Solid electrolytes for sodium-ion batteries have gained significant attention as potential enablers for safer and higher-energy-density energy storage systems. Three prominent materials in this category are β-alumina, Na3PS4, and Na3Zr2Si2PO12, each offering distinct advantages and challenges in terms of ionic conductivity, synthesis methods, and electrochemical stability. These materials aim to address the limitations of liquid electrolytes, including flammability, dendrite formation, and narrow electrochemical windows, while maintaining high ionic transport properties suitable for practical battery applications.

β-alumina is one of the earliest studied solid electrolytes for sodium-ion conduction, with a hexagonal crystal structure that provides fast ion transport along two-dimensional conduction planes. The room-temperature ionic conductivity of β-alumina typically ranges between 0.01 and 0.1 S/cm, making it competitive with liquid electrolytes. However, its anisotropic conductivity and brittle mechanical properties pose challenges for thin-film processing and integration into solid-state cells. Synthesis of β-alumina often involves high-temperature solid-state reactions or sol-gel methods, requiring precise control of stoichiometry to avoid secondary phases that degrade performance. Compatibility with sodium metal anodes is generally good due to its electrochemical stability, but interfacial resistance can still develop over cycling. Recent advances focus on doping with elements like lithium or magnesium to enhance mechanical strength and reduce grain boundary resistance.

Na3PS4 belongs to the thiophosphate family and exhibits a cubic structure with isotropic sodium-ion conduction. Its ionic conductivity at room temperature reaches approximately 0.1 to 0.5 mS/cm, which is lower than β-alumina but still sufficient for certain applications. A key advantage of Na3PS4 is its relatively low synthesis temperature, often achieved through mechanochemical methods or solution-based routes, enabling scalable production. However, this material suffers from poor stability against sodium metal, forming resistive interphases that increase cell impedance over time. Moisture sensitivity is another critical limitation, as exposure to humid environments leads to decomposition and release of toxic H2S gas. Research efforts have explored halogen doping (e.g., Na3PS4-xClx) to improve air stability and composite approaches with polymers or oxides to enhance mechanical robustness.

Na3Zr2Si2PO12, a NASICON-type electrolyte, offers a balance between ionic conductivity and chemical stability. Its three-dimensional framework supports sodium-ion diffusion with conductivities in the range of 0.1 to 1 mS/cm at ambient temperatures. The material demonstrates better moisture resistance compared to Na3PS4 and maintains stable interfaces with both high-voltage cathodes and sodium metal anodes, though interfacial engineering is still required to minimize resistance. Synthesis of Na3Zr2Si2PO12 typically involves solid-state reactions at elevated temperatures, with careful control of ZrO2 impurities that can hinder ion transport. Doping strategies, such as partial substitution of Zr with Y or Ca, have been employed to optimize sintering behavior and enhance total conductivity. Mechanical properties remain a concern due to inherent brittleness, prompting investigations into flexible composite membranes incorporating binders or fibrous scaffolds.

A critical challenge across all three solid electrolytes is achieving durable interfaces with electrode materials. Sodium metal anodes tend to form uneven electrodeposits, leading to dendrite penetration and short-circuiting in solid-state cells. Cathode compatibility varies depending on the operating voltage, with some oxide-based cathodes inducing detrimental side reactions at high potentials. Composite electrolytes combining multiple materials, such as β-alumina particles embedded in polymer matrices, have shown promise in mitigating interfacial issues while maintaining adequate ionic transport. Similarly, bilayer or multilayer architectures can isolate reactive components and extend cycle life.

Moisture sensitivity remains a major hurdle, particularly for sulfide-based electrolytes like Na3PS4. Even brief exposure to ambient humidity can degrade performance, necessitating stringent dry-room conditions during cell fabrication and operation. Advances in protective coatings and hydrophobic additives aim to improve handling without compromising ionic conductivity. For example, thin Al2O3 layers deposited via atomic layer deposition have demonstrated effectiveness in shielding Na3PS4 from moisture while permitting sodium-ion migration.

Mechanical stability is another area requiring optimization. Solid electrolytes must withstand stack pressure during cell assembly and volume changes during cycling without cracking or delaminating. Composite approaches incorporating flexible polymers or ductile inorganic phases help alleviate stress concentrations, though trade-offs in ionic conductivity must be carefully managed. Recent work on glass-ceramic composites has yielded materials with improved fracture toughness while preserving high ionic mobility.

Recent progress in doping and interface engineering has pushed the performance boundaries of these solid electrolytes. For β-alumina, tailored sintering aids and grain boundary modifiers have reduced interfacial resistance, enabling more efficient charge transfer. In Na3PS4, aliovalent doping with elements like Ge or Sn has enhanced both ionic conductivity and phase stability. Na3Zr2Si2PO12 variants with optimized stoichiometries exhibit lower activation energies for ion hopping, contributing to better low-temperature performance. These advances, combined with scalable synthesis techniques, are critical for transitioning from lab-scale demonstrations to commercial viability.

Despite these improvements, key challenges persist in achieving the necessary combination of high ionic conductivity, electrochemical stability, and mechanical robustness for widespread adoption in sodium-ion batteries. Continued research into novel material compositions, advanced characterization techniques, and innovative cell designs will be essential to overcome these barriers. The development of solid electrolytes tailored for sodium-ion systems holds significant promise for next-generation energy storage, offering potential improvements in safety, energy density, and cycle life compared to conventional liquid electrolyte systems.