The solvation structure, dynamics, and transport properties, as well as thermal and electrochemical stabilities of “solvent-in-salt” (SIS) electrolytes, also known as highly concentrated electrolytes, are far from fully understood. Furthermore, these special types of electrolytes are almost without exception based on fluorinated salts. In contrast, here we report on fluorine-free SIS electrolytes comprising ambient temperature liquid sodium bis(2-(2-ethoxyethoxy)ethyl)phosphate (NaDEEP) salt and tris(2-(2-ethoxyethoxy)ethyl)phosphate (TEOP) solvent, for which the ionic conductivities and ion diffusivities are altered profoundly as the salt concentration is increased. A careful molecular level analysis reveals a microstructure with a “solvent-rich” phase with almost an order of magnitude faster ion diffusion than in a “salt-rich” phase. Aggregated ionic structures in these SIS electrolytes lead to higher ionic conductivities alongside lower glass transition temperatures, <−80 °C, but also agreeable thermal stabilities, up to 270 °C, and improved anodic stabilities, possibly up to 7.8 V vs. Na/Na+ and at least >5 V vs. Na/Na+. Altogether, this provides a foundation for both better understanding and further development of fluorine-free SIS electrolytes for sodium batteries.

A series of orthoborate-based ionic materials of bis(glycolato)borate ([BGB]) and bis(ethylene-1,2-dioxy)borate anions ([BEDB]) coupled with tetrabutylphosphonium ([P4444]) and tetrabutylammonium ([N4444]) cations have been synthesized, and their physicochemical properties are characterized. The ionic materials based on the most popular orthoborate anion, bis(oxalato)borate anion ([BOB]), which contains four carbonyl groups, are all liquid at ambient temperature, while the bis(glycolato)borate ([BGB]) anion, with two carbonyl groups, and the bis(ethylene-1,2-dioxy)borate ([BEDB]) anion, without carbonyl groups, render solids at ambient temperature. The ionic materials based on the [BGB] anion display the highest decomposition temperatures, and those based on the BEDB anion are the lowest. The [P4444][BGB], [P4444][BEDB], and [N4444][BEDB] salts feature significantly wider plastic phase I temperature ranges than their analogues. FTIR spectroscopy, multinuclear (15N, 31P, 13C, and 11B) solid-state NMR spectroscopy, and single-crystal X-ray diffraction were all used to unveil the ionic interactions and structural features, which display weaker ionic interactions for [BEDB] compared to [BGB] when bearing the same cation and present relatively higher crystallinity of [P4444][BGB] among the ionic materials.

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 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.