Issue 3, 2024

Fluorine-free “solvent-in-salt” sodium battery electrolytes: solvation structure and dynamics

Abstract

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.

Graphical abstract: Fluorine-free “solvent-in-salt” sodium battery electrolytes: solvation structure and dynamics

Supplementary files

Article information

Article type
Paper
Submitted
02 Jan 2024
Accepted
08 Feb 2024
First published
09 Feb 2024
This article is Open Access
Creative Commons BY license

Energy Adv., 2024,3, 564-573

Fluorine-free “solvent-in-salt” sodium battery electrolytes: solvation structure and dynamics

Y. Xu, A. Filippov, S. Bhowmick, P. Johansson and F. U. Shah, Energy Adv., 2024, 3, 564 DOI: 10.1039/D4YA00002A

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