Towards safer electrolytes: comparing the air stability and electrochemical properties of NaPF 6 , NaTFSI and Na[B(hfip) 4 ]·DME for sodium-ion batteries

The air stability, electrochemical properties and battery performance of NaPF 6 , NaTFSI and Na[B(hfip) 4 ]·DME have been compared. NaPF 6 was found to have excellent tolerance to air while 1 M NaTFSI gave similar cycling performance to 1 M NaPF 6 . 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(CF 3 ) 2 (O i Pr F ), DME = 1,2-dimethoxyethane] as alternative electrolyte salts to the current benchmark standard NaPF 6 and compares their air stability, electrochemical properties and performance in sodium-ion coin cells. Multinuclear NMR spectroscopic experiments found that NaPF 6 and NaTFSI were stable to atmospheric air after one month, whereas Na[B(hfip) 4 ]·DME showed signs of degradation. The air stability of NaPF 6 was compared to LiPF 6 , 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 NaPF 6 has the highest bulk conductivity. Cyclic voltammetry experiments showed 1 M NaPF 6 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% NaPF 6 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 NaPF 6 and 1 M NaTFSI electrolytes. Thus, 1 M NaTFSI is a viable alternative to 1 M NaPF 6 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.