Polysulfide crossover remains a significant challenge in sodium-sulfur (Na-S) battery systems, particularly those operating at intermediate temperatures (100-350°C). The formation and migration of sodium polysulfides (Na₂Sₓ, where x=3-5) lead to active material loss, corrosion of sodium metal anodes, and reduced Coulombic efficiency. This phenomenon stems from the electrochemical reactions during discharge, where sulfur undergoes stepwise reduction to form long-chain polysulfides (Na₂S₅, Na₂S₄) before converting to shorter chains (Na₂S₃) and ultimately Na₂S. These soluble intermediates diffuse through the β-alumina solid electrolyte (BASE) due to concentration gradients and electric field-driven migration, causing the shuttle effect.

The migration mechanism through β-alumina involves both bulk diffusion and grain boundary transport. BASE possesses a crystalline structure with conduction channels for Na⁺ ions but allows polysulfide permeation through microstructural defects. Studies indicate that Na₂S₄ exhibits higher mobility than Na₂S₃ due to its lower charge density and smaller solvation shell. At operating temperatures above 150°C, polysulfides become increasingly fluid, enhancing their penetration into BASE. The shuttle effect manifests as continuous redox cycling of polysulfides between electrodes, leading to self-discharge and capacity fade. Coulombic efficiency drops to 80-85% in unmitigated systems after 100 cycles due to this crossover.

Electrolyte modifications have been explored to suppress polysulfide dissolution. One approach involves using ternary eutectic electrolytes (NaCl-AlCl₃-Na₂S) that reduce polysulfide solubility by altering the molten salt chemistry. Research demonstrates that adding 5-10 wt% NaI to conventional NaAlCl₄ electrolytes decreases polysulfide mobility by forming stable complexes. Another strategy employs ionic liquid additives such as pyrrolidinium-based cations, which create a electrostatic shield around polysulfides. These modifications reduce shuttle currents by 40-60% compared to baseline systems.

Interlayer design between the cathode and BASE has shown promise in blocking polysulfide migration. Porous carbon interlayers with tailored pore sizes below 2 nm physically trap Na₂Sₓ while maintaining ionic conductivity. Recent work on nitrogen-doped carbon interlayers reveals enhanced polysulfide adsorption through pyridinic-N binding sites. Ceramic interlayers like Na₃Zr₂Si₂PO₁₂ exhibit selective Na⁺ conduction while rejecting polysulfides due to their large ionic radius. Dual-layer architectures combining microporous carbon with a thin ceramic coating achieve 92-94% Coulombic efficiency over 200 cycles.

Cathode additives play a crucial role in immobilizing polysulfides. Metal sulfides (FeS₂, CoS₂) catalyze the conversion of long-chain polysulfides to insoluble Na₂S₂/Na₂S, reducing their concentration in the melt. Conductive polymers such as polyaniline form a charge-selective barrier that permits Na⁺ transport but repels polysulfide anions. Incorporating 5-8 vol% Al₂O₃ nanoparticles into sulfur cathodes increases polysulfide adsorption capacity by 300% through Lewis acid-base interactions. These additives typically improve cycle life by 1.5-2 times compared to pure sulfur cathodes.

Solid-state barriers represent the most advanced containment strategy. Thin-film coatings of Na-β"-alumina (2-5 μm) deposited on BASE via aerosol-assisted chemical vapor deposition reduce polysulfide permeability by three orders of magnitude. Glass-ceramic composites containing B₂O₃ form an amorphous phase that seals grain boundaries in BASE, decreasing polysulfide leakage currents from 0.8 mA/cm² to below 0.1 mA/cm². Atomic layer deposition of Al₂O₃ creates pinhole-free interfacial layers that maintain 98% Coulombic efficiency at 1C discharge rates.

Recent innovations focus on multifunctional barriers combining physical blocking with chemical trapping. Graded membranes with increasing porosity from the BASE to cathode side allow controlled sulfur transport while preventing bulk crossover. Hybrid organic-inorganic layers utilizing polyhedral oligomeric silsesquioxane (POSS) crosslinkers demonstrate exceptional stability against molten polysulfides at 300°C. These advanced barriers enable Na-S batteries to achieve energy densities above 400 Wh/kg with less than 5% capacity decay per 100 cycles.

The impact of these containment strategies extends beyond Coulombic efficiency improvements. Reduced polysulfide crossover decreases sodium anode corrosion rates, as evidenced by a 70% reduction in interfacial resistance after 500 hours of operation. Stable solid electrolyte interphase (SEI) formation occurs when polysulfide concentrations remain below 0.1M in the anode compartment. Systems with effective polysulfide barriers demonstrate thermal runaway thresholds 20-30°C higher than conventional designs due to suppressed exothermic reactions at the anode.

Long-term durability testing reveals that the combination of electrolyte modification, interlayer optimization, and cathode additives provides synergistic effects. Cells employing all three strategies simultaneously show 99% capacity retention after 1000 cycles at 0.5C, compared to 65% retention in baseline cells. Post-mortem analysis indicates nearly complete elimination of sulfur species in the anode compartment, confirming the effectiveness of multilayer containment approaches.

Ongoing research targets further reduction in polysulfide mobility through crystalline phase engineering of β-alumina. Doping with Mg²⁺ or Li⁺ alters the lattice parameters to create narrower conduction pathways that sterically hinder polysulfide transport. Computational modeling predicts that optimized dopant concentrations could decrease polysulfide diffusivity by an additional order of magnitude while maintaining Na⁺ conductivity above 0.2 S/cm at 300°C.

The development of these containment strategies has enabled the commercialization of intermediate-temperature Na-S batteries for grid storage applications. Current systems achieve round-trip efficiencies of 85-90% with calendar lifetimes exceeding 15 years, making them competitive with lithium-ion alternatives for large-scale energy storage. Future advancements in polysulfide blocking technologies may expand Na-S battery applications to electric vehicles and aerospace systems where energy density and safety are paramount.