Multifunctional Additives Improve the Electrolyte Properties of Magnesium Borohydride Toward Magnesium–Sulfur Batteries

Highly reductive magnesium borohydride [Mg(BH₄)₂] is compatible with metallic Mg, making it a promising Mg-ion electrolyte for rechargeable Mg batteries. However, pure Mg(BH₄)₂ in ether-based solutions displays very limited solubility (0.01 M), low oxidative stability (<1.8 V vs Mg), and nucleophilic characteristic, all of which preclude its practical utilization for any battery applications. Herein, we present a multifunctional additive of tris(2H-hexafluoroisopropyl)borate (THFPB) for preparing Mg(BH₄)₂-based electrolytes. By virtue of the strong electron-acceptor ability of the THFPB molecule, a transparent and high-concentration Mg(BH₄)₂/THFPB-diglyme (DGM) electrolyte (0.5 M, almost 50 times higher than that of the pristine Mg(BH₄)₂-DGM electrolyte) is first obtained, which shows dramatic performance improvements, including high ionic conductivity (3.72 mS cm–¹ at 25 °C) and high Mg plating/stripping Coulombic efficiency (>99%). The newly-generated active cation and anion species revealed by Raman, NMR and MS spectra, increase the electrochemical potential window from 1.8 V to 2.8 V vs Mg on stainless steel electrode, rendering electrolytes the ability to examine high voltage cathodes. More importantly, on account of the non-nucleophilicity of active electrolyte species, we present the first example of magnesium–sulfur (Mg–S) batteries using Mg(BH₄)₂-based electrolytes, which exhibit a high discharge capacity of 955.9 and 526.5 mA h g–¹ at the initial and 30th charge/discharge cycles, respectively. These achievements not only provide an efficient and specific strategy to eliminate the major roadblocks facing Mg(BH₄)₂-based electrolytes but also highlight the profound effect of functional additives on the electrochemical performances of unsatisfied Mg-ion electrolytes.