Sodium Tetraphenylborate: A Fluorine-Free Electrolyte Salt for Sodium-Ion Batteries

Abstract

Sodium-ion batteries are a promising emerging technology due to the low cost and wide abundance of sodium, but commonly use fluorine-containing electrolyte salts, such as sodium hexafluorophosphate (NaPF6), which pose safety and toxicity concerns (e.g. HF formation). This work investigates using sodium tetraphenylborate (NaBPh4) as a non-fluorinated electrolyte salt for sodium-ion batteries. Multinuclear NMR spectroscopy studies revealed NaBPh4 has excellent stability towards atmospheric air and thermogravimetric analysis (TGA) showed this salt has good thermal stability, providing convenient handling, transport and storage. Bulk conductivity measurements using NaBPh4 in ethylene carbonate: diethyl carbonate (EC:DEC 1:1 v/v) at different concentrations found 0.5 M NaBPh4 gave the highest conductivity. Battery cycling in coin cells using a Prussian white (Na2Fe[Fe(CN)6]) cathode and hard carbon anode with an upper cut-off voltage of 3.6 V showed poor capacity retention, however, lowering the upper voltage limit to 3.0 V gave stable cycling, albeit at lower capacity. Solution-state NMR spectroscopy studies on the post-cycled electrolyte found degradation of NaBPh4 occurs during cycling, with triphenylborane and biphenyl hypothesised as likely electrochemical oxidation products.