A fluorinated dialkoxide-based magnesium-ion electrolyte

Abstract

Efficient large scale electrochemical energy storage systems, such as those based on multivalent ions, are a prerequisite for the realization of intermittent renewable energy sources. From the perspectives of both cost and environmental concerns, it is of critical importance that components of these systems are synthesized using sustainable chemical processes starting from their initial conception. Herein, we report on a fluorinated dialkoxide-based magnesium-ion electrolyte that is synthesized through an atom-efficient and scalable process without the use of any metal alkyls. The electrolyte composition results in high solution conductivity (4.77 mS cm À1 at 26.3 C), low overpotentials, ca. 100% coulombic efficiency for electrodeposition/ dissolution, and good performance in full battery cells using Chevrel phase Mo 6 S 8 . Multivalent battery chemistries represent one possible route for future energy storage in applications such as transportation and the electrical grid. 1,2 The high volumetric energy density (3832 mA h cm À3 ) 3 and suitably negative reduction potential (À2.356 V vs. SHE) 3 of magnesium metal has drawn a great deal of attention in the eld of rechargeable batteries. Unfortunately, the realization of batteries that use this chemistry has proved difficult to achieve in practice, mostly due to the propensity of Mg to passivate either intrinsically or from environmental factors such as the electrolyte with which it comes in contact. 1,4 As such, one of the main issues in this area is the rational design of electrolytes capable of activating the metal surface for reversible Mg electrodeposition while also being able to withstand oxidizing voltages. Earlier methods to create electroactive Mg solutions relied on the use of Grignard-type compounds in combination with Lewis acids such as AlCl 3 in ether-based solvents. 5,6 Due to the high degree of reactivity associated with these solutions, research efforts in the past few years have been dedicated to nding alternatives. One such option is Mg(HMDS) 2 :AlCl 3 (HMDS ¼ hexamethyldisilazide), 7 though unfortunately the synthesis of Mg(HMDS) 2 also requires the use of metal alkyls. 8 Additionally, while some so-called "traditional" electrolyte anions such as perchlorates have been shown to lead to anode passivation, 9 others such as bis(triuoromethane) sulfonamide (TFSI À ) 3,10 have recently been shown to support reversible Mg ion electrochemistry, though they suffer from signicant plating and stripping overpotentials in addition to being synthetically exigent. 11,12 Electrolytes that utilize magnesium alkoxides have also been of interest in the Mg-ion community, given the less nucleophilic nature of alkoxides compared to amides. Mg compounds of the type ROMgCl 13-15 (R ¼ alkyl, aryl), when combined with AlCl 3 or MgCl 2 , have been demonstrated as Mg-ion electrolytes with desirable electrochemical properties. Expanding on this idea, a recent paper shows that mixtures of uorinated alkoxymagnesium halides with AlCl 3 perform more favorably than their n...

Mg Cathode Materials and Electrolytes for Rechargeable Mg Batteries: A Review

“…MoO 3 , [14] V 2 O 5 , [56,[104][105] MnO 2 , [106][107][108][109][110] and TiO 2 [111][112][113] have been prepared with different structures. Most of these transition metal compounds are nanosized, to provide a high surface area with more active sites for cation intercalation as well as short ion-transport paths to facilitate cation diffusion.…”

Remarkable electrochemical and ion-transport characteristics of magnesium-fluorinated alkoxyaluminate–diglyme electrolytes for magnesium batteries

“…[9][10][11] Halidecontaining ethereal solutions consisting of different magnesium sources and strong Lewis acids such as AlCl 3 , AlCl 2 (C 2 H 5 ), BPh 3 [Ph = phenyl group], and B(HFIP) 3 [HFIP = hexafluoro-isopropoxyl group] support reversible magnesium deposition/ dissolution with excellent energy and coulombic efficiencies. 10,[12][13][14][15][16][17][18][19][20] However, these electrolytes also exhibit severe drawbacks such as insufficient anodic stability, high corrosivity, incompatibility with typical transition-metal-oxide cathodes, and moisture sensitivity-all of which extremely hinder practical battery materialization. To overcome these drawbacks, many attempts have been made to develop highly efficient magnesium-ion conductive electrolytes.…”

Non-Nucleophilic Electrolyte Based on Ionic Liquid and Magnesium Bis(diisopropyl)amide for Rechargeable Magnesium-Ion Batteries

“…The low CE and high overpotential for the reversible Mg deposition process in this electrolyte are found to be improved by involving boron clusters such as magnesium monocarborane (MMC) in tetraglyme . Recently, electrolytes based on fluorinated alkoxide and dialkoxide are reported to show reversible Mg deposition/stripping with nearing 100% CE with high anodic stability (>3.0 V vs Mg/Mg 2+ ) on glassy carbon and gold working electrodes. , …”

“…29 Recently, electrolytes based on fluorinated alkoxide and dialkoxide are reported to show reversible Mg deposition/stripping with nearing 100% CE with high anodic stability (>3.0 V vs Mg/Mg 2+ ) on glassy carbon and gold working electrodes. 30,31 Most of the magnesium electrolytes contain organometallic magnesium complexes, which are highly hazardous and corrosive. These issues are averted in inorganic electrolyte solutions based on the reaction of MgCl 2 and AlCl 3 in THF, known as magnesium aluminum chloride complexes (MACC) .…”