ABSTRACT
Sodium-ion batteries (SIBs) are a promising grid-level storage technology due to the abundance and low cost of sodium. The development of new electrolytes for SIBs is imperative since it impacts battery life and capacity. Currently, sodium hexafluorophosphate (NaPF6 ) is used as the benchmark salt, but is highly hygroscopic and generates toxic HF. This work describes the synthesis of a series of sodium borate salts, with electrochemical studies revealing that Na[B(hfip)4 ]â DME (hfip=hexafluoroisopropyloxy, Oi PrF ) and Na[B(pp)2 ] (pp=perfluorinated pinacolato, O2 C2 (CF3 )4 ) have excellent electrochemical performance. The [B(pp)2 ]- anion also exhibits a high tolerance to air and water. Both electrolytes give more stable electrode-electrolyte interfaces than conventionally used NaPF6 , as demonstrated by impedance spectroscopy and cyclic voltammetry. Furthermore, they give greater cycling stability and comparable capacity to NaPF6 for SIBs, as shown in commercial pouch cells.
ABSTRACT
Sodium-ion batteries represent a promising alternative to lithium-ion systems. However, the rapid growth of sodium-ion battery technology requires a sustainable and scalable synthetic route to high-grade sodium hexafluorophosphate. This work demonstrates a new multi-gram scale synthesis of NaPF6 in which the reaction of ammonium hexafluorophosphate with sodium metal in THF solvent generates the electrolyte salt with the absence of the impurities that are common in commercial material. The high purity of the electrolyte (absence of insoluble NaF) allows for concentrations up to 3â M to be obtained accurately in binary carbonate battery solvent. Electrochemical characterization shows that the degradation dynamics of sodium metal-electrolyte interface are different for more concentrated (>2â M) electrolytes, suggesting that the higher concentration regime (above the conventional 1â M concentration) may be beneficial to battery performance.