Electrolytes For Magnesium Batteries Research Articles

Recent Advances in Electrolytes for Magnesium Batteries: Bridging the Gap Between Chemistry and Electrochemistry.

Rechargeable magnesium batteries (RMBs) have the potential to provide a sustainable and long-term solution for large-scale energy storage due to high theoretical capacity of magnesium (Mg) metal as an anode, its competitive redox potential (Mg/Mg2+: -2.37 V vs. SHE) and high natural abundance. To develop viable magnesium batteries with high energy density, the electrolytes must meet a range of requirements: high ionic conductivity, wide electrochemical potential window, chemical compatibility with electrode materials and other battery components, favourable electrode-electrolyte interfacial properties and cost-effective synthesis. In recent years, significant progress in electrolyte development has been made. Herein, a comprehensive overview of these advancements is presented. Beginning with the early developments, we particularly focus on the chemical aspects of the electrolytes and their correlations with electrochemical properties. We also highlight the design of new anions for practical electrolytes, the use of electrolyte additives to optimize anode-electrolyte interfaces and the progress in polymer electrolytes.

Mixed-Anion Contact Ion-Pair Formation Enabling Improved Performance of Halide-Free Mg-Ion Electrolytes.

Discovery of stable and efficient electrolytes that are compatible with magnesium metal anodes and high-voltage cathodes is crucial to enabling energy storage technologies that can move beyond existing Li-ion systems. Many promising electrolytes for magnesium anodes have been proposed with chloride-based systems at the forefront; however, Cl-containing electrolytes lack the oxidative stability required by high-voltage cathodes. In this work, we report magnesium trifluoromethanesulfonate (triflate) as a viable coanion for Cl-free, mixed-anion magnesium electrolytes. The addition of triflate to electrolytes containing bis(trifluoromethane sulfonyl) imide (TFSI-) anions yields significantly improved Coulombic efficiency, up to a 100 mV decrease in the plating/stripping overpotential, improved tolerance to trace H2O, and improved oxidative stability (0.35 V improvement compared to that of hybrid TFSI-Cl electrolytes). Based on 19F nuclear magnetic resonance and Raman spectroscopy measurements, we propose that these improvements in performance are driven by the formation of mixed-anion contact ion pairs, where both triflate and TFSI- are coordinated to Mg2+ in the electrolyte bulk. The formation of this mixed-anion magnesium complex is further predicted by the density functional theory to be thermodynamically driven. Collectively, this work outlines the guiding principles for the improved design of next-generation electrolytes for magnesium batteries.

Differentiating between Ion Transport and Plating–Stripping Phenomena in Magnesium Battery Electrolytes Using Operando Raman Spectroscopy

Understanding metal plating–stripping and mass transport processes is necessary for the development of new electrolytes for post-lithium energy storage applications. Operando vibrational spectroscopy is a valuable analytical tool for this purpose, enabling structural and chemical changes at electrode–electrolyte interfaces to be probed dynamically, under battery cycling conditions. In this work we apply operando Raman spectroscopy to characterize the behavior of the Mg based “all phenyl complex” [Mg2Cl3]+[AlPh4]− in tetrahydrofuran (THF), an exemplar electrolyte for emerging Mg battery technologies. We demonstrate that the observed electrolyte Raman band intensities vary reversibly with electrochemical cycling due to anion migration in response to the applied electric field, while Mg plating and stripping can be monitored independently through the broad background scattering intensity. Spectral measurements across different sites of the platinum working electrode indicate that the ion transport response is spatially heterogeneous, while the plating and stripping response is ubiquitous.

Magnesium Battery Electrolytes with Improved Oxidative Stability Enabled by Selective Solvation in Fluorinated Solvents

Magnesium Battery Electrolytes with Improved Oxidative Stability Enabled by Selective Solvation in Fluorinated Solvents

Amelioration of ionic conductivity (303 K) with the supplement of MnO2 filler in the chitosan biopolymer electrolyte for magnesium batteries

In the present work, the significance of adding MnO2 filler towards the development of biopolymer electrolyte and chitosan with enhanced ionic conductivity has been reported. The complexation that has been taken place with the inclusion of MgNO3.6H2O salt and MnO2 filler in the chitosan matrix has been investigated through the Fourier transform infrared (FTIR) spectra analysis. Further, the transport parameter values such as number of charge carrier densities (n), mobility (μ) and diffusion coefficient (D) have been calculated from the deconvoluted FTIR spectra. X-Ray diffraction (XRD) and differential scanning calorimetric (DSC) examination revealed that the inclusion of filler significantly reduced the degree of crystallinity and the glass transition Tg values, respectively. These findings show that the inclusion of filler increases the segmental mobility of the polymer chains, allowing for faster ion transport. The conduction properties of the prepared electrolytes have been determined using alternating current (AC) impedance analysis, with the electrolyte containing 60 wt% magnesium salt (2.6 ± 0.08) × 10−4 S cm−1 achieving the maximum ionic conductivity value at room temperature (303 K). This value is further increased by one order of magnitude with the addition of MnO2 filler into the polymer matrix (1.25 ± 0.09) × 10−3 S cm−1. To elucidate the individual contributions of ions and electrons in the conduction process, the direct current (DC) polarization method has been used to determine the transference number (tion) of the produced electrolytes. The filler-added chitosan polymer exhibits an extended electrochemical stability window, 1.7 V, as determined by linear sweep voltammetry (LSV).

In Situ Grignard Metalation Method for the Synthesis of Hauser Bases.

The in situ Grignard Metalation Method (iGMM) is a straightforward one‐pot procedure to quickly produce multigram amounts of Hauser bases R2N‐MgBr which are valuable and vastly used metalation reagents and novel electrolytes for magnesium batteries. During addition of bromoethane to a suspension of Mg metal and secondary amine at room temperature in an ethereal solvent, a smooth reaction yields R2N‐MgBr under evolution of ethane within a few hours. A Schlenk equilibrium is operative, interconverting the Hauser bases into their solvated homoleptic congeners Mg(NR2)2 and MgBr2 depending on the solvent. Scope and preconditions are studied, and side reactions limiting the yield have been investigated. DOSY NMR experiments and X‐ray crystal structures of characteristic examples clarify aggregation in solution and the solid state.

Towards Solid‐State Magnesium Batteries: Ligand‐Assisted Superionic Conductivity

AbstractSolid‐state inorganic magnesium batteries are considered as potential high energy storage devices of the future. Here we present a series of magnesium borohydride tetrahydrofuran (THF) composites, Mg(BH4)2 ⋅ xTHF(−MgO), 0≤x≤3, as solid‐state electrolytes for magnesium batteries. Three new monoclinic compounds were identified, Mg(BH4)2 ⋅ 2/3THF (Cc), α‐Mg(BH4)2 ⋅ 2THF (P21/c) and β‐Mg(BH4)2 ⋅ 2THF (C2), and the detailed structures of α‐ and β‐Mg(BH4)2 ⋅ 2THF are presented. The magnesium ionic conductivity of composites formed by these compounds were several orders of magnitude higher than that of the distinct compounds, x=0, 2/3, 2, and 3. The nanocomposite stabilized by MgO nanoparticles (∼50 nm), Mg(BH4)2 ⋅ 1.5THF−MgO(75 wt%), displayed the highest Mg2+ conductivity, σ(Mg2+)∼10−4 S cm−1 at 70 °C, a high ionic transport number of tion=0.99, and cyclic voltammetry revealed an oxidative stability of ∼1.2 V vs. Mg/Mg2+. The electrolyte was stable towards magnesium electrodes, which allowed for stable Mg plating/stripping for at least 100 cycles at 55 °C with a current density of 0.1 mA cm−2. Finally, a proof‐of‐concept rechargeable solid‐state magnesium battery was assembled with a magnesium metal anode and a TiS2 cathode. A maximum discharge capacity of 94.2 mAh g−1 was displayed, which corresponds to y=0.2 in MgyTiS2.

Prospects for Improved Magnesocene‐Based Magnesium Battery Electrolytes

AbstractMagnesium batteries are currently attracting a lot of interest as a next generation battery technology. One critical issue is to find a suitable electrolyte and herein we explore an electrolyte based on magnesocene (MgCp2) in tetrahydrofuran (THF), aiming for low‐voltage Mg batteries, with respect to: Mg plating characteristics, electrochemical stability windows, electrolyte speciation, and electrolyte decomposition reactions; both experimentally and computationally. Overall, the electrolyte does not seem to decompose on a Mg metal anode and most likely reduced solvation of Mg2+ by the Cp− anion is important and species such as MgCp2THF2 may play an important role for Mg plating with small overpotential. The oxidation limit is largely determined by the Cp− anion and density functional theory predicted oxidation reactions point to polymerized end‐products to be possible. Furthermore, in silico substitution studies enable us to establish the prospects of some Cp− anion derivatives to further improve the oxidative stability, but still the Mg2+ solvation must be monitored for ease of reduction and Mg plating.

Structure of Magnesium Chloride Complexes in Ethereal Systems: Computational Comparison of THF and Glymes as Solvents for Magnesium Battery Electrolytes

AbstractThe structure of the electrolyte is crucial for the performance of rechargeable magnesium batteries. Doubly charged cations interact much stronger with both anions and solvent molecules, forming different size clusters. Here, we apply DFT calculations to investigate salt solvation by altering the first solvation shell of the magnesium‐chloride complexes in different ethereal solvents: tetrahydrofuran, monoglyme, diglyme, triglyme and tetraglyme. The analysis was performed by looking for the most stable structures, considering mono‐, di‐ and trimeric clusters of MgxCly. The determination of clusters geometries, together with their energies, resulted in a comprehensive picture of the thermodynamically preferred state of the electrolyte, and allowed for a simple assessment of the electrochemical activity of the electrolyte. Our analysis shows that clustering is beneficial for desolvation of magnesium from the cluster, but causes overpotentials due to hindered electron transfer.

Electrolytes for Magnesium Batteries: Which Way to Go?

Magnesium battery technology is one of the promising post-lithium technology, that has potential to reduce our lithium dependence, and at the same time outperform current Li-ion technology. Magnesium is more abundant, hence cheaper, and comes with promises of higher energy density and lower tendency to dendrite formation – possibility of metallic anode application. Higher capacity of such battery comes from double charge of magnesium ion, that also causes battery chemistry more difficult. Especially electrode/electrolyte interface becomes tricky, and simple transfer of electrolyte concept from lithium technology is not possible. Reversible plating/stripping of magnesium has been demonstrated many times, however in most of the cases from highly reactive and/or aggressive electrolytes. That put some obstacles on the way for future commercialization and a key challenge on the road to a rechargeable magnesium batteries is the development of non-corrosive magnesium electrolytes.For some time, role of chloride in electrode process was considered to be essential for reduction of magnesium ions. However new studies show possible operation of magnesium battery with Cl-free electrolyte. Conditions for that are not completely clear, and here we are going to put some light on the processes occurring at electrode/electrolyte interfaces. DFT calculations are used to understand desolvation of magnesium ion, which is a key step prior to reduction at the electrode. Comparison of both systems, with standard and Cl-free electrolytes, shows the possibility to tune electrode process by changes in electrolyte composition, indicating the way towards the less reactive, and less corrosive systems. Presented results were obtained within E-MAGIC project, which received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 824066.

Exploiting cation aggregation in new magnesium amidohaloaluminate electrolytes for magnesium batteries

The active species in magnesium electrolytes is regularly assigned as being the thermodynamically stable dinuclear [Mg2Cl3]+ cation. By deliberately targeting other implicated aggregates, their effect on Mg-ion battery performance is easily ascertained.

Application of Single-Ion Conducting Gel Polymer Electrolytes in Magnesium Batteries

The development of polymer electrolytes for magnesium batteries has been hindered by difficulties related to achieving sufficient magnesium ion conduction and magnesium electrodeposition. The high charge density of the magnesium cation causes it to interact with both the polar polymer matrix and any counteranions, challenging ion transport. Solvents and salts that are known to be incompatible with the magnesium electrode have widely been used in prior reports to increase ionic conductivity. Herein we report the use of a single-ion conducting magnesium gel polymer electrolyte consisting of a poly(ethylene glycol) dimethacrylate (PEGDMA) crosslinker that is copolymerized with an anionic monomer and subsequently swelled in solvents compatible with magnesium metal. Sufficient magnesium ion conductivities (>10–4 S/cm at 25 °C) were able to be achieved with specific solvents and solvent mixtures. Constant potential holds were used to interrogate magnesium electrodeposition from these electrolytes. Small amounts...

Computational Insights into Mg‐Cl Complex Electrolytes for Rechargeable Magnesium Batteries

AbstractDFT calculations were conducted to provide unprecedented thermodynamic insights on tetrahydrofuran (THF) solvation, isomerization, and complexation of possible Mg−Cl coordination species for popular Mg−Cl electrolytes for magnesium batteries. Computational results using the M06‐2x functional with the 6‐31+G(d) basis set indicate that trigonal bipyramidal e,e‐cis‐tbp‐MgCl2(THF)3 dichloride species and octahedral [MgCl(THF)5]+ monochloride species are the dominant mononuclear species. These two can combine to form the dinuclear species [(μ‐Cl)3Mg2(THF)6]+ with a free energy −6.30 kcal/mol, which is calculated to be the dominant Mg−Cl species in solution. Two mono‐cation species, [(μ‐Cl)3Mg2(THF)6]+ and [MgCl(THF)5]+, have comparable LUMO energies, thus both of them can act as active species for Mg deposition. However, the significant dominance of the dinuclear species in the electrolyte indicates that it is the primary species involved in reversible Mg deposition.

Electrolyte Additive Enabling Conditioning-Free Electrolytes for Magnesium Batteries.

Most electrolytes for rechargeable Mg batteries require time-consuming conditioning or precycling process to achieve a fully reversible Mg deposition/dissolution, which hinders the normal operation of Mg batteries. This study details a simple and effective method for eliminating this conditioning behavior using heptamethyldisilazane (HpMS) as an electrolyte additive. It was found that the HpMS additive greatly increases the current density and Coulombic efficiency of Mg deposition/dissolution from the initial cycles in various sulfone and glyme solutions containing MgCl2 or Mg(TFSI)2. The beneficial effect of HpMS was ascribed to its ability to scavenge trace water in the electrolytes and remove Mg(OH)2 and Mg(TFSI)2-decomposition products from the Mg surface. Considering its applicability for a wide range of Mg electrolytes, the use of HpMS is expected to accelerate the development of practical Mg batteries.

Structural and Conductivity Mechanism of a Boron-Based Ionic Liquid Electrolyte for Magnesium Batteries

In the last years, massive improvements have been achieved in the field of battery technology opening the door to more efficient electric vehicles and electric power storage. Nevertheless, further research is needed to overcome lithium drawbacks and enter in the such called “beyond Li-ion” technology [1]. In this perspective, magnesium secondary batteries are promising electrochemical storage devices due to their a) high volumetric capacity (3832 mAh·cm-3), b) great abundance and homogeneous spread of Mg in the Earth’s crust, c) safety and environmentally friendliness, and d) low standard reduction potential (-2.36 V). The main roadblock to the enter of these devices in the global market is the development of a safe and efficient electrolyte. In the last 3 years it has been demonstrated that electrolytes obtained by dissolving a suitable magnesium salt (δ-MgCl2) into ionic liquids (ILs) are able to efficiently deposit and strip magnesium with very low overpotentials and high current densities [2-4]. In the present work a new family of electrolytes is proposed, based on 1-Butyl-1-methylpyrrolidinium chloride and boron trichloride, doped with δ-MgCl2. The thermal stability and transitions of the proposed materials is studied using HR-TGA and MDSC. The structure and interactions of the materials are investigated by vibrational spectroscopies (FT-IR and confocal micro-Raman). Finally, Broadband Electrical Spectroscopy is exploited to elucidate the electric response of materials and to detect the long-range charge migration mechanisms of the electrolytes.

Polymer Gel Electrolytes for Magnesium Batteries: Ion Transport and Electrochemical Characterization

The development of new rechargeable battery systems employing novel chemistries is imperative to meet the increasing energy storage demands of emerging technologies. Magnesium metal batteries are one attractive type of beyond lithium-ion system, due to the widespread abundance, gravimetric and volumetric charge capacity, and electrodeposition potential of magnesium. However, the electrolyte formulations that enable magnesium deposition and dissolution are few in comparison with lithium. Here we report on the ion transport and electrochemical properties of magnesium polymer gel electrolytes based crosslinked, ionomeric networks that are swelled. The nature of the solvent(s) and salt(s) of the liquid medium, and the charge density of the ionomer network, are all found to heavily influence the properties of the gel electrolyte. Magnesium polymer gel electrolytes are created by swelling the ionic networks in electrolytes containing AlCl3, MgCl2, and/or Mg(HMDS)2. Interestingly, higher ionic conductivities and higher current densities for magnesium deposition/dissolution are observed for some gel electrolytes compared to the properties of the pure liquid component. This is hypothesized to be due to the speciation of the free complex ions, which is altered by the presence of the ionic polymer network. These gels are further applied to the Mg-S system, and it is shown that the structure and charge density of the gels influence anionic polysulfide transport. Gel formulations are identified where polysulfides are preferentially rejected while complex magnesium cations are facilely transported across the gel. Structure-property correlations will be presented as found from the combination of SAXS/WAXS, diffusion, elemental analysis, and conductivity measurements. Single-ion conducting magnesium polymer electrolytes are also created by swelling the ionomeric networks in pure solvents. High ionic conductivities are achievable (> 10^-4 S/cm). However, even when these electrolytes have high magnesium conductivity and are electrochemical stable (non-passivating) against magnesium metal, no magnesium electrodeposition is induced at reasonable potentials. This is suspected to be due to the difficulty of the desolvation of Mg+2.

Conditioning-Free Electrolytes for Magnesium Batteries Using Sufone–Ether Mixtures with Increased Thermal Stability

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTConditioning-Free Electrolytes for Magnesium Batteries Using Sufone–Ether Mixtures with Increased Thermal StabilityLaura C. MerrillLaura C. MerrillDepartment of Chemical and Biomolecular Engineering, University of Notre Dame, 205 McCourtney Hall, Notre Dame, Indiana 46556, United StatesMore by Laura C. Merrill and Jennifer L. Schaefer*Jennifer L. SchaeferDepartment of Chemical and Biomolecular Engineering, University of Notre Dame, 205 McCourtney Hall, Notre Dame, Indiana 46556, United States*J. L. Schaefer. E-mail: [email protected], Tel: 15746315114.More by Jennifer L. Schaeferhttp://orcid.org/0000-0003-4293-6328Cite this: Chem. Mater. 2018, 30, 12, 3971–3974Publication Date (Web):May 31, 2018Publication History Received1 February 2018Revised31 May 2018Published online31 May 2018Published inissue 26 June 2018https://doi.org/10.1021/acs.chemmater.8b00483Copyright © 2018 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views1199Altmetric-Citations18LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit Read OnlinePDF (2 MB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Cations,Deposition,Electrolytes,Magnesium,Sulfones Get e-Alerts

Electrochemical Properties and Speciation in Mg(HMDS)2-Based Electrolytes for Magnesium Batteries as a Function of Ethereal Solvent Type and Temperature.

Magnesium batteries are a promising alternative to lithium-ion batteries due to the widespread abundance of magnesium and its high specific volumetric energy capacity. Ethereal solvents such as tetrahydrofuran (THF) are commonly used for magnesium-ion electrolytes due to their chemical compatibility with magnesium metal, but the volatile nature of THF is a concern for practical application. Herein, we investigate magnesium bis(hexamethyldisilazide) plus aluminum chloride (Mg(HMDS)2-AlCl3) electrolytes in THF, diglyme, and tetraglyme at varying temperature. We find that, despite the higher thermal stability of the glyme-based electrolytes, THF-based electrolytes have better reversibility at room temperature. Deposition/stripping efficiency is found to be a strong function of temperature. Diglyme-based Mg(HMDS)2-AlCl3 electrolytes are found to not exchange as quickly as THF and tetraglyme, stabilizing AlCl2+ and facilitating undesired aluminum deposition. Raman spectroscopy, 27Al NMR, and mass spectrometry are used to identify solution speciation.

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