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 new electrolytes for SIBs is imperative since it impacts battery life capacity. Currently, sodium hexafluorophosphate (NaPF6) used as benchmark salt, but highly hygroscopic generates toxic HF. This work describes synthesis series borate salts, with electrochemical studies revealing that Na[B(hfip)4]⋅DME (hfip=hexafluoroisopropyloxy, OiPrF) Na[B(pp)2] (pp=perfluorinated pinacolato, O2C2(CF3)4) have excellent performance. [B(pp)2]− anion also exhibits high tolerance air water. Both give more stable electrode-electrolyte interfaces than conventionally NaPF6, demonstrated by impedance spectroscopy cyclic voltammetry. Furthermore, they greater cycling stability comparable capacity NaPF6 SIBs, shown in commercial pouch cells.