Tuning Lewis acid-base interactions in molten sodium batteries to enhance charge transport

Low temperature (<150 °C) molten sodium batteries promise a cost-effective approach to grid-scale energy storage. The molten salt NaI-AlCl 3 catholyte achieves over 200 Wh kg −1 , but its accessible capacity is limited at low rates due to electrode blocking at the catholyte–electrode interface. To improve NaI solubility and charge transport, several Lewis acid additives were investigated, ranging from the hard Lewis acid AlCl 3 to the soft base NaI. Spectroscopic and electrochemical evaluations were conducted on the control catholyte (60/40 mol% NaI/AlCl 3 ) and formulations where 1 mol% AlCl 3 had been replaced with AlI 3 , BiCl 3 , GaCl 3 , SbCl 3 , SnCl 2 , or SnI 2 . All additives affected the relative concentrations of AlX 4 − (X = Cl, I) species. Raman spectroscopic analysis also revealed GaCl 3 and SbCl 3 additives weakened Al-X bonds, correlating with improved electrochemical performance at the electrode-molten salt interface, evidenced by decreased Tafel slopes, increased current during oxidation, and more stable potentials under constant current. Molten sodium batteries were assembled at 135 °C using GaCl 3 , SbCl 3 , AlI 3 (a poor performer), and a control (no additive). GaCl 3 reduced the separator-salt interfacial resistance from 24.2 to 9.40 Ω cm 2 and enabled cycling at 50 mA cm −2 for 11.3 mAh cm −2 . The cyclable capacity increased from 60% to 70% (119 mAh cm −2 ) at 25 mA cm −2 charge and 5 mA cm −2 discharge. Long-term cycling (100 cycles, 1200 h) revealed mixed improvements with GaCl 3 or SbCl 3 . Performance differences are rationalized by the differing Lewis acidities and their ability to promote charge transfer across the electrode-catholyte and NaSICON-catholyte interfaces, creating high-performing, low-temperature molten sodium batteries.