Lithium-sulfur (Li-S) batteries are a promising next-generation energy storage technology due to their high theoretical energy density and low material cost. However, the polysulfide shuttle effect remains a critical challenge that significantly limits their cycle life and practical viability. This phenomenon occurs when soluble lithium polysulfides (LiPS), intermediate species formed during charge and discharge, migrate between the sulfur cathode and lithium anode, leading to active material loss, electrolyte degradation, and anode corrosion. Understanding and mitigating the shuttle effect is essential for improving the performance of Li-S batteries.

The polysulfide shuttle effect arises from the complex redox chemistry of sulfur. During discharge, elemental sulfur (S₈) undergoes a series of reduction reactions to form long-chain polysulfides (Li₂Sₙ, 4 ≤ n ≤ 8), which are highly soluble in common ether-based electrolytes. These dissolved species diffuse to the lithium anode, where they are further reduced to insoluble short-chain sulfides (Li₂S₂/Li₂S). During charging, the short-chain sulfides are oxidized back to long-chain polysulfides, which can diffuse to the cathode. However, incomplete conversion and parasitic reactions lead to continuous shuttling, resulting in rapid capacity fade, low Coulombic efficiency, and increased internal resistance.

The shuttle effect has several detrimental impacts on Li-S battery performance. First, it causes active material loss as sulfur species become trapped in the electrolyte or on the anode surface, reducing the available capacity over cycles. Second, the reduction of polysulfides at the anode forms a passivating layer of Li₂S, increasing interfacial resistance and promoting uneven lithium deposition, which can lead to dendrite formation and safety risks. Third, the continuous redox cycling of polysulfides consumes electrolyte and lithium, further degrading cycle life. Studies have shown that uncontrolled shuttle effects can reduce Coulombic efficiency to below 90% and limit cycle life to fewer than 100 cycles in conventional Li-S cells.

To suppress the polysulfide shuttle, researchers have developed three primary strategies: interlayers, adsorbents, and electrolyte additives. Each approach targets different aspects of the shuttle mechanism, and their effectiveness depends on material selection and integration into the battery design.

Interlayers are physical barriers placed between the cathode and anode to block polysulfide migration while allowing lithium-ion transport. These materials often combine porous structures with polar surfaces to adsorb polysulfides chemically or physically. For example, carbon-based interlayers, such as graphene or carbon nanotubes, provide conductive pathways and physical confinement, but their non-polar surfaces exhibit weak interactions with LiPS. To enhance adsorption, polar materials like metal oxides (TiO₂, MnO₂), metal sulfides (CoS₂, MoS₂), or metal-organic frameworks (MOFs) are incorporated. These materials form strong Lewis acid-base interactions with polysulfides, effectively anchoring them near the cathode. Interlayers can improve cycle life by over 200% while maintaining high sulfur utilization.

Adsorbents are materials integrated into the cathode or separator to immobilize polysulfides. Unlike interlayers, adsorbents are dispersed within the electrode or coated onto the separator, providing a larger surface area for polysulfide trapping. Common adsorbents include heteroatom-doped carbons (N, O, S), which introduce polar sites for polysulfide binding, and conductive polymers (PEDOT:PSS, PANI), which combine adsorption with electronic conductivity. Advanced adsorbents like single-atom catalysts (SACs) have shown exceptional performance by facilitating polysulfide conversion kinetics. For instance, nitrogen-coordinated iron SACs can achieve nearly 100% Coulombic efficiency for over 500 cycles by accelerating Li₂S precipitation and decomposition.

Electrolyte additives are another effective method to mitigate the shuttle effect. These compounds modify the electrolyte chemistry to reduce polysulfide solubility or stabilize the anode interface. Lithium nitrate (LiNO₃) is a widely used additive that forms a protective solid-electrolyte interphase (SEI) on the lithium anode, preventing polysulfide reduction. However, LiNO₃ is consumed over time, limiting its long-term effectiveness. Alternative additives, such as ionic liquids (e.g., Pyr₁₄TFSI) or fluorinated ethers (e.g., TTFE), can suppress polysulfide dissolution through solvation energy tuning. Recent studies have also explored redox mediators like organosulfur compounds, which facilitate reversible polysulfide conversion, reducing shuttle-related losses.

The choice of shuttle mitigation strategy depends on the specific requirements of the Li-S battery system. Interlayers and adsorbents are more suitable for high-sulfur-loading cathodes, where polysulfide concentration is high, while electrolyte additives are essential for stabilizing the anode interface. Combining multiple approaches, such as a polar interlayer with a optimized electrolyte, has shown synergistic effects in prolonging cycle life. For example, cells with TiO₂-coated separators and LiNO₃ additives have demonstrated over 80% capacity retention after 300 cycles at practical sulfur loadings (> 3 mg/cm²).

Despite these advances, challenges remain in scaling up shuttle mitigation techniques. Interlayers and adsorbents often increase cell weight and complexity, reducing energy density. Electrolyte additives may introduce compatibility issues with other cell components or increase cost. Future research should focus on optimizing material designs for minimal trade-offs between performance and practicality. For instance, lightweight polymer-based interlayers or multifunctional additives that simultaneously address shuttle effects and lithium dendrite growth could provide more holistic solutions.

In summary, the polysulfide shuttle effect is a major obstacle to the commercialization of Li-S batteries, causing rapid capacity fade and poor cycle life. Interlayers, adsorbents, and electrolyte additives have proven effective in suppressing shuttle behavior, but each method has limitations that must be addressed. By continuing to refine these strategies and exploring new materials, researchers can unlock the full potential of Li-S batteries for high-energy-density applications.