Non-aqueous organic electrolytes play a critical role in the performance of sodium-ion batteries, influencing key parameters such as ionic conductivity, electrochemical stability, and interfacial compatibility. These electrolytes typically consist of sodium salts dissolved in organic solvent blends, often enhanced with functional additives to improve cycling stability and safety. The optimization of these components directly impacts the formation of a stable solid-electrolyte interphase (SEI), which governs long-term cell performance.

**Solvent Blends: Carbonates and Ethers**
The choice of solvents determines the electrolyte's viscosity, dielectric constant, and sodium-ion solvation capability. Carbonate-based solvents, such as ethylene carbonate (EC), propylene carbonate (PC), and dimethyl carbonate (DMC), are widely used due to their high dielectric constants and ability to dissolve sodium salts. EC, in particular, facilitates the formation of a stable SEI on anode materials like hard carbon due to its reductive decomposition at low potentials. Binary or ternary blends, such as EC:DMC or EC:PC:DMC, balance high ionic conductivity with wide electrochemical stability windows. Ether-based solvents, including dimethoxyethane (DME) and tetrahydrofuran (THF), exhibit lower viscosity and faster ion transport but suffer from narrower stability windows and higher flammability. Their use is often limited to specific applications requiring high-rate capability.

**Sodium Salts: Conductivity and Stability**
The selection of sodium salts affects ionic dissociation, thermal stability, and interfacial reactions. Sodium hexafluorophosphate (NaPF6) is commonly employed due to its high ionic conductivity and moderate stability against hydrolysis. However, it can decompose at elevated temperatures, releasing corrosive HF. Sodium perchlorate (NaClO4) offers superior thermal and electrochemical stability but poses safety concerns due to its oxidative nature. Sodium bis(fluorosulfonyl)imide (NaFSI) and sodium triflate (NaOTf) are emerging alternatives with higher thermal stability and reduced moisture sensitivity. The concentration of these salts also plays a role; moderately concentrated electrolytes (1–3 M) typically achieve optimal conductivity, while highly concentrated systems (>3 M) can enhance SEI stability but increase viscosity.

**Additive Effects: SEI Modifiers and Stabilizers**
Functional additives are incorporated in small quantities (1–5 wt%) to address specific challenges. Fluoroethylene carbonate (FEC) is a widely studied additive that preferentially decomposes to form a robust, fluorine-rich SEI layer, improving cyclability and reducing irreversible capacity loss. Vinylene carbonate (VC) similarly enhances SEI formation by polymerizing at the electrode surface, creating a more elastic interface resistant to cracking. Other additives, such as succinonitrile (SN), improve thermal stability, while cesium hexafluorophosphate (CsPF6) can suppress dendrite growth on sodium metal anodes. The synergistic effects of multiple additives are often explored to achieve balanced performance.

**Decomposition Mechanisms and SEI Formation**
The reduction of carbonate solvents and salts at the anode surface leads to SEI formation, a process critical for preventing continuous electrolyte breakdown. EC decomposes at around 0.8 V vs. Na/Na+, generating species like sodium ethylene dicarbonate (SEDC) and polycarbonates. NaPF6 contributes inorganic components such as NaF and P-O compounds, which enhance mechanical stability. The SEI's composition and morphology depend on the electrolyte formulation; excessive decomposition can lead to thick, resistive layers, while controlled reactions yield thin, ion-conductive films. In ether-based systems, the SEI tends to be more organic-rich and less stable against sodium plating.

**Ionic Conductivity Optimization**
Achieving high ionic conductivity requires balancing salt dissociation and solvent mobility. The Walden rule suggests that higher dielectric constants (e.g., from EC or PC) improve salt dissociation, while low-viscosity solvents (e.g., DMC or DME) enhance ion mobility. Optimal blends, such as EC:DMC (1:1 by volume) with 1 M NaPF6, typically achieve conductivities in the range of 8–12 mS/cm at 25°C. Temperature also plays a significant role; conductivity decreases sharply below -20°C due to increased viscosity and reduced ion mobility. Additives like FEC can mitigate this by modifying the solvation structure and reducing freezing points.

**Challenges and Trade-offs**
While carbonate blends dominate commercial research, their high flammability remains a safety concern. Ethers offer safer alternatives but require stabilization against oxidation at high voltages. Sodium salts like NaPF6 demand stringent moisture control during handling, while NaClO4 introduces safety risks in large-scale systems. Additives improve SEI quality but may increase costs or reduce energy density. Future research focuses on novel solvent-salt-additive combinations that simultaneously address conductivity, stability, and cost.

In summary, non-aqueous organic electrolytes for sodium-ion batteries rely on carefully engineered solvent blends, sodium salts, and additives to achieve high performance. The interplay between these components dictates SEI quality, ionic transport, and overall cell longevity. Continued advancements in formulation strategies will be essential for enabling the widespread adoption of sodium-ion batteries in energy storage applications.