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

Abstract

• 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.