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Sodium-ion batteries (SIBs) with Earth-abundant elements are promising for global electrification, but electrolyte stability impacts electrochemical performance and safety. This study compares non-fluorinated 1,2-diethoxyethane (DEE) and fluorinated 1,2-bis(2,2-difluoroethoxy)ethane (F4DEE) as electrolyte solvents in Na_0.97Ca_0.03[Mn_0.39Fe_0.31Ni_0.22Zn_0.08]O₂(NCMFNZO)/hard carbon (HC) pouch cells up to 4.0 V. Fluorination slightly reduces ionic conductivity and increases viscosity but significantly enhances electrochemical stability and safety. Cells with F4DEE exhibit lower impedance, reduced gas evolution, and less voltage decay during high-voltage storage at 40 °C. Long-term cycling shows ∼85% capacity retention after 500 cycles at 25 °C and ∼80% at 40 °C with less transition metal dissolution, outperforming DEE-based cells. Isothermal microcalorimetry reveals lower parasitic heat generation with F4DEE, while soft X-ray absorption spectroscopy confirms stabilized Ni and Mn oxidation states, indicating suppressed electrolyte oxidation. Accelerating rate calorimetry reveals improved thermal stability with F4DEE. These findings highlight fluorinated ether solvents as a promising approach to enhance SIB lifespan and safety, with ongoing challenges requiring further solvent and additive optimization. 

Despite extensive research efforts to develop non-aqueous sodium-ion batteries (SIBs) as alternatives to lithium-based energy storage battery systems, their performance is still hindered by electrode-electrolyte side reactions. As a feasible strategy, the engineering of electrolyte additives has been regarded as one of the effective ways to address these critical problems. In this review, we provide a comprehensive overview of recent progress in electrolyte additives for non-aqueous SIBs. We classify the additives based on their effects on specific electrode materials and discuss the functions and mechanisms of each additive category. Finally, we propose future directions for electrolyte additive research, including studies on additives for improving cell performance under extreme conditions, optimizing electrolyte additive combinations, understanding the effect of additives on cathode-anode interactions, and understanding the characteristics of electrolyte additives.