversion with two distinct charge-storage steps. Te cathode undergoes Te/TeCl4 conversion involving Cl- as charge carriers, during which a tellurium subchloride phase is presented as an intermediate. Significantly, the Te cathode achieves a high specific capacity of 543 mAh gTe -1 and an outstanding energy density of 850 Wh kgTe -1, outperforming most of the previously reported cathodes. Our electrolyte analysis indicates that the addition of Mg(TFSI)2 reduces the overall ion-molecule interaction and mitigates the strength of ion-solvent aggregation within the MACC electrolyte, which implies the facilized Cl- dissociation from the electrolyte. Besides, Mg(TFSI)2 is verified as an essential buffer to mitigate the corrosion and passivation of Mg anodes caused by the consumption of the electrolyte MgCl2 in Mg//Te dual-ion cells. These findings provide crucial insights into the development of advanced Mg-based dual-ion batteries.

Unlocking Four‐electron Conversion in Tellurium Cathodes for Advanced Magnesium‐based Dual‐ion Batteries

AbstractMagnesium (Mg) batteries hold promise as a large‐scale energy storage solution, but their progress has been hindered by the lack of high‐performance cathodes. Here, we address this challenge by unlocking the reversible four‐electron Te0/Te4+ conversion in elemental Te, enabling the demonstration of superior Mg//Te dual‐ion batteries. Specifically, the classic magnesium aluminum chloride complex (MACC) electrolyte is tailored by introducing Mg bis(trifluoromethanesulfonyl)imide (Mg(TFSI)2), which initiates the Te0/Te4+ conversion with two distinct charge‐storage steps. Te cathode undergoes Te/TeCl4 conversion involving Cl− as charge carriers, during which a tellurium subchloride phase is presented as an intermediate. Significantly, the Te cathode achieves a high specific capacity of 543 mAh gTe−1 and an outstanding energy density of 850 Wh kgTe−1, outperforming most of the previously reported cathodes. Our electrolyte analysis indicates that the addition of Mg(TFSI)2 reduces the overall ion‐molecule interaction and mitigates the strength of ion‐solvent aggregation within the MACC electrolyte, which implies the facilized Cl− dissociation from the electrolyte. Besides, Mg(TFSI)2 is verified as an essential buffer to mitigate the corrosion and passivation of Mg anodes caused by the consumption of the electrolyte MgCl2 in Mg//Te dual‐ion cells. These findings provide crucial insights into the development of advanced Mg‐based dual‐ion batteries.

Combining experimental and theoretical vibrational spectroscopy to study magnesium aluminum chloride complex electrolytes

Measurements of the Far-IR and Raman spectra of 1:1 MACC-ether electrolytes were carried out. [(μ-Cl)2Mg2]2+ is the major dimer in DME solution and the bands at 250 and 222 cm−1 may be used as markers in the respective IR and Raman spectra. On the other hand, [MgCl]+ stands out in the THF-based electrolyte, where an IR band at 286 cm−1 is here reported for the first time in the literature and may be employed as sensor. The [(μ-Cl)3Mg2]+ dimer, which is usually formed at the 2:1 MACC-THF electrolyte and deemed the electrochemically active species on the electrode, is also identified at the 1:1 ratio. AIMD-simulated IR spectra of the dimer and monomer are in good agreement with the experimental data and were pivotal to characterize the [MgCl(THF)5]+ complex. QC calculations helped assign a few bands in the Raman spectra and Gibbs free energies explained the preferential formation of Mg species in each medium.

A Review of Magnesium Aluminum Chloride Complex Electrolytes for Mg Batteries

AbstractDeveloping suitable electrolytes with high oxidation decomposition potential, low cost, and good compatibility with electrode materials has been a critical challenge in realizing practical magnesium batteries. The emerging magnesium aluminum chloride complex (MACC) electrolytes based on inorganic chloride salts exhibit high Coulombic efficiencies for magnesium batteries. This review summarizes recent studies of MACC electrolytes, focusing on the synthesis, characterization, and chemical environment of Mg species, electrolytic conditioning of electrolytes, and their application in typical magnesium batteries. The electrolyte evolution and influencing factor of electrolytic conditioning are discussed, and several kinds of conditioning‐free MACC electrolytes are further introduced. Finally, future trends and perspectives in this field are discussed.

Activating Magnesium Electrolytes through Chemical Generation of Free Chloride and Removal of Trace Water.

Mg batteries are attractive next-generation energy storage systems due to their high natural abundance, inexpensive cost, and high theoretical capacity compared to conventional Li-ion based systems. The high energy density is achieved by electrodeposition and stripping of a Mg metal anode and requires the development of effective electrolytes enabled by a mechanistic understanding of the charge-transfer mechanism. The magnesium aluminum chloride complex (MACC) electrolyte is a good model system to study the mechanism as the solution phase speciation is known. Previously, we reported that minor addition of Mg(HMDS)2 to the MACC electrolyte causes significant improvement in the Mg deposition and stripping voltammetry resulting in good Coulombic efficiency on cycle one and, therefore, negating the need for electrochemical conditioning. To determine the cause of the improved electrochemistry, here we probe the speciation of the electrolyte after Mg(HMDS)2 addition using Raman spectroscopy, 27Al nuclear magnetic resonance spectroscopy, and 1H-29Si heteronuclear multiple bond correlation spectroscopy on MACC + Mg(HMDS)2 at various Mg(HMDS)2 concentrations. Mg(HMDS)2 scavenges trace H2O, but it also reacts with MACC complexes, namely, AlCl4-, to form free Cl-. We suggest that although both the removal of H2O and the formation of free Cl- improve electrochemistry by altering the speciation at the interface, the latter has a profound effect on electrodeposition and stripping of Mg.

Composition dependence on the spectral behavior of magnesium aluminum chloride complex electrolytes

A composition-dependent study is presented for 1:1 and 2:1 MgCl2:AlCl3 (MACC)-tetrahydrofuran (THF) electrolytes and the findings very well support electrochemical data reported in the literature. Quantitative Raman analyses in the region characteristic of the νCC and νCO modes of THF at around 915 cm−1 were performed and correlated with the low wavenumber region. The νAl-O and νMg-O vibrations are reported for the first time and are in excellent agreement with the solvates identified in the IR and Raman spectra. Although AlCl3.xTHF (x = 1–2) are present in these solutions, the systems are governed by the [AlCl4]− anion. In light of our spectroscopic results, [Mg (THF)6]2+ is the major species and responsible for the lower electrochemical performance of the 1:1 electrolyte, whereas [MgCl2 (THF)4] and [(μ-Cl)3Mg2(THF)6]+ are the complexes related with the higher electrochemical activity of the 2:1 proportion.

Conditioning-Free MgCl2/AlCl3 Electrolyte for Magnesium Batteries By Chemical Formation of Solvated Chloride

Mg metal is a promising alternative anode material that offers higher theoretical specific capacity than conventional Li-ion batteries (LIBs), and the high natural abundance of Mg makes it more economically appealing than other post-LIB anode materials. The greater energy density hinges on a Mg-metal anode, and reversible Mg electrodeposition and stripping require an effective Mg electrolyte. Various electrolyte systems that support reversible have been introduced, but many of them contain drawbacks, such as narrow electrochemical window, reactivity with electrode materials, or poor Coulombic efficiency. In addition, challenges remain in understanding the relationship between electrochemistry at the Mg anode and the electrolyte composition due to the complicated interface of Mg metal and the electrolyte.To probe the effect of the interface on Mg electrodeposition, an electrolyte with a known solution phase composition, the magnesium aluminum chloride complex (MACC) electrolyte, is explored. The MACC electrolyte undergoes electrolytic conditioning to support reversible Mg electrodeposition and stripping. During conditioning, free Cl- is released electrolytically while Al3+ is irreversibly deposited, and the concentration of the active species [Mg2(µ−Cl)3·6(THF)]+ increases, activating the MACC electrolyte.Here, we show that a small concentration of Mg(HMDS)2 to the MACC electrolyte suppresses Al3+ deposition and promotes reversible Mg electrodeposition and stripping on the first cycle. Such a drastic change from a small concentration suggests that changes are localized at the electrode-electrolyte interface. We show spectroscopically that Mg(HMDS)2 scavenges trace water in the solution. Spectroscopic results also suggest that Mg(HMDS)2 reacts with AlCl4 - in the MACC electrolyte to form free Cl-. Based on the spectroscopic and electrochemical results, we suggest that the formation of Cl- is the primary cause for improved behavior on cycle one, highlighting the role of Cl- in Mg deposition and stripping.

Conditioning-Free Mg Electrolyte by the Minor Addition of Mg(HMDS)2.

Mg-based batteries are an attractive next-generation energy storage chemistry due to the high natural abundance and inexpensive cost of Mg, along with the high theoretical energy density compared to that of conventional Li-ion chemistry. The greater energy density is predicated on a Mg metal anode, and pathways to achieving reversible Mg electrodeposition and stripping are reliant on the development of Mg electrolytes. Although Mg electrolyte chemistry has advanced significantly from the reactive Grignards of the 1920s to the carboranes of this decade, there remains significant challenges in correlating the Mg metal anode electrochemistry with the composition of the electrolyte salts as a result of the complicated interface of Mg metal and the electrolyte. To probe the effect of the interface on Mg electrodeposition, we turn to an electrolyte with a known solution-phase composition: the magnesium aluminum chloride complex (MACC) electrolyte. The MACC electrolyte requires electrolytic conditioning to support reversible Mg electrodeposition and stripping. Here, we show that a small concentration (2-5 mM) of Mg(HMDS)2 with respect to the MACC electrolyte salts suppresses Al3+ deposition and promotes reversible Mg electrodeposition and stripping in the first cycle. The significant effect of a small concentration of additive is attributed to changes to the electrode interface. The impact of the Mg interface on the observed electrochemical performance is discussed.

Effect of the Electrolyte Solvent on Redox Processes in Mg-S Batteries.

Mg-S batteries are attractive for next-generation energy storage because of their high theoretical capacity and low cost. The foremost challenge in Mg-S batteries is designing electrolytes that support reversible electrochemistry at both electrodes. Here, we target a solution-mediated reduction pathway for the S8 cathode by tailoring the electrolyte solvent. Varying the solvent in Mg-based systems is complicated because of the active nature of the solvent in solvating Mg2+and the complex dynamics of electrolyte-Mg interfaces. To understand the effect of the solvent on the S8 reduction processes in the Mg-S cell, the magnesium-aluminum chloride complex (MACC) electrolyte was prepared in different ethereal solvents. Reversible Mg electrodeposition is demonstrated in the MACC electrolyte in several solvent systems. The electrodeposition overpotentials and current densities are found to vary with the solvent, suggesting that the solvent plays a noninnocent role in the electrochemical processes at the Mg interface. Mg-S cells are prepared with the electrolytes to understand how the solvent affects the reduction of S8. A reductive wave is present in all linear-sweep voltammograms, and the peak potential varies with the solvent. The peak potential is approximately 0.8 V versus Mg/Mg2+, lower than the expected reduction potential of 1.7 V. We rule out passivation of the Mg anode as the cause for the low voltage peak potential, making processes at the S8 cathode the likely culprit. The ability to oxidize MgS with the MACC electrolyte is also examined, and we find that the oxidation current can be attributed to side reactions at the C-electrolyte interface.

Stability of Tetraglyme for Reversible Magnesium Deposition from a Magnesium Aluminum Chloride Complex

The electrochemical behavior of Mg deposition and stripping at Au-electrode in magnesium aluminum chloride complex (MACC) tetraglyme electrolyte has been studied using differential electrochemical mass spectrometry (DEMS) and scanning tunneling microscopy (STM). During Mg deposition no dendrites are formed. MACC electrolyte supports reversible Mg deposition with high coulombic efficiency. Despite of the high coulombic efficiencies that has been observed in MACC, appreciable ethylene formation due to the decomposition of tetraglyme has been detected solely during Mg stripping. The mechanism of organic solvent decomposition will be discussed. This ethylene formation is quantitatively responsible for the missing charge balance of Mg-deposition and dissolution. Possible origins will be discussed.

Comparison of the Interfacial Reaction Kinetics and Plating Morphology of Lithium and Magnesium Anodes

Li dendrite growth is a critical safety issue for batteries with lithium metal anodes. Mg metal electrodes offer a promising way to overcome this safety limitation. Yao et.al. has observed that Li and Mg plating show different morphology, where Li plating is more dendritic but Mg plating is more faceted [1]. The difference of the morphology evolution of the plated Li and Mg is related to the interfacial reactions kinetics at the electrode and electrolyte interface. One major difference is that an electrical insulating but ionic conducting solid electrolyte interphase (SEI) always covers Li anode, but Mg electrode surface is believed to be SEI free[2]. Here, atomistic simulations are combined with phase-field model to evaluate the effect of interfacial reaction kinetics on the plated Li/Mg structural morphology. Reactivity of Li with liquid electrolytes, typically ethylene carbonate (EC), forms SEI layer on the Li metal. Magnesium aluminum chloride complex in tetrahydrofuran (THF) is one of the few electrolytes showing good reversible Mg deposition/stripping with a relatively large anodic stability (~3.1 V)[3]. Thus we considered Li/SEI/EC-electrolyte and Mg/THF-electrolyte interface models. Using both static first-principles calculations and ab initio molecular dynamics, the interfacial structures, de-solvation processes and Li+/Mg2+ ion energy landscapes at Li/SEI/EC-electrolyte and Mg/THF-electrolyte, and the effects of electric field on them were investigated. The preliminary results showed that the Li and Mg metal are all negatively charged at zero voltage, but the negative charge density of Li metal is more than that of Mg metal, which is due to SEI can block the electrons into electrolyte from anode. While the Mg plating free energy change for the electrode reaction is higher than Li due to its larger solvation energy (2.5 eV for Mg vs 1.2 eV for Li from thermodynamic cycle). Coupling with atomistic calculations, phase-field simulations were performed to investigate the Li/Mg plating morphological evolution from mesoscale. With thermodynamic and kinetic parametric inputs from first-principles calculation (i.e. interfacial energy and anisotropy of Li/SEI (Li2CO3), surface energy and anisotropy of Mg, diffusion coefficient of Li+ in SEI (Li2CO3), and relative difference in charge-transfer energy barrier of Li/Mg plating), our non-linear phase-field model captures the notable morphological evolution difference between dendritic Li plating and faceted Mg plating. Furthermore, the simulation results indicate that, among all possible parameters for Li/Mg electrodeposition processes, the plating charge-transfer energy barrier difference is the major contributing factor that accounts for the morphological discrepancy. Specifically, in our systematical tests on the impact of charge-transfer energy barrier on plating morphology (a range of 0.5-10 times of the Li plating reaction energy barrier height were tested), dendrites were prominently suppressed from around 2 times and faceted morphology (hexagonal for Mg) were produced at about 4 times. This study result fundamentally explains the origin of the Li/Mg plating morphological difference, and reveals that the proper elevation of the charge-transfer barrier height (e.g. by anodic surface treatment, protective coating, upgraded electrolyte, etc.) is a promising approach to dendrite suppression in Li metal batteries.

Simultaneous Suppression of Metal Corrosion and Electrolyte Decomposition by Graphene Oxide Protective Coating in Magnesium-Ion Batteries: Toward a 4-V-Wide Potential Window.

Despite remarkable developments in electrolyte systems over the past 2 decades, magnesium-ion batteries still suffer from corrosion susceptibility and low anodic limits. Herein we describe how graphene oxide (GO), coated onto non-noble metals (Al, Cu, and stainless steel) via electrophoretic deposition, can solve this problem. In all phenyl complex electrolytes, GO coating results in a significant suppression of corrosion and extends the anodic limits (up to 4.0 V vs Mg/Mg2+) with no impact on reversible Mg plating/stripping reactions. The same effect of GO coating is also established in magnesium aluminum chloride complex electrolytes. This remarkable improvement is associated with the electrostatic interaction between the ionic charges of electrolytes and the surface-functional groups of GO. In addition, GO coating does not aggravate the cathode performance of Mo6S8, which allows the use of non-noble metals as current collectors. We also discuss the role of GO in increasing anodic limits when it is hybridized with α-MnO2.

Effect of Concentration on the Electrochemistry and Speciation of the Magnesium Aluminum Chloride Complex Electrolyte Solution.

Magnesium batteries offer an opportunity to use naturally abundant Mg and achieve large volumetric capacities reaching over four times that of conventional Li-based intercalation anodes. High volumetric capacity is enabled by the use of a Mg metal anode in which charge is stored via electrodeposition and stripping processes, however, electrolytes that support efficient Mg electrodeposition and stripping are few and are often prepared from highly reactive compounds. One interesting electrolyte solution that supports Mg deposition and stripping without the use of highly reactive reagents is the magnesium aluminum chloride complex (MACC) electrolyte. The MACC exhibits high Coulombic efficiencies and low deposition overpotentials following an electrolytic conditioning protocol that stabilizes species necessary for such behavior. Here, we discuss the effect of the MgCl2 and AlCl3 concentrations on the deposition overpotential, current density, and the conditioning process. Higher concentrations of MACC exhibit enhanced Mg electrodeposition current density and much faster conditioning. An increase in the salt concentrations causes a shift in the complex equilibria involving both cations. The conditioning process is strongly dependent on the concentration suggesting that the electrolyte is activated through a change in speciation of electrolyte complexes and is not simply due to the annihilation of electrolyte impurities. Additionally, the presence of the [Mg2(μ-Cl)3·6THF]+ in the electrolyte solution is again confirmed through careful analysis of experimental Raman spectra coupled with simulation and direct observation of the complex in sonic spray ionization mass spectrometry. Importantly, we suggest that the ∼210 cm-1 mode commonly observed in the Raman spectra of many Mg electrolytes is indicative of the C3v symmetric [Mg2(μ-Cl)3·6THF]+. The 210 cm-1 mode is present in many electrolytes containing MgCl2, so its assignment is of broad interest to the Mg electrolyte community.

(Battery Division Postdoctoral Associate Research Award Address, sponsored by MTI Corporation and the Jiang Family Foundation) The Solvation Structure of Active Cations in Next-Generation Battery Electrolytes

Next-generation batteries offer the promise of higher performance using materials that are less expensive and more abundant than those in convention Li-ion cells. From a fundamental point-of-view, the new chemistries involved in these also systems allow for exploration of the unknown and generation of new, fundamental knowledge. To that end, this talk will explore the local structure of the active cations in electrolyte solutions for Li-S and Mg batteries and the effect of this structure on the charge storage reactions and ultimately on the battery metrics. The first part of the talk will discuss the emerging solvate electrolytes as possible electrolyte solutions for Li-S batteries. The solvate electrolytes are characterized by extremely high quantities of salt resulting in interesting electrochemistry at the S cathode. Because the behavior of the electrolyte solution is governed by the complexes in the solution, we will explore the local solvation structure of the active cation, Li+, and how it is affected by the addition of what are commonly thought to be innocuous cosolvents. The addition of the cosolvents to the solvate electrolyte results in subtle but important changes in the Li+ solvation structure that affect the kinetics of the S reduction.The second part of the talk will focus on electrolytes for Mg batteries. We will explore the local structure of Mg2+ in the seemingly simple MgCl2 + AlCl3 in THF electrolyte known as the magnesium aluminum chloride complex (MACC) electrolyte. The importance of Mg speciation in active electrolytes and the effect these complexes have on the charge transfer processes at the electrode-electrolyte interface will be addressed. We will also explore the role of chloride in facilitating the electrodeposition of divalent Mg2+ and the implications of using halogenated electrolytes in full cells. Figure 1

Exploration of Novel Magnesium Battery Electrolytes based on Inorganic Salts

We present a method to produce a novel magnesium/aluminum chloride complex (MaCC) electrolyte, involving a new promoter, CuCl2. The chemically formed electrolytic solution is observed to perform Mg deposition/dissolution with high coulombic efficiency. We further propose a new mechanism for MaCC synthesis, which suggests an alternative way to synthesize Mg-conductive electrolytes, by reacting metal chloride salts with nonaqueous suspensions of metallic magnesium. A set of preliminary experiments verifies that efficient Mg deposition/dissolution can be achieved from an electrolytic solution formed by direct reaction of FeCl2 with a nonaqueous suspension of Mg powder.

Exploration of Novel Magnesium Battery Electrolytes based on Inorganic Salts

The theoretical volumetric capacity of Mg-ion batteries is far higher than state-of-the-art Li-ion batteries (3833mAh/cm3 for Mg vs. 850 mAh/cm3for Li). The development of suitable Mg-conductive electrolytic solutions has been a major barrier to Mg-ion technology: at present there is a need for liquids with wide anodic stability windows, whose interfaces allow plating and stripping of Mg with high coulombic efficiency and low deposition overpotential. We have produced a number of Mg-conductive electrolytes by direct reaction of inorganic chloride salts with suspensions of Mg powder in nonaqueous solvents. The Mg-carrying cation in these electrolytes is presumably similar to the one that provides conductivity in the recently discovered magnesium aluminum chloride complex (MACC) electrolyte [1,2], but there is apparently no metal-complex anion, and no need for an auxiliary Lewis acid during synthesis. The chemically synthesized Mg-conductive electrolyte formed by reacting ZnCl2 with Mg in tetrahydrofuran is stable up to 3.0 V vs. Mg/Mg2+, and produces an ionic conductivity of 0.24 mS/cm at room temperature. Cyclic voltammetry shows reversible Mg plating and stripping on platinum. The deposition overpotential of freshly synthesized electrolyte is below 200mV, and the coulombic efficiency of plating and stripping approaches 100%. Over a period of a week, the coulombic efficiency of plating/stripping only decays by 5%; the deposition overpotential increases more substantially with aging, however. Electrochemical Impedance Spectroscopy (EIS) was used to analyze the transport properties of the solutions. Other properties, such as composition and structure of electrolytic solutions, the morphology of Mg deposits, and thermal stability will be also discussed to explore the precise mechanism by which this and Mg electrolytes formed by other chloride salts form, as well as their feasibility for future magnesium battery applications. Barile, Christopher J. et al. J. Phys. Chem. C 118.48 (2014): 27623-27630 Vardar, Gulin, et al. Chem. Mater. 28.21 (2016): 7629-7637 Figure 1

In-situ Multimodal Imaging and Spectroscopy of Mg Electrodeposition at Electrode-Electrolyte Interfaces

We report the study of Mg cathodic electrochemical deposition on Ti and Au electrode using a multimodal approach by examining the sample area in-situ using liquid cell transmission electron microscopy (TEM), scanning transmission X-ray microscopy (STXM) and X-ray absorption spectroscopy (XAS). Magnesium Aluminum Chloride Complex was synthesized and utilized as electrolyte, where non-reversible features during in situ charging-discharging cycles were observed. During charging, a uniform Mg film was deposited on the electrode, which is consistent with the intrinsic non-dendritic nature of Mg deposition in Mg ion batteries. The Mg thin film was not dissolvable during the following discharge process. We found that such Mg thin film is hexacoordinated Mg compounds by in-situ STXM and XAS. This study provides insights on the non-reversibility issue and failure mechanism of Mg ion batteries. Also, our method provides a novel generic method to understand the in situ battery chemistry without any further sample processing, which can preserve the original nature of battery materials or electrodeposited materials. This multimodal in situ imaging and spectroscopy provides many opportunities to attack complex problems that span orders of magnitude in length and time scale, which can be applied to a broad range of the energy storage systems.

Mg/O2 Battery Based on the Magnesium–Aluminum Chloride Complex (MACC) Electrolyte

Mg/O2 cells employing a MgCl2/AlCl3/DME (MACC/DME) electrolyte are cycled and compared to cells with modified Grignard electrolytes, showing that performance of magnesium/oxygen batteries depends strongly on electrolyte composition. Discharge capacity is far greater for MACC/DME-based cells, while rechargeability in these systems is severely limited. The Mg/O2-MACC/DME discharge product comprises a mixture of Mg(ClO4)2 and MgCl2, with the latter likely formed from slow decomposition of the former. The presence of Cl in these compounds suggests that the electrolyte participates in the cell reaction or reacts readily with the initial electrochemical products. A rate study suggests that O2 diffusion in the electrolyte limits discharge capacities at higher currents. Formation of an insulating product film on the positive electrodes of Mg/O2-MACC/DME cells following deep discharge increases cell impedance substantially and likely explains the poor rechargeability. An additional impedance rise consistent with f...

The Speciation of Mg and Al in Chloride-Containing Mg Battery Electrolyte Solutions

Magnesium batteries are theoretically capable of achieving up to five times higher volumetric energy densities compared to Li ion batteries. The enhanced volumetric capacity is enabled by the use of a Mg metal anode that undergoes Mg electrodeposition and stripping processes during battery cycling. Fortunately, Mg electrodeposition largely results in morphologically smooth surfaces that bypass the risk of battery shorts due to dendrite formation. The challenge, therefore, lies in the development of electrolyte solutions from which Mg electrodeposition and stripping readily occurs. Mg metal is highly reactive and thus precludes the use of conventional battery electrolyte components such as carbonates. Currently, most electrolyte solutions from which Mg electrodeposition and stripping occur almost always contain chloride including the traditional Grignard and organohaloaluminate electrolytes in addition to the more recently developed all-inorganic magnesium aluminum chloride complex (MACC) electrolyte. Here we describe a multimodal approach to explore the activity of chloride in Mg electrolyte solutions. The MACC electrolyte solution is taken as a model electrolyte system upon which a variety of spectroscopic techniques are applied. The benefit of MACC lies in the electrolytic conditioning process that is required to activate the electrolyte providing both an “off” state and “on” state from which relative changes can be elucidated. Raman spectroscopy, 27Al NMR, 35Cl NMR, and pair distribution function analysis reveal the change in the Mg and Al related complexes in the as-prepared (off) versus conditioned (on) electrolyte solution. The Al concentration drops as the electrolyte is conditioned due to the irreversible Al plating occurring during the initial conditioning. The drop in Al concentration is accompanied by an increase in Mg concentration due to oxidation of the Mg counter electrode. The Mg complexes in both the as-prepared and conditioned electrolyte, however, are confirmed to be the elusive Mg dimer complex [Mg2(μ-Cl)3·6THF]+ commonly implicated as the active Mg complex. The Mg dimer is a common moiety found in the solid state crystal structures of dried Mg electrolytes, however, this is the first time the Mg dimer has been characterized in solution. The fact that the dimer is present in both the inactive and active electrolyte suggests that its mere presence is not the cause for increased activity. Instead, the formation of free chloride in the MACC electrolyte is responsible for enhanced Mg electrodeposition and stripping as observed by surface enhanced Raman spectroscopy. The ability of chloride to promote Mg electrodeposition and stripping has important implications for the development of Mg battery electrolytes and cathode materials. Recently, the field has been moving away from electrolytes that contain chloride due to the inherent instability of oxide cathodes with chloride-containing electrolytes. The role of chloride in the Mg electrodeposition and stripping processes must be elucidated further to determine if alternative enhancing agents can be designed.

Deciphering the Structure of the Magnesium Aluminum Chloride Electrolyte in Bulk and Near the Anode Electrode

Electrical energy storage is a key technology for a clean energy economy, but currently requires significant improvement in energy density beyond the capabilities of traditional Li-ion batteries. Mg-ion batteries are an exciting alternative in terms of the amount of energy that can be delivered, safety, manufacturing and disposal costs, with limited environmental impact. The electrochemical functions of the Mg-ion battery rely on the choice of the electrolyte, which is limited by the distinct chemistry of Mg. To date, very few electrolytes can reversibly plate and strip Mg [1,2]. The magnesium aluminum chloride complex (MACC) electrolyte can reversibly plate and strip Mg with significantly higher voltage (3.1 V) as compared to other electrolytes, but there is a pressing need to address critical questions about the structural evolution of this electrolyte during electrochemical cycling [3]. While it is well established that Mg can be deposited reversibly from complex ethereal solutions of Mg-Al chloride complex reagents, the stripping/deposition mechanism at the anode is still debated [2,3]. In this poster we present a detailed atomistic DFT study of the electrolyte in bulk and when in contact with Mg-anode surfaces. These models are essential to clarify the structure of the electrolyte under various environmental conditions as well as to provide insight on the electron-transfer mechanism occurring at the electrode/solvent interface [4,5]. [1] D. Aurbach, Z. Lu, A. Schechter, Y. Gofer, H. Gizbar, R. Turgeman, Y. Cohen, M. Moshkovich, and E. Levi, Nature 407, 724–727 (2002). [2] N. Pour, Y. Gofer, D. T Major, and D. Aurbach, J. Am. Chem. Soc. 133, 6270–6278 (2011). [3] R. E. Doe, R. Han, J. Hwang, A. J. Gmitter, I. Shterenberg, H. D. Yoo, N. Pour, and D. Aurbach, Chem. Commun. 50, 243–245 (2014). [4] P. Canepa, G. S. Gautam, R. Malik, S. Jayraman, Z. Rong, K. R. Zavadil, K. Persson, G. Ceder, Chem. Mater. 27 (9), 3317–3325 (2015). [5] P. Canepa, S. Jayaram, L. Cheng, N. N. Rajput, W. D. Richards, S. G. Gautam, L. A. Curtiss, K. A. Persson and G. Ceder, Energy Environ. Sci. 8, 3718–3730 (2015).