Electrolytes For Mg Batteries Research Articles

A Novel and Highly Efficient Indolyl‐Based Electrolyte for Mg Batteries

Magnesium‐based batteries are one of the most promising candidates as postlithium electrochemical energy storage devices. Despite considerable research progress, suitable electrolytes supporting long‐term reversible stripping/deposition of magnesium metal in a low content or halide‐free electrolyte are still of great interest. Moreover, the electrolytes should possess certain properties to be a good match with candidate cathode materials. Herein, the electrochemical performance of a novel indolyl‐based electrolyte is presented. Theoretical modeling combined with NMR analysis identifies the magnesium species in the electrolyte. Reversible stripping/deposition of Mg metal in indolyl‐based electrolyte shows long‐term cycling with high coulombic efficiency of >99.5% and yields dendrite‐free magnesium deposition in compact and smooth layers. The X‐ray diffraction shows different preferential structural orientations depending on the electrode substrate. Finally, a proof‐of‐concept Mg║Mo6S8 full cell using the newly developed electrolyte is proposed.

Probing the effect of the stoichiometric ratio of Mg(CF3SO3)2/AlCl3 on optimizing the electrolyte performance

ABSTRACT Low cost and safety qualify magnesium (Mg) metal batteries as one of the utmost post-lithium batteries. Nonetheless, most Mg electrolytes suffer from conditionality and incompatibility with most useable cathodes, hindering realising applicable Mg batteries. Herein, we study the effect of the stoichiometric ratio of Mg(CF3SO3)2/AlCl3 on the physical, electrical and electrochemical performance of an electrolyte system based on Mg(CF3SO3)2 in acetonitrile (ACN) and tetraethylene glycol dimethyl ether (G4). The physicochemical analyses include Fourier-transform infrared spectroscopy (FTIR), UV–Vis irradiation, electrochemical impedance spectroscopy (EIS) and Mg plating/stripping of electrolytes exhibit the optimum stoichiometric ratio is 0.28MAlCl3/0.17MMg(CF3SO3)2. The present study offers new approaches for designing promising efficient electrolytes for Mg batteries. The work extends to using an unconventional electrode based on sulphur, silicon carbide, and barium titanate to demonstrate the performance of the full cell. This optimised electrolyte coupled with the unconventional cathode allows a high initial specific discharge capacity, ~ 241 mAhg−1. However, the dissolution of polysulphides issue still exists and leads to irreversible cycling.

Theory-experiment partnership applied to the spectroscopic analysis of a promising conditioning-free electrolyte for Mg batteries

The addition of Mg to the 1:1 MgCl2:AlCl3-dimethoxyethane system allowed to observe for the first time at the νMgCl region, besides Raman bands at 213 and 221 cm−1 of the respective MgCl2(DME)2 and [(μ-Cl)2Mg2(DME)4]2+ complexes, a new band at 236 cm−1, which is belonging to the [(μ-Cl)4Mg3(DME)5]2+ species, as well supported by ab initio molecular dynamics (AIMD) simulations and “static” quantum-chemical (QC) calculations. Another band at 555 cm−1 is experimentally observed and shows good relationship with theoretical bands identified at 543 and 547 cm−1, which correspond to the dimer and trimer, respectively. From this work on it may be used as a sensor to monitor DME coordination, once power spectra reveal strong contribution of the Mg-O-C-C-O- δS mode as well as larger Raman scattering cross section for the trimeric species. This investigation’s results evidence that reductant Mg powder acts not only as a scavenger to remove deleterious species present in the electrolyte, but also leads to the formation of complexes considered electroactive.

Sustainable and cost-effective ternary electrolyte Et3NHCl-AlCl3-Mg(DEP)2 for high-performance rechargeable magnesium batteries.

Cost-effective and sustainable battery materials for large-scale batteries are the need of the hour to garner renewable energy with high-performance metal battery technologies. Here, we report the high-performance and long cycle life electrolyte prepared from low-cost triethylamine hydrochloride (Et3NHCl) and aluminum chloride (AlCl3) termed as (TA) with different concentrations of magnesium diethylphosphate (Mg(DEP)2) salt. The optimized ratio of the 0.1 M Mg(DEP)2 electrolyte has shown a high ionic conductivity of 4.5 × 10-3 S cm-1 at ambient temperature and good anodic stability of 2.41 V vs. Mg/Mg2+. The dissolution/deposition of magnesium (Mg) on a Pt working electrode was systematically analyzed in this electrolyte. Cyclic voltammetry (CV) of the Mg-graphite battery was used to probe the intercalation/de-intercalation of Mg-AlCl4- ions into/from the graphite layer structure. This was confirmed by various analytical techniques, such as energy dispersive X-ray spectroscopy, X-ray diffraction technique and X-ray photoemission spectroscopy. Notably, during the galvanostatic study analysis, the assembled Mg cell delivered a high discharge capacity of 115 mA h g-1 at a high C/10 rate, with more than 180 cycles at >80% coulombic efficiency. This electrolyte will be helpful in grid-scale power storage systems in future generations.

Molecular Insight into Microstructural and Dynamical Heterogeneities in Magnesium Ionic Liquid Electrolytes.

Ionic liquids (ILs) are promising designer solvents for multivalent electrolytes, enabling the modulation of molecular-level interactions of solvate species. The molecular mechanism of multivalent-ion clustering and its impact on electrolytes properties is far less studied than that of ion pairs. Herein, we explore the effect of ion clusters on the transport and electrochemical behavior of IL-based electrolytes for Mg batteries. Simulation and small-angle X-ray scattering results indicate that ILs with higher denticity effectively suppress ion agglomeration and parasitic reactions of the Mg electrolytes. Although ion clustering reduces the diffusivity of Mg2+, the Coulombic efficiency for the reversible Mg deposition/stripping process is improved, highlighting the importance of microstructural and dynamical heterogeneities in the rational design of enhanced multivalent electrolytes.

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.

Approaching Electrochemical Limits of Mg x Cl y z+ Complex-Based Electrolytes for Mg Batteries by Tailoring the Solution Structure

The future demand for energy storage requires the development of next generation batteries, e.g. based on magnesium (Mg). Mg as anode material offers great advantages such as low costs and a high volumetric capacity compared to state-of-the-art anodes. However, the lower standard potential of Mg∣Mg2+ (−2.36 V vs SHE) compared to Li∣Li+ (−3.04 V vs SHE) or Li+ intercalation/deintercalation into/from graphite (≈−2.95 V vs SHE) emerges the need for high voltage cathodes and suitable electrolytes to achieve competitive cell energy values. The oxidative stabilities of less than 3.5 V vs Mg∣Mg2+ for most of those electrolytes which enable Mg electrodeposition/-dissolution is too low to facilitate needed high-voltage Mg-based batteries. In this study, we therefore investigate the limits of oxidative stability of a commonly used Mg(TFSI)2- and MgCl2-based electrolyte by variation of solvents (ethers and ionic liquids) and salt ratios. Further on, we highlight the underlying reasons for the oxidative stability limits.

Al-compatible boron-based electrolytes for rechargeable magnesium batteries

Galvanic couple-assisted dissolution of Mg metal in an ethereal solution containing tris(2H-hexafluoroisopropyl)borate was utilized to prepare an efficient Al-compatible electrolyte for rechargeable Mg batteries. The electrolyte exhibited conditioning-free Mg deposition, high oxidative stability (3.5 V vs. Mg/Mg2+), and a superb electrochemical performance with various cathodes on an Al current collector.

Solvate Ionic Liquid Electrolytes for Mg Batteries

We report here the structures and physicochemical properties of Mg[N(SO2CF3)2]2 (MgTFSA2)/glyme solvate ionic liquid electrolytes for Mg batteries. MgTFSA2 and certain glymes, such as triglyme (G3), tetraglyme (G4), and pentaglyme (G5), form equimolar complexes.1 The melting points of [Mg(G3)]TFSA2, [Mg(G4)]TFSA2, and [Mg(G5)]TFSA2 complexes are 70, 138, and 141 °C, respectively. X-ray crystallography revealed that each glyme wraparound a Mg2+ ion and forms a [Mg(glyme)]2+ complex cation in the crystalline solvate. To decrease the melting points of (MgTFSA2)/glyme complexes, asymmetric structure was introduced to the ligand (glyme). One of the terminal methyl groups of G3 molecule was substituted with other alkyl groups having various chain lengths. In this study, we used triethylene glycol ethyl methyl ether (G3Et) triethylene glycol butyl methyl ether (G3Bu) as ligands. As we expected, the melting point of [Mg(G3Et)]TFSA2 is 57.3 °C, which is lower than that of [Mg(G3)]TFSA2.2 The entropic term of melting increased with increasing asymmetric alkyl chain length, indicating that conformational flexibility successfully decreased the melting point. [Mg(G3Bu)]TFSA2 became a glass forming liquid and maintained liquid state at room temperature. Raman spectra for the [Mg(G3Bu)]TFSA2 indicated that the 1:1 complex structure of [Mg(G3Bu)]2+ is kept even in the liquid state. [Mg(G3Bu)]TFSA2 is thermally stable and does not decompose up to 251 °C. The oxidative stability of [Mg(G3Bu)]TFSA2 was analyzed by linear sweep voltammetry. [Mg(G3Bu)]TFSA2 possesses a higher oxidative stability compared with uncoordinated glymes. This is because the HOMO energy level of glyme is lowered by the complexation with Mg2+ ion.1,3 Mg metal deposition and dissolution are also possible in this electrolyte. Physicochemical properties of [Mg(glyme)]TFSA2 complexes will be compared with those of other glyme-based solvate ionic liquids such as [Li(glyme)]TFSA and [Na(glyme)]TFSA. Acknowledgements This study was supported in part by JSPS KAKENHI (Grant Nos. 18H03926) from the Japan Society for the Promotion of Science (JSPS), the MEXT program “Elements Strategy Initiative to Form Core Research Center” of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan and the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST). References S. Tsuzuki, T. Mandai, S. Suzuki, W. Shinoda, T. Nakamura, K. Ueno, S. Seki, Y. Umebaysashi, K. Dokko, M. Watanabe, Phys. Chem. Chem. Phys., 2017, 19, 18262-18272.K. Hashimoto, S. Suzuki, Morgan L. Thomas, T. Mandai, S. Tsuzuki, K. Dokko, Phys. Chem. Chem. Phys., 2018, 20, 7998-8007.T. Mandai, K. Dokko, M. Watanabe, Chem. Rec. 2019, 19, 708-722.

(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

Hexafluorophosphate-Based Solutions for Mg Batteries and the Importance of Chlorides.

The selection of viable conventional magnesium salts in electrolyte solutions for Mg secondary batteries is very limited. Reversible magnesium deposition was demonstrated with only MgTFSI2, in ethereal solutions. A recent report has suggested that Mg can be reversibly deposited from a solution of Mg(PF6)2 in tetrahydrofuran and acetonitrile. In this paper, we dispute that claim and show that PF6- anions passivate Mg anodes and completely inhibit any Mg deposition/dissolution process. We show that addition of chlorides suppresses the passivation phenomena and allows reversible Mg deposition/dissolution processes to commence. The Mg deposits have been examined via elemental analysis, scanning electron microscopy, and X-ray diffraction measurements, depicting a highly oriented, preferential Mg growth. This study evaluates the feasibility of employing PF6-based electrolytes for Mg batteries and exemplifies the aptitude of chlorides for suppressing passivation phenomena.

Synthesis, Crystal Structure, and Electrochemical Properties of a Simple Magnesium Electrolyte for Magnesium/Sulfur Batteries

AbstractMost simple magnesium salts tend to passivate the Mg metal surface too quickly to function as electrolytes for Mg batteries. In the present work, an electroactive salt [Mg(THF)6][AlCl4]2 was synthesized and structurally characterized. The Mg electrolyte based on this simple mononuclear salt showed a high Mg cycling efficiency, good anodic stability (2.5 V vs. Mg), and high ionic conductivity (8.5 mS cm−1). Magnesium/sulfur cells employing the as‐prepared electrolyte exhibited good cycling performance over 20 cycles in the range of 0.3–2.6 V, thus indicating an electrochemically reversible conversion of S to MgS without severe passivation of the Mg metal electrode surface.

Synthesis, Crystal Structure, and Electrochemical Properties of a Simple Magnesium Electrolyte for Magnesium/Sulfur Batteries.

Most simple magnesium salts tend to passivate the Mg metal surface too quickly to function as electrolytes for Mg batteries. In the present work, an electroactive salt [Mg(THF)6 ][AlCl4 ]2 was synthesized and structurally characterized. The Mg electrolyte based on this simple mononuclear salt showed a high Mg cycling efficiency, good anodic stability (2.5 V vs. Mg), and high ionic conductivity (8.5 mS cm(-1) ). Magnesium/sulfur cells employing the as-prepared electrolyte exhibited good cycling performance over 20 cycles in the range of 0.3-2.6 V, thus indicating an electrochemically reversible conversion of S to MgS without severe passivation of the Mg metal electrode surface.

Dynamic Surface Stress Response during Reversible Mg Electrodeposition and Stripping

Through the use of in situ electrochemical surface stress measurements, Mg deposition and stripping processes in electrolytes for Mg batteries are studied. We examine four electrolytes: PhMgCl+AlCl3/THF, (DTBP)MgCl–MgCl2/THF, MgCl2+AlCl3/THF, and Mg(BH4)2+LiBH4/diglyme. Each of these electrolytes exhibits common surface stress response features, indicating that the mechanisms of Mg deposition and stripping are similar among the different electrolytes. Combining the measurements with density functional theory calculations, each part of the stress-potential curve is assigned to steps in the deposition and stripping reactions. The analysis suggests the following mechanism: (1) Mg2+/anion/solvent complexes adsorb on the substrate prior to the deposition; (2) Mg deposits as random nuclei and the deposition continues without a recrystallization process; (3) during the initial stage of Mg stripping, less coordinated Mg(0) is converted to soluble Mg(II) species and to partially oxidized species, MgOx; (4) as the anodic reactions proceed further, Mg continues to dissolve and MgOx is removed via chemical processes; (5) due to the strong interaction between Mg and the noble metal substrate atoms, the Mg layer directly bound to the substrate are the last to be anodically converted (and desorb).

Practical Stability Limits of Magnesium Electrolytes

The development of a Mg ion based energy storage system could provide several benefits relative to today's Li-ion batteries, such as improved energy density. The electrolytes for Mg batteries, which are typically designed to efficiently plate and strip Mg, have not yet been proven to work with high voltage cathode materials that are needed to achieve high energy density. One possibility is that these electrolytes are inherently unstable on porous electrodes. To determine if this is indeed the case, the electrochemical properties of a variety of electrolytes were tested using a porous carbon coating on graphite foil and stainless steel electrodes. It was determined that the oxidative stability limit on these porous electrodes is considerably reduced as compared to those found using polished platinum electrodes. Furthermore, the voltage stability was found to be about 3 V vs. Mg metal for the best performing electrolytes. These results imply the need for further research to improve the stability of Mg electrolytes to enable high voltage Mg batteries.

The Interplay of Al and Mg Speciation in Advanced Mg Battery Electrolyte Solutions.

Mg batteries are an attractive alternative to Li-based energy storage due to the possibility of higher volumetric capacities with the added advantage of using sustainable materials. A promising emerging electrolyte for Mg batteries is the magnesium aluminum chloride complex (MACC) which shows high Mg electrodeposition and stripping efficiencies and relatively high anodic stabilities. As prepared, MACC is inactive with respect to Mg deposition; however, efficient Mg electrodeposition can be achieved following an electrolytic conditioning process. Through the use of Raman spectroscopy, surface enhanced Raman spectroscopy, (27)Al and (35)Cl nuclear magnetic resonance spectroscopy, and pair distribution function analysis, we explore the active vs inactive complexes in the MACC electrolyte and demonstrate the codependence of Al and Mg speciation. These techniques report on significant changes occurring in the bulk speciation of the conditioned electrolyte relative to the as-prepared solution. Analysis shows that the active Mg complex in conditioned MACC is very likely the [Mg2(μ-Cl)3·6THF](+) complex that is observed in the solid state structure. Additionally, conditioning creates free Cl(-) in the electrolyte solution, and we suggest the free Cl(-) adsorbs at the electrode surface to enhance Mg electrodeposition.

Mg Battery Electrolytes from Aluminate and Borate Complexes in Ionic Liquids

Despite the theoretical advantages of using Mg-based electrochemistry, a rechargeable magnesium battery suitable for practical applications has yet to be realized.1,2 One of the biggest factors hampering Mg battery development is discovery of an electrolyte that is compatible with reversible deposition/dissolution at the anode as well as the chosen electrochemical process at the cathode. One of the big breakthroughs in this field came when Grignard, and Grignard like, reagents were combined with RxAlCl3-x to form Mg-organohaloaluminate complexes.3 The resulting transmetallation reactions result in electrolytes with greater conductivity and oxidative stability than their pure Grignard predecessors. Boron-based systems have also shown promising electrochemical behavior.4,5 All of the reversible Mg electrolytes have thus far only been shown to work in ether based solvents, and mainly reported specifically in THF. There are a few practical reasons for this, which will be discussed in this presentation. However, our group was interested in testing the Al and B-based systems in Ionic liquids (ILs), which have several advantageous electrochemical characteristics of their own.6 We therefore report on studies of Mg-organohaloaluminate/borate complexes in ILs. Speciation conditions within the IL media will be discussed and correlated with the electrochemical behavior. Figure 1 shows one such example in which PhMgCl was combined with AlCl3 at a 2:1 molar equivalent ratio within butyl-methylpyrrolidinium bis(trifluorosulfonyl)imide (BMPyrTFSI). This work builds on efforts in our group and elsewhere that has shown a great difficulty with obtaining reversible deposition/dissolution of Mg in ionic liquids using more conventional Mg salts.7 Data like that of figure 1 demonstrate that the appropriate combination of ionic liquid and Mg complex could provide a capable electrolyte for Mg batteries. The unusual looping from the current response in figure 1 is a result of conditions unique to ionic liquids and will be discussed. Further conditions to be considered here will be the effect of the R group, IL cation/anion, temperature, concentration and Mg:Al or Mg:B ratio in the solutions.

Evaluation of (CF3SO2)2N− (TFSI) Based Electrolyte Solutions for Mg Batteries

MgTFSI2 is the only ether-soluble “simple” magnesium salt. The poor electrochemical performance of Mg electrodes in its solutions hinders its practicality as a viable electrolyte for Mg batteries. MgTFSI2/DME solutions were demonstrated to dissolve large quantities of MgCl2 and produce electrolyte solutions with superior performance, though the electrochemical performance, mainly in terms of reversibility, of MgTFSI2/MgCl2 (DME) solutions cannot yet compete with that of organometallic based electrolyte solutions. We believe that the solutions' purity level governs the overall electrochemical performance, especially in solutions where a strong reductant (i.e Grignard reagent) is not present to act as an impurity scavenger. In this work, we alter the performance of the MgTFSI2/MgCl2 (DME) solutions through chemical and electrochemical conditioning and demonstrate the effect on the solutions’ electrochemical characteristics. We demonstrate relatively high reversible behavior of Mg deposition/dissolution with crystalline uniformity of the Mg deposits, complemented by a fully reversible intercalation/de-intercalation process of Mg ions into Mo6S8 cathodes. We also investigated LiTFSI/MgCl2 solutions which exhibited even higher reversibility than MgTFSI2/MgCl2 (DME) solutions, which we attribute to the higher purity level available for the LiTFSI salt.

High Lithium Ionic Mobility in Mo6S8 Chevrel Phase Cathodes

Rechargeable magnesium batteries have been recognized as an alternative solution to lithium ion batteries because of their advantages of excellent safety and cost [1]. Chevrel phase compounds, MxMo6S8(M = metal), are promising cathode materials in Mg batteries, because they display exceptionally fast cation transport for multi-valent ions compared to any other host materials. However, depending upon the composition and synthesized temperature of the Chevrel phase, the kinetics of Mg diffusion is highly affected at both room temperature and elevated temperature.In addition, there are many on-going efforts to discover improved electrolytes for Mg batteries. It has been recently demonstrated that using an electrolyte with a concentrated lithium salt could benetit the thermodynamic and kinetic in a lithium-sulfur cell [2]. Here, we report the experimental and computational calculation results to explain unusually high ionic mobility of lithium in Mo6S8 chevrel phase cathode materials. When a concentrated lithium salt of 0.5M LiCl is added to the conventional APC electrolytes, almost full theoretical capacity can be achieved at room temperature in Mo6S8 Chevrel phase cathode materials at very high C-rates up to 20, 30C.

A study of a fluorine substituted phenyl based complex as a 3 V electrolyte for Mg batteries

A stable 3 V Mg battery electrolyte with high efficiency for Mg deposition/dissolution was prepared based on the reaction between fluorine substituted complex 4-F-PhMgBr and AlCl3 in THF solvent.