Enhanced Stability of the Carba-closo-dodecaborate Anion for High-Voltage Battery Electrolytes through Rational Design

Abstract

Future energy applications rely on our ability to tune liquid intermolecular interactions and achieve designer electrolytes with highly optimized properties. In this work, we demonstrate rational, combined experimental-computational design of a new carba- closo-dodecaborate-based salt with enhanced anodic stability for Mg energy storage applications. We first establish, through a careful examination using a range of solvents, the anodic oxidation of a parent anion, the carba- closo-dodecaborate anion at 4.6 V vs Mg (2.0 vs Fc), a value lower than that projected for this anion in organic solvent-based electrolytes and lower than weakly associating bis(trifluoromethylsulfonyl)imide and tetrafluoroborate anions. Solvents such as acetonitrile, 3-methylsulfolane, and 1,1,1,3,3,3-hexafluoroisopropanol are shown to enable the direct measurement of carba- closo-dodecaborate oxidation, where the resultant neutral radical drives passive film formation on the electrode. Second, we employ computational screening to evaluate the impact of functionalization of the parent anion on its stability and find that replacement of the carbon-vertex proton with a more electronegative fluorine or trifluoromethyl ligand increases the oxidative stability and decreases the contact-ion pair formation energy while maintaining reductive stability. This predicted expansion of the electrochemical window for fluorocarba- closo-dodecaborate is experimentally validated. Future work includes evaluation of the viability of these derivative anions as efficient and stable carriers for energy storage as a function of the ionic transport through the resulting surface films formed on candidate cathodes.

A Stable, Non-corrosive Perfluorinated Pinacolatoborate Mg Electrolyte for Rechargeable Mg Batteries

“…Mg and astripping potential at Angewandte Chemie Zuschriften À0.111 V, giving overpotential of 197 mV,w hich is comparable with Mg/MgCl 2 /AlCl 3 (MMAC) electrolytes, [3f] but lower than reported chloride-free Mg electrolytes. Furthermore,t he Mg-FPB electrolyte exhibited an outstanding anodic stability at more than 4.0 Vv s. Mg, which is comparable with reported chloride-free Mg electrolytes, [4][5][6][7] but significantly improved over other reported Mg electrolytes. Furthermore,t he Mg-FPB electrolyte exhibited an outstanding anodic stability at more than 4.0 Vv s. Mg, which is comparable with reported chloride-free Mg electrolytes, [4][5][6][7] but significantly improved over other reported Mg electrolytes.…”

“…[2] However,t he research into rechargeable Mg batteries is still in its infancy, and development of Mg batteries is primarily hampered by two grand challenges,t he lack of high performance Mg 2+ electrolyte [2b,c] and energy-dense cathode materials. [3] The reported Mg 2+ electrolytes applied in Mg batteries are mainly based on the combination of aMg-containing Lewis base (for example,Grignard reagents and MgCl 2 )and aLewis acid (for example,A lCl 3 [4] Mg(CB 11 H 11 F) 2, [5] Mg[Al-(HFIP) 4 ] 2 , [6] and Mg[B(HFIP) 4 ] 2 (HFIP =-OCH(CF 3 ) 2 ). [3] The reported Mg 2+ electrolytes applied in Mg batteries are mainly based on the combination of aMg-containing Lewis base (for example,Grignard reagents and MgCl 2 )and aLewis acid (for example,A lCl 3 [4] Mg(CB 11 H 11 F) 2, [5] Mg[Al-(HFIP) 4 ] 2 , [6] and Mg[B(HFIP) 4 ] 2 (HFIP =-OCH(CF 3 ) 2 ).…”

A Stable, Non-corrosive Perfluorinated Pinacolatoborate Mg Electrolyte for Rechargeable Mg Batteries

“…[2a] As Mg 2+ electrolytes play ap ivotal role in rechargeable Mg batteries,intensive efforts have been made in developing high-performance Mg 2+ electrolytes in the past decade. [3] The reported Mg 2+ electrolytes applied in Mg batteries are mainly based on the combination of aMg-containing Lewis base (for example,Grignard reagents and MgCl 2 )and aLewis acid (for example,A lCl 3 [4] Mg(CB 11 H 11 F) 2, [5] Mg[Al-(HFIP) 4 ] 2 , [6] and Mg[B(HFIP) 4 ] 2 (HFIP =-OCH(CF 3 ) 2 ). [7] However,M g(CB 11 H 12 ) 2 is expensive and not suitable for broad Mg battery applications because the synthesis of carborane anion (CB 11 H 12 À )-containing precursors is not trivial and they are very expensive with very limited commercial availability.M g[B(HFIP) 4 ] 2 lacks chemical stability under ambient conditions as it undergoes chemical decomposition in the presence of moisture, [7a] which is believed to be the same for Mg[Al(HFIP) 4 ] 2 .T herefore,t he development of chloride-free Mg electrolytes from inexpensive commercially available chemicals is required to simultaneously deliver excellent electrochemical performance and chemical stability.H erein, we report ah ighly electrochemically active,a nodically stable,a nd non-corrosive magnesium fluorinated pinacolatoborate,M g[B(O 2 C 2 (CF 3 ) 4 ) 2 ] 2 (abbreviated as Mg-FPB,F igure 1), which has strongly coordinating perfluorinated pinacolato bidentate ligands to stabilize the boron center.…”

“…Mg and astripping potential at Angewandte Chemie Zuschriften À0.111 V, giving overpotential of 197 mV,w hich is comparable with Mg/MgCl 2 /AlCl 3 (MMAC) electrolytes, [3f] but lower than reported chloride-free Mg electrolytes. [4][5][6][7] Ah igh Coulombic efficiencyo f9 5% was obtained at 50 mV s À1 with ag lassy carbon electrode (Supporting Information, Figure S2). Furthermore,t he Mg-FPB electrolyte exhibited an outstanding anodic stability at more than 4.0 Vv s. Mg, which is comparable with reported chloride-free Mg electrolytes, [4][5][6][7] but significantly improved over other reported Mg electrolytes.…”

“…[4][5][6][7] Ah igh Coulombic efficiencyo f9 5% was obtained at 50 mV s À1 with ag lassy carbon electrode (Supporting Information, Figure S2). Furthermore,t he Mg-FPB electrolyte exhibited an outstanding anodic stability at more than 4.0 Vv s. Mg, which is comparable with reported chloride-free Mg electrolytes, [4][5][6][7] but significantly improved over other reported Mg electrolytes. [2c] Chloride-containing Mg electrolytes are corrosive to common metallic current collectors,s uch as SS and Al, which makes them not practically attractive.…”