Sodium Borates: Expanding the Electrolyte Selection for Sodium-Ion Batteries

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 ]⋅DME (hfip=hexafluoroisopropyloxy, Oⁱ Pr^F ) 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.