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

Abstract

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.

Modeling of Electron‐Transfer Kinetics in Magnesium Electrolytes: Influence of the Solvent on the Battery Performance

“… [2] Therefore, longer glymes CH 3 O−(C 2 H 4 O)n−CH 3 (Gn) with n>1 are considered more and more frequently as solvents for magnesium batteries. [ 20 , 25 , 31 , 32 , 33 ] The multidenticity of glymes is beneficial for the solubility of magnesium salts, but especially the higher order glymes also significantly reduce the mobility of the ions due to their higher viscosity. [11] Moreover, the properties of the solvent and the structure of the electrochemically active specie can significantly impact the desolvation behaviour and consequently the magnesium deposition.…”

Advancing Electrolyte Solution Chemistry and Interfacial Electrochemistry of Divalent Metal Batteries

“…It is now well-accepted that the Lewis base of organomagnesium halide does not exist in ethereal solvents as single compounds, but rather an equilibrium mixture of various compounds (i. e. Schlenk equilibrium) including high nucleophilic R 2 Mg or Ph 2 Mg. [9] To expand the cathode material pool limited by these nucleophilic species, the same transmetalation route was employed to prepare a non-nucleophilic electrolyte of [Mg 2 (μ-Cl) 3 • 6THF][HMDSAlCl 3 ] (HMDS: hexamethyldisilazide) that has an electrochemical window up to 3.2 V on Pt, [14] in which HMDSMgCl is a non-nucleophilic Lewis base and AlCl 3 is the Lewis acid. Other solutions using alkoxides, [15] borates, [16] coupled with AlCl 3 , [17] have been proposed to prepare electrolytes with favorable characteristics. Later, the inorganic Mg aluminum chloride complex (i. e. MACC) was further presented which comprises the acid-base reaction products between MgCl 2 and AlCl 3 in THF.…”

“…To expand the cathode material pool limited by these nucleophilic species, the same transmetalation route was employed to prepare a non‐nucleophilic electrolyte of [Mg 2 (μ‐Cl) 3 ⋅ 6 THF ][HMDSAlCl 3 ] ( HMDS : hexamethyldisilazide) that has an electrochemical window up to 3.2 V on Pt, [14] in which HMDSMgCl is a non‐nucleophilic Lewis base and AlCl 3 is the Lewis acid. Other solutions using alkoxides, [15] borates, [16] coupled with AlCl 3 , [17] have been proposed to prepare electrolytes with favorable characteristics. Later, the inorganic Mg aluminum chloride complex ( i. e .…”

Triiodide Anion as a Magnesium-ion Transporter for Low Overpotential Battery Cycling in Iodine-Containing Mg(TFSI)2 Electrolyte

“…16,17 Grignardfree electrolytes have been developed, yet many still rely on a strong Lewis acids and still frequently include chloride ions. 18,19 These electrolytes are effective in cycling magnesium with low overpotentials and afford long-term stability, but their complicated syntheses, extreme sensitivity to air and moisture, high corrosiveness, and incompatibility with oxide based cathodes motivated the need to develop magnesium simple salt electrolytes incorporating anions such as TFSI − , ClO 4 − , and PF 6 − . 7 Simple salt electrolytes are incredibly attractive for magnesium batteries, but they typically display reductive incompatibility with magnesium metal anodes, leading to electrolyte decomposition that gives rise to forming a passivating layer, which then results in high overpotentials for cycling.…”