Fluorine-Free Bis(glycolato)borate Anion-Based Salts and Electrolytes: Structures, Properties, and Lithium Compatibility

Abstract

A number of bis(glycolato)borate (BGB) anion-based salts, comprising Li⁺, Na⁺, K^+, Mg²⁺ and Ca²⁺ cations, has been synthesized and characterized. Fluorine-free electrolytes based on LiBGB and organic solvents, such as dimethyl sulfoxide (DMSO), triethyl phosphate (TEP), and trimethyl phosphate (TMP) have been created and their transport properties, thermal and electrochemical stabilities, and lithium compatibility are examined. The ionic conductivities of the 1 M LiBGB-TEP and 1 M LiBGB-TMP electrolytes are ca. 2–3 times lower than for the 1 M LiBGB-DMSO electrolytes (2.05, 2.65 vs. 5.70 mS cm⁻¹ at 25 °C), and as compared to the state-of-the-art 1 M lithium hexafluorophosphate (LiPF₆) in EC:DEC (EC:DEC=1 : 1 in vol., LP40) they display lower ionic conductivities, but the formers’ redox stability on aluminum (Al) and glassy carbon electrodes are much better. Concentrated (>1 M) LiBGB-DMSO electrolytes display enhanced redox stability, but worse Al passivation. Among the electrolytes, 1 M LiBGB-TMP achieves the best long-term stability over 300 h at 0.1 mA/cm² for Li plating-stripping while the Li compatibility needs to be further improved. Overall, this study introduces a family of versatile fluorine-free orthoborate salts and electrolytes for mono- and divalent batteries, and a fundamental understanding of their transport and electrochemical properties, aiming towards battery applications.