Electrolytes For Rechargeable Magnesium Batteries Research Articles

Machine learning molecular dynamics insight into high interface stability and fast kinetics of low-cost magnesium chloride amine electrolyte for rechargeable magnesium batteries

The development of rechargeable magnesium batteries (RMBs) is hindered by the lack of long-lifespan and low-cost electrolytes. Moreover, due to lacking of an in-depth understanding of accurate dynamic solvation structures, the relationship between the interface kinetics behavior and a stable anode interface is still unclear. Herein, we develop a novel low-cost electrolyte containing the MgCl2 salt and the 3-methoxypropylamine solvent (denoted as the MgCl2-S2 electrolyte), which exhibits higher interface stability and faster kinetics compared to the reference Mg(TFSI)2-S2 electrolyte. The powerful machine learning molecular dynamics (MLMD) is employed to systematically investigate the differences of the fine solvation structures and explore the fundamental reason for better interfacial dynamics processes. The results show that the degree of dissociation in MgCl2-S2 electrolyte is less than 1 %, much lower than 95.1 % in Mg(TFSI)2-S2 electrolyte. The solvation structures dominated by Cl− ions can induce Mg2+ ions to preferentially acquire electrons in both kinetics and thermodynamics, which thus improving the deposition kinetics and enhancing the stability of the anode interface. This work inspires a new paradigm from MLMD to investigate the relationship between interface stability and accurate solvation structures for RMBs.

Toward Improved Anodic Stability of Ether-Based Electrolytes for Rechargeable Magnesium Batteries

Rechargeable magnesium batteries (RMBs) are one of the promising energy-storage technologies for sustainable energy storage due to the abundant resources and intrinsically remarkable energy-storage properties of magnesium metal. However, to compete with alternative technologies, such as present lithium-ion batteries, there is a need to improve their energy density. One of the approaches to accomplish the above demand is to use high-voltage cathodes. The poor anodic stability of the current ether-based electrolytes compatible with magnesium metal anodes limits their working voltage and the choice of electrode materials. In this study, we explored different organic solvent-based electrolytes to design anodically stable ether-based electrolyte solutions for RMB applications. Through comprehensive experimental and computational surveys, we found that the intrinsic electrochemical/chemical stabilities against magnesium metal and the well-balanced solvating ability were necessary to achieve the desired functionality. Based on this knowledge, we designed and synthesized glyme analogues bearing trifluoroalkyl groups. Consequently, we developed anodically stable electrolytes that support electrochemical magnesium deposition/dissolution by combining suitable fluorinated glyme-based solvents with appropriate conducting salts. These electrolytes showed a remarkable anodic limit of 4.4 V vs Mg2+/Mg (the highest ever reported to the best of our knowledge) and effectively suppressed the undesired corrosion of Al current collectors. However, these electrolytes could not be applied to RMBs with high-voltage oxide-based cathodes. Fragility against oxide-based cathodes caused undesired catalytic decomposition of the fluorinated solvents during charging.

Unexplored Class of Eutectic Electrolytes for Rechargeable Magnesium Batteries

Unexplored Class of Eutectic Electrolytes for Rechargeable Magnesium Batteries

Efficient single-perfluorinated borate-based electrolytes for rechargeable magnesium batteries

• Two efficient boron-based (MFBB and MCBB) electrolytes with a simpler preparation process and a low cost are proposed. • Two electrolytes have the same anionic group [B(Otfe) 4 ] − and different cationic groups of [Mg(DME) n ] 2+ in MFBB and [Mg 2 Cl 2 (THF) 6 ] 2+ in MCBB. • The active cationic groups are the main reason for the different electrochemical performances and the unique advantage of chlorine-containing cations is theoretically explained. • MCBB electrolyte exhibits excellent mg plating/stripping properties without a conditioning process and good rate performance coupled with Mo 6 S 8 cathode. Rechargeable magnesium batteries (RMBs) are considered a highly promising energy storage system among post-lithium-ion batteries due to the large earth abundance, high volumetric capacity, and less tendency for forming dendrites of Mg metal anode. However, the lack of low-cost and effective synthesis approaches for magnesium electrolytes seriously hinders the development of RMBs. Herein, according to an in-situ one-pot reaction of simple magnesium salts (MgF 2 , MgCl 2 ) with tris(2,2,2-trifluoroethyl)borate (B(Otfe) 3 ), two non-nucleophilic MgF 2 -B(Otfe) 3 (MFBB) and MgCl 2 -B(Otfe) 3 (MCBB) boron-based electrolytes are successfully synthesized. The mass spectrometry and nuclear magnetic resonance analysis confirm that both electrolytes contain the same bulky and weakly coordinated [B(Otfe) 4 ] − anion, while different solvated cations ([M g (DME) 3 ] 2+ for MFBB, and [Mg 2 Cl 2 (THF) 6 ] 2+ for MCBB). The density functional theory calculation demonstrates a lower de-solvation energy barrier of the chlorine-containing cation compared to the chlorine-free cation on magnesium anode. As a result, the MCBB electrolyte exhibits excellent Mg plating/stripping properties with a low overpotential (< 0.1 V) and a high Coulombic efficiency (> 97%) from the first cycle, different from the conditioning process of the MFBB electrolyte. Both MFBB and MCBB electrolytes present decent oxidation stability on inert Pt (>3.0 V vs. Mg/Mg 2+ ), meanwhile, the MCBB electrolyte also shows available anodic stability on non-noble SS (>2.5 V vs. Mg/Mg 2+ ). With small voltage hysteresis and stable rate cyclability, the proposed MCBB electrolyte exhibits superior compatibility with Chevrel phase Mo 6 S 8 cathode through the catalyzed assistance of open metal sites at the surface of Mo 6 S 8 , which demonstrates the limitations of the chlorine-containing solvated cations to insertion cathodes.

An efficient Hauser-base electrolyte for rechargeable magnesium batteries

Rechargeable magnesium batteries (RMBs) are considered the promising candidates for post lithium-ion batteries due to the abundant storage, high capacity, and dendrite-rare characteristic of Mg anode. However, the lack of practical electrolytes impedes the development and application of RMBs. Here, through a one-step reaction of LiCl congenital-containing Knochel–Hauser base TMPL (2,2,6,6-tetramethylpiperidinylmagnesium chloride lithium chloride complex) with Lewis acid AlCl3, we successfully synthesized an efficient amino-magnesium halide TMPLA electrolyte. Raman and mass spectroscopy identified that the electrolyte comprises the typical di-nuclear copolymer [Mg2Cl3·6THF]+ cation group and [(TMP)2AlCl2]− anion group, further supported by the results of density functional theory calculations (DFT) and the Molecular dynamics (MD) simulations. The TMPLA electrolyte exhibits promising electrochemical performance, including available anodic stability (>2.65 V vs. SS), high ionic conductivity (6.05 mS cm−1), and low overpotential (<0.1 V) as well as appropriate Coulombic efficiency (97.3%) for Mg plating/stripping. Both the insertion Mo6S8 cathode and conversion CuS cathode delivered a desirable electrochemical performance with high capacity and good cycling stability based on the TMPLA electrolyte. In particular, when compatible with low cost and easily synthesized CuS, the CuS||Mg cell displayed an extremely high discharge capacity of 458.8 mAh g−1 for the first cycle and stabilized at 170.2 mAh g−1 with high Coulombic efficiency (99.1%) after 50 cycles at 0.05 C. Our work proposes an efficient electrolyte with impressive compatibility with Mg anode and insertion/conversion cathode for practical RMBs and provides a more profound knowledge of the Lewis acid–base reaction mechanisms.

Boron–Based Electrolytes for Rechargeable Magnesium Batteries: Biography and Perspective

AbstractRechargeable magnesium batteries (RMBs) are a promising alternative in the post–lithium era for their high volumetric capacity, crustal abundance, and metallic safety. Electrolytes for RMBs have experienced a development process from the initial Grignard compound to today's diversified single magnesium salt. Among them, the boron‐based electrolyte plays a significant connecting link between the preceding and the following. This review retrospectively combed the origin and development course of boron–magnesium electrolyte and focuses on the application of three representatives: borohydride, boron‐center fluorinated alkoxylate, and closo–borane cluster. Fundamental understandings and typical concerns correlative with the Mg−B electrolyte designs are highlighted and briefly discussed. Finally, future direction and possible proposals for practical Mg−B electrolytes are put forward.

Comparative Study of Conventional Electrolytes for Rechargeable Magnesium Batteries

AbstractConventional magnesium salts are generally considered to be passivating to the magnesium anode, hence chloride‐based electrolyte additives are often added to circumvent this issue. As a result, the performance of conventional magnesium salts has rarely been assessed in a chloride‐free system. This work provides a comparative study on the electrochemical performance and interfacial reaction of four commercially available magnesium salts with tetrabutylammonium borohydride as moisture scavenger. Magnesium bis(hexamethyldisilazide) was found to be the most reductively stable and enabled excellent magnesium plating/stripping without chloride additive. Investigation of the solid electrolyte interphase revealed a thin organic polyether layer that was conducive to magnesium‐ion migration. The results also showed that a minuscule amount of chloride (20 mM) improved the reversibility of magnesium plating/stripping but was still corrosive towards the current collector, thus establishing that chloride‐free electrolyte is most appropriate for future development of rechargeable magnesium batteries.

N2O as a Universal Reaction Gas to Overcome Spectral Interference in Determining Metal Impurities in Mg(TFSI)2 Electrolytes for Rechargeable Magnesium Batteries by Inductively Coupled Plasma Tandem Mass Spectrometry.

A new strategy for the determination of metal impurities in magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)2) electrolytes for rechargeable magnesium batteries using inductively coupled plasma tandem mass spectrometry (ICP-MS/MS) was proposed. Mg(TFSI)2 was dissolved in 1,2-dimethoxyethane (DME), and 13 metal impurity elements were directly determined. Since N2 has a lower O atom affinity (1.6 eV) than the O atom (5.2 eV), N2O was a more effective O atom transfer gas than O2. In the MS/MS mode, N2O was selected as the reaction gas, and high sensitivities and low limits of detection (LODs) of analytes were obtained by mass shift methods. The accuracy of proposed analytical methods was assessed by the spike recovery experiments and comparative analyses using sector field inductively coupled plasma mass spectrometry (SF-ICP-MS). LODs were in the range of 0.18-26.6 ng kg-1, the recoveries were 92.5%-107%, and the relative standard deviation (RSD) was 2.0%-5.3%. No significant difference was observed between the ICP-MS/MS and SF-ICP-MS results at a 95% confidence level. The measurement realized the rapid determination of 13 metal impurity elements in Mg(TFSI)2 using N2O as a reaction gas with high sensitivity, accuracy, and precision. The method was applied for the analysis of Mg(TFSI)2 products with satisfactory results.

A High-Performance Magnesium Triflate-based Electrolyte for Rechargeable Magnesium Batteries

Summary The quest for a suitable electrolyte formulation is pivotal to the success of rechargeable magnesium batteries. A simple conventional electrolyte having high compatibility with magnesium anode and cathode material is in great demand. Herein, we report a simple yet effective electrolyte formulation, comprising magnesium triflate (Mg(OTf)2) and magnesium chloride in monoglyme, that can enable highly reversible, conditioning-free, and homogeneous magnesium deposition. Galvanostatic Mg plating/stripping demonstrates an average Coulombic efficiency of 99.4% over 1,000 cycles. The cells show excellent performance at current densities up to 3 mA cm−2 and areal capacities up to 5 mAh cm−2. Post mortem analysis unveils the formation of a robust solid electrolyte interphase, which leads to improved kinetics at the magnesium electrode. A prototype Mg battery with Mo6S8 cathode demonstrates stable cycling performance over 100 cycles. This study shows that rational design of Mg(OTf)2-based electrolytes is a promising direction toward the realization of high-performance magnesium batteries.

Improved Non-Grignard Electrolyte Based on Magnesium Borate Trichloride for Rechargeable Magnesium Batteries

The high anodic stability of electrolytes for rechargeable magnesium batteries enables the use of new positive electrodes, which can contribute to an increase in energy density. In this study, novel Ph3COMgCl-, Ph3SiOMgCl-, and B(OMgCl)3-based electrolytes were prepared with AlCl3 in triglyme. The Ph3COMgCl-based electrolyte showed anodic stability over 3.0 V vs. Mg but was chemically unstable, whereas the Ph3SiOMgCl-based electrolyte was chemically stable but featured lower anodic stability than the Ph3COMgCl-based electrolyte. Advantageously, the B(OMgCl)3-based electrolyte showed both anodic stability over 3.0 V vs. Mg (possibly due to the Lewis acidic nature of B in B(OMgCl)3) and chemical stability (possibly due to the hard acid character of B(OMgCl)3). B(OMgCl)3, which was prepared by reacting boric acid with a Grignard reagent, was characterized by nuclear magnetic resonance (NMR) spectroscopy, Fourier-transform infrared spectroscopy (FTIR), and X-ray absorption spectroscopy (XAS). The above analyses showed that B(OMgCl)3 has a complex structure featuring coordinated tetrahydrofuran molecules. 27Al NMR spectroscopy and Al K-edge XAS showed that when B(OMgCl)3 was present in the electrolyte, AlCl3 and AlCl2+ species were converted to AlCl4−. Mg K-edge XAS showed that the Mg species in B(OMgCl)3-based electrolytes are electrochemically positive. As a rechargeable magnesium battery, the full cell using the B(OMgCl)3-based electrolyte and a Mo6S8 Chevrel phase cathode showed stable charge-discharge cycles. Thus, B(OMgCl)3-based electrolytes, the anodic stability of which can be increased to ~3 V by the use of appropriate battery materials, are well suited for the development of practical Mg battery cathodes.

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.

Ion Agglomeration and Transport in MgCl2-Based Electrolytes for Rechargeable Magnesium Batteries.

Magnesium halide salts are an exciting prospect as stable and high-performance electrolytes for rechargeable Mg batteries (RMBs). By nature of their complex equilibria, these salts exist in solution as a variety of electroactive species (EAS) in equilibrium with counterions such as AlCl4-. Here we investigated ion agglomeration and transport of several such EAS in MgCl2 salts dissolved in ethereal solvents under both equilibrium and operating conditions using large-scale atomistic simulations. We found that the solute morphology is strongly characterized by the presence of clusters and is governed by the solvation structures of EAS. Specifically, the isotropic solvation of Mg2+ results in the slow formation of a bulky cluster, compared with chainlike analogues observed in the Cl-containing EAS such as Mg2Cl3+, MgCl+, and Mg2Cl22+. We further illustrate these clusters can reduce the diffusivity of charge-carrying species in the MgCl2-based electrolyte by at least an order of magnitude. Our findings for cluster formation, morphology, and kinetics can provide useful insight into the electrochemical reactions at the anode-electrolyte interface in RMBs.

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.

Prototype rechargeable magnesium batteries using ionic liquid electrolytes

Understanding Mg-ion speciation is of utmost importance to the rational design of electrolytes for rechargeable magnesium batteries. Herein, we report an effort to better understand Mg-ion speciation in ionic liquid (IL) electrolytes through the design of alkoxy-functionalized cations with different alkoxy substituent. In contrast with previous studies focusing on the coordination sphere of Mg2+, the comparison of Raman spectroscopy, electrochemical and DFT calculation results of various IL-based electrolytes suggests that the coordination sphere of transient Mg+ plays a key role in Mg reversible deposition/dissolution process. Finally, a prototype Mg/V2O5 cell using the noncorrosive IL-based electrolyte is demonstrated for the first time, exhibiting a remarkable initial discharge capacity of 140 mAh g−1 and reversible capacity of 100 mAh g−1.

Synthesis of magnesium chloride complex electrolyte: Galvanic couple assisted catalytic dissolution of magnesium in ethereal solution

Herein, we reveal the critical role of CrCl3 and the mechanism for the synthesis of magnesium chloride complex (MaCC), an advanced conditioning-free electrolyte for rechargeable magnesium batteries. This involves a catalytic dissolution of Mg metal by nanoscale bimetallic galvanic couples in an ethereal solution: At the initial stage, nanoscale amorphous Cr-rich ‘islands’ form on Mg surface, creating numerous Cr-Mg galvanic couples. These Cr-rich islands act as local cathodic sites due to partial electron transfer from Mg metal substrate. Furthermore, the first-principles calculation shows that Al prefers to bind at Cr-rich regions rather than Mg. These trigger a heterogeneous catalysis for the selective deposition of Al on Cr-rich islands and a dramatic increase in the dissolution rate of Mg metal on the neighboring region. This leads to an ultrahigh Mg2+-to-Al3+ concentration ratio in the resultant solution, a key property of the conditioning-free electrolyte. This study is potentially applicable to many other fields like metal-air batteries and corrosion protection of metals, where a subtle manipulation of passive layer is required.

Strategic combination of Grignard reagents and allyl-functionalized ionic liquids as an advanced electrolyte for rechargeable magnesium batteries

A novel strategic approach for preparing highly efficient electrolytes for rechargeable Mg batteries is proposed through the reaction between Grignard reagents and allyl-functionalized ionic liquids.

New Class of Non-Corrosive, Non-Nucleophilic and Highly Efficient Electrolytes for Rechargeable Magnesium Batteries

It will be shown that fluorinated alkoxyborates based magnesium electrolytes are weakly coordinated systems, which exhibit a high anodic stability (4.3 V vs. Mg), high ionic conductivity (7 mS/cm) and high Coulombic efficiency (>99%) of reversible magnesium deposition. Owing to the non-corrosive, chemically stable nature and the robust, economic synthesis in one step from commercially available materials, this design concept of new, weakly coordinated ion conducting salts opens a promising avenue towards the realization of high-energy magnesium batteries. Synthesis, chemical and structural analysis and electrochemical properties will be presented, as well as its application to Mg-S batteries, where it shows superior performance.

Studies on the Effect of Nano-Sized MgO in Magnesium-Ion Conducting Gel Polymer Electrolyte for Rechargeable Magnesium Batteries

Magnesium-ion conducting gel polymer electrolytes (GPEs) with different contents of nano-sized MgO have been prepared and investigated by various electrical and electrochemical techniques. The Mg2+ ion conduction in GPEs was confirmed from cyclic voltammetry and impedance analysis. It was found that doping appropriate nano-sized MgO in the GPE can induce significant improvements in both the electrochemical and the mechanical properties of GPEs. The composite GPE with 7% MgO shows a high ionic conductivity of 4.6 × 10−3 S/cm with electrochemical stability up to 4.7 V versus Mg2+/Mg at room temperature. Furthermore, it is free-standing and flexible with high tensile strength (9.7 ± 0.1 MPa) and elongation at break (91.7 ± 0.2%), further ensuring their potential applications as GPEs for rechargeable Mg batteries.

Non-Grignard and Lewis Acid-Free Sulfone Electrolytes for Rechargeable Magnesium Batteries

A major challenge for developing rechargeable Mg-ion batteries (MIB) is the lack of suitable electrolytes. We report herein dialkyl sulfones as non-Grignard and Lewis acid-free MIB electrolytes. In particular, a dipropyl sulfone (DPSO)/tetrahydrofuran (THF) (1/1, v/v) solution with MgCl2 salt exhibits high ionic conductivity (1.1 mS cm–1 at 30 °C), Mg cycling efficiency (>90%), and anodic stability (ca. 3.0 V vs Mg). As evidenced by single crystal X-ray diffraction analysis, a novel [Mg(DPSO)6]2+ cation complex balanced by two [MgCl3(THF)]− anions is identified in the DPSO/THF solution. The DPSO/THF electrolyte also enables excellent cycle performance (>300 cycles) of a Chevrel phase Mo6S8 cathode and displays a decent compatibility with an organic cathode (3,4,9,10-perylenetetracarboxylic dianhydride, PTCDA). Along with the superior electrochemical properties of the DPSO/THF electrolyte, its innate chemical stability and eco-friendly nature make it a promising MIB electrolyte.

A new class of non-corrosive, highly efficient electrolytes for rechargeable magnesium batteries

Easily synthesized fluorinated magnesium alkoxyborates with non-corrosive and chemically stable features show high electrochemical performance as magnesium ion conducting salts.