Engineering optimization approach of nonaqueous electrolyte for sodium ion battery with long cycle life and safety

Electrolyte design strategies are closely related to the capacities, cycle life and safety of sodium–ion batteries. In this study, we aimed to optimize electrolyte with the focus on engineering aspects. The basic physicochemical properties including ionic conductivity, viscosity, wettability and thermochemical stability of the electrolytes using NaPF₆ as the solute and the mixed solvent with different components of EMC, DMC or DEC in PC or EC were systematically measured. Ah pouch cell with NaNi_1/3Fe_1/3Mn_1/3O₂/hard carbon electrodes was used to evaluate the performance of the prepared electrolytes. By using the Inductive Coupled Plasma Emission Spectrometer (ICP), X-ray photoelectron spectroscopy (XPS), Thermogravimetric-differential scanning calorimetry (TG-DSC) and Accelerating Rate Calorimeter (ARC), we show that an optimized electrolyte can effectively promote the formation of a protective interfacial layer on two electrodes, which not only retards parasitic reactions between the electrodes and electrolyte but also suppresses dissolution of metal ions from the cathode. With an optimized electrolyte, a NaNi_1/3Fe_1/3Mn_1/3O₂/hard carbon cell can attain 56.16% capacity retention under the low temperature of −40 °C, and can be able to retain 80% capacity retention after more than 2500 cycles while presenting excellent thermal safety.