Corrosion in sodium-sulfur (Na-S) battery systems presents a significant challenge to long-term operational reliability, particularly due to the aggressive chemical environment created by molten sodium and sodium polysulfides at elevated operating temperatures of 300–350°C. The degradation mechanisms affect metallic components such as current collectors, cell housings, and seals, ultimately compromising structural integrity and electrochemical performance. Understanding these mechanisms and developing mitigation strategies is critical for advancing Na-S battery technology.

The primary corrosion pathways in Na-S systems stem from the reactivity of molten sodium and the highly corrosive nature of sodium polysulfides (Na2Sx, where x ranges from 3 to 5). Molten sodium, while an excellent conductor, aggressively attacks many metals, leading to embrittlement and intergranular corrosion. Meanwhile, sodium polysulfides, formed during discharge cycles, are particularly corrosive to steel and other common structural materials. These species induce sulfidation, a process where sulfur penetrates the metal lattice, forming brittle metal sulfides that weaken mechanical properties.

Current collectors and cell housings, typically constructed from stainless steel or nickel-based alloys, are vulnerable to both uniform and localized corrosion. At operating temperatures, chromium in stainless steel forms a protective oxide layer, but prolonged exposure to polysulfides leads to chromium depletion and sulfide formation. For example, Type 316 stainless steel exhibits significant weight loss and surface pitting after extended operation in Na-S cells due to the breakdown of its passive layer. Nickel-based alloys, while more resistant, still suffer from sulfidation at grain boundaries, leading to crack propagation under thermal cycling.

Material solutions to mitigate corrosion include chromium-coated steel, refractory metals, and ceramic coatings. Chromium-coated steel leverages a dense, adherent chromium layer to provide a barrier against sulfur penetration. Laboratory tests show that chromium coatings of at least 20 µm thickness significantly reduce corrosion rates, though long-term stability requires further validation. Refractory metals such as molybdenum and tantalum offer superior resistance due to their high melting points and thermodynamic stability with sulfur. However, their high cost and manufacturing complexity limit widespread adoption. Ceramic coatings, particularly alumina (Al2O3) and yttria-stabilized zirconia (YSZ), provide excellent chemical inertness and thermal stability. Plasma-sprayed YSZ coatings have demonstrated corrosion resistance exceeding 10,000 hours in simulated Na-S environments, though adhesion to metallic substrates remains a challenge under thermal cycling.

Seal degradation represents another critical failure mode in Na-S batteries. The seals, often made from glass or ceramic-to-metal composites, must withstand thermal expansion mismatches while preventing sulfur vapor permeation. Field failures have been documented where sulfur penetration through microcracks in seals led to rapid corrosion of external metallic components. Advanced sealing solutions include compressive seals using molybdenum or graphite gaskets, which maintain integrity under thermal cycling while minimizing sulfur leakage.

Corrosion monitoring in Na-S batteries employs several techniques to assess material degradation in real time. Electrochemical impedance spectroscopy (EIS) tracks interfacial resistance changes caused by corrosion product buildup. High-temperature microscopy and X-ray diffraction (XRD) provide in-situ analysis of phase transformations and sulfide formation. Post-mortem studies using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) reveal elemental redistribution and corrosion morphology. These methods collectively inform predictive models for lifetime estimation.

Lifetime prediction models integrate corrosion kinetics with operational parameters such as temperature, current density, and cycling frequency. Empirical models based on Arrhenius-type equations correlate corrosion rates with temperature, while mechanistic models account for polysulfide concentration gradients and diffusion-limited sulfide growth. Accelerated aging tests at elevated temperatures help validate these models, though extrapolation to real-world conditions requires careful consideration of thermal gradients and mechanical stresses.

Case studies of field failures highlight the consequences of inadequate corrosion protection. In one instance, a grid-scale Na-S battery installation experienced premature failure due to sulfur permeation through a compromised seal, leading to rapid corrosion of the external steel housing. Post-failure analysis revealed extensive sulfide formation and localized wall thinning. Another case involved current collector failure in a high-power Na-S system, where sulfidation-induced cracking disrupted electrical connectivity. These failures underscore the need for robust material selection and corrosion monitoring protocols.

Future advancements in Na-S battery corrosion resistance may explore novel alloy compositions, such as high-entropy alloys with tailored sulfidation resistance, or advanced coating techniques like atomic layer deposition (ALD) for ultra-thin, conformal barriers. Additionally, integrating real-time corrosion sensors with battery management systems could enable proactive maintenance and extend operational lifetimes.

In summary, corrosion in Na-S batteries arises from the synergistic effects of molten sodium and polysulfides, necessitating material innovations and rigorous monitoring. Chromium coatings, refractory metals, and ceramic barriers offer promising solutions, though long-term durability under cycling conditions remains an area of active research. Field failures emphasize the importance of seal integrity and sulfur containment. By combining advanced materials with predictive modeling, the reliability and commercial viability of Na-S battery systems can be significantly enhanced.