Synthesis of sodium (oxalate) difluoro borate-based hybrid electrolyte salts with enhanced interfacial properties for NaNi0.3Fe0.4Mn0.3O2 cathodes

Abstract

sodium (oxalate) difluoro borate (NaODFB) stands out as a highly promising sodium salt and an influential additive in sodium-ion battery electrolytes. However, the prevailing synthesis methodology suffers from some limitations, notably the generation of undesirable by-products and the inherent safety hazards. Furthermore, NaODFB exhibits only suboptimal performance as a standalone sodium salt in sodium-ion batteries, which has so far hampered the widespread use of this compound. In this study, we present a groundbreaking approach involving a one-step reaction to synthesize a NaODFB-based and NaBF₄-assisted hybrid sodium salt (hereafter referred to as H-NaODFB) in acetonitrile (ACN). This synthesis leverages aluminum trichloride (AlCl₃) as a crucial reaction aid, while sodium tetrafluoroborate (NaBF₄) and anhydrous oxalic acid (H₂C₂O₄) serve as the primary reaction raw materials within the framework of an organic solvent solid-phase method. The prepared H-NaODFB salt was meticulously incorporated into an electrolyte tailored for application in Na/NaNi_0.3Fe_0.4Mn_0.3O₂(NFM) batteries. Comparative analysis with a commercially procured high-purity NaODFB electrolyte revealed that 1 mol per L H-NaODFB electrolyte exhibited superior initial capacity, enhanced cycling stability, and superior kinetic performance. Furthermore, our electrolyte demonstrated diminished interfacial impedance, giving rise to the development of stable cathodic electrolyte interphase (CEI) membranes characterized by excellent ion transport properties predominantly governed by inorganic components. Conclusively, our pioneering work successfully realized a one-step synthesis methodology for NaODFB-based hybrid sodium salts. Integration of H-NaODFB with high-voltage cathode materials in the electrolyte resulted in notable improvements in the long-cycle and multiplicity performance of sodium-ion batteries. This achievement signified a notable advancement in the synthesis and application of electrolyte salts for sodium-ion batteries, which opens up a novel avenue for future advancements in this critical domain.