Water has a dynamic and significant role in the stability of NaPF₆-based non-aqueous electrolytes for sodium-ion batteries. These electrolytes are highly susceptible to the formation of HF in the presence of water. Most remarkably, HF formation can be observed in battery grade electrolytes which contain <20 ppm H₂O. Fundamental electrolyte characterization is important as electrolyte has a critical role in batteries’ stability and performance. In this work, we studied the degradation mechanism of NaPF₆-based electrolytes with different solvent mixtures (cyclic and acyclic carbonates and varying amounts of water or HF). NMR spectroscopy reveals the electrolyte degradation in these various conditions. In addition, we have conducted fundamental studies on the stability of the sodium-based electrolytes in the presence of a phosphazene-based (FM2) or fluorinated carbonate (FEC) additive. In these systems, additives offer a way to alter the degradation rate and byproducts. Our results indicate that the phosphazene additive acts as a “HF scavenger” in electrolytes containing water. The effects on degradation mitigation can be demonstrated through the electrochemical performance of a NaNi_0.4Fe_0.2Mn_0.4O₂ (NFM)/hard carbon full cell in electrolytes containing FM2 and up to 100 ppm additional water. Without the FM2 additive, the ability of the cell to hold capacity decreased from 91 ± 0.5 mAh/g to 14 ± 2 mAh/g. Remarkably, when FM2 is present in the electrolytes which contain the same concentration of water the discharge capacity was maintained at 44 ± 3 mAh/g at a rate of 15 mA/g. The result suggests that the byproducts of the degradation were detrimental to the battery performance. Through this work, a link can be drawn between the degradation NaPF₆-based non-aqueous electrolytes, the purity of the electrolytes as well as additive effect.