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

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 (OⁱPr^F), 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.