Weakly Solvating Ester Electrolyte for High Voltage Sodium-Ion Batteries

Ethyl acetate (EA) was identified as a promising electrolyte solvent for sodium-ion batteries (SIBs), exhibiting low viscosity, cost-effectiveness, and low toxicity. Despite a significant portion of aggregation being linked to the weak solvation of Na+/EA as revealed by molecular dynamics (MD) simulations, pulsed-field gradient nuclear magnetic resonance (pfg-NMR) analysis identified a noteworthy Na+ diffusion coefficient of 3.95×10−10 m2 s−1 at 25°C in the presence of 1 m NaPF6 salt. Employing fluoroethylene carbonate (FEC) as a film-forming additive to create electrode-electrolyte interphase, this electrolyte surprisingly made ∼210 mAh Na0.97Ca0.03[Mn0.39Fe0.31Ni0.22Zn0.08]O2 (NCMFNZO)/hard carbon (HC) pouch cells achieve a lengthy cycling lifetime of 250 cycles with ∼80 % capacity retention, cycled up to 4.0 V at 40°C. X-ray photoelectron spectroscopy (XPS) revealed increasing interphasial organic species over cycling, augmenting charge transfer resistance on both cathode and anode, particularly during fast charging or low temperatures (<10°C), promoting Na plating. Gas chromatography-mass spectrometry combined with density functional theory identified CO2 as the major gas generated from charged cathode/electrolyte interactions, exhibiting temperature/voltage dependence.