Towards safer electrolytes: comparing the air stability and electrochemical properties of NaPF6, NaTFSI and Na[B(hfip)4]·DME for sodium-ion batteries

Ould, Darren M. C., Courtney, James M., Morgan, David J. ORCID: https://orcid.org/0000-0002-6571-5731, Curtis, Daniel J., Orzech, Marcin W., Kiani, Sajad, de Boode, Brent, Grey, Clare P., Wright, Dominic S. and Margadonna, Serena 2026. Towards safer electrolytes: comparing the air stability and electrochemical properties of NaPF6, NaTFSI and Na[B(hfip)4]·DME for sodium-ion batteries. EES Batteries 10.1039/d6eb00011h

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Abstract

Sodium-ion batteries (SIBs) are a promising post lithium-ion battery (LIB) technology, which offer advantages in improved sustainability. This work investigates using NaTFSI [TFSI = bis(trifluoromethylsulfonyl)imide] and Na[B(hfip)4]·DME [hfip = OCH(CF3)2 (OiPrF), DME = 1,2-dimethoxyethane] as alternative electrolyte salts to the current benchmark standard NaPF6 and compares their air stability, electrochemical properties and performance in sodium-ion coin cells. Multinuclear NMR spectroscopic experiments found that NaPF6 and NaTFSI were stable to atmospheric air after one month, whereas Na[B(hfip)4]·DME showed signs of degradation. The air stability of NaPF6 was compared to LiPF6, where the latter underwent complete decomposition after 24 hours. Electrochemical investigations in 1 M solutions of ethylene carbonate : diethyl carbonate (EC : DEC) solvent revealed 1 M NaPF6 has the highest bulk conductivity. Cyclic voltammetry experiments showed 1 M NaPF6 and 1 M Na[B(hfip)4]·DME are compatible with aluminium foils up to 4.2 V vs. Na/Na+, whereas 1 M NaTFSI underwent aluminium corrosion. Corrosion could be supressed by either limiting cut-off voltage or by the addition of 2 wt% NaPF6 as an additive, both applicable mitigation strategies. Stable long-term cycling at 1C rate in cells using a Prussian white cathode and hard carbon anode occured with both 1 M NaPF6 and 1 M NaTFSI electrolytes. Thus, 1 M NaTFSI is a viable alternative to 1 M NaPF6 in SIBs with a Prussian white cathode, offering a potentially safer electrolyte choice by limiting HF generation on account of the strong C–F bonds in NaTFSI.

Item Type:	Article
Date Type:	Published Online
Status:	In Press
Schools:	Schools > Chemistry
Additional Information:	License information from Publisher: LICENSE 1: URL: https://creativecommons.org/licenses/by/3.0/, Start Date: 2026-01-21
Publisher:	Royal Society of Chemistry
Date of First Compliant Deposit:	5 February 2026
Date of Acceptance:	21 January 2026
Last Modified:	05 Feb 2026 10:15
URI:	https://orca.cardiff.ac.uk/id/eprint/184439

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