Lithium-sulfur (Li-S) batteries are a promising next-generation energy storage technology due to their high theoretical energy density and low cost. However, the polysulfide shuttle effect and sluggish redox kinetics hinder their practical application. Two-dimensional (2D) materials, particularly transition metal dichalcogenides (TMDCs) and MXenes, have emerged as effective catalysts for polysulfide conversion, addressing these challenges through their unique structural and electronic properties.

The polysulfide shuttle effect arises from the dissolution of lithium polysulfides (LiPS) in the electrolyte, leading to active material loss and rapid capacity degradation. TMDCs and MXenes mitigate this issue through strong adsorption and catalytic conversion of LiPS. TMDCs, such as MoS2 and WS2, possess exposed transition metal sites and sulfur-rich surfaces that chemically bind LiPS, preventing their diffusion. MXenes, like Ti3C2Tx, combine high conductivity with abundant surface functional groups (-O, -F, -OH), enhancing both adsorption and electrocatalytic activity. The synergy between adsorption and catalysis is critical—effective immobilization of LiPS on the catalyst surface must be coupled with accelerated conversion kinetics to suppress the shuttle effect.

Conductive scaffold designs further optimize the performance of 2D material catalysts. A key challenge is ensuring efficient electron and ion transport while maintaining high catalytic activity. MXenes excel in this regard due to their metallic conductivity, which facilitates rapid electron transfer during redox reactions. Hybrid architectures integrating TMDCs with conductive carbon networks (e.g., graphene, carbon nanotubes) improve charge transport and prevent restacking of 2D layers. For example, vertically aligned MoS2 nanosheets on graphene provide abundant edge sites for LiPS adsorption while maintaining a porous structure for electrolyte infiltration. Similarly, MXene-carbon composites enhance polysulfide confinement and redox kinetics, leading to improved sulfur utilization and cycling stability.

Operando studies have provided critical insights into the reaction pathways and catalytic mechanisms of 2D materials in Li-S batteries. In situ X-ray absorption spectroscopy (XAS) and Raman spectroscopy reveal dynamic changes in sulfur speciation during cycling, confirming the role of TMDCs and MXenes in promoting Li2S nucleation and decomposition. Operando electrochemical impedance spectroscopy (EIS) demonstrates reduced charge transfer resistance in the presence of these catalysts, indicating faster reaction kinetics. These studies highlight that the catalytic activity of 2D materials is closely tied to their electronic structure—defect engineering and heteroatom doping can further enhance their performance by modulating adsorption energies and active site density.

Shuttle effect mitigation is achieved through both physical confinement and chemical interactions. The layered structure of TMDCs and MXenes provides a large surface area for LiPS adsorption, while their tunable surface chemistry enables strong binding with polysulfides. For instance, sulfur-terminated MoS2 exhibits higher binding energy for LiPS compared to its pristine counterpart, reducing shuttle effects. MXenes, with their negatively charged surfaces, effectively trap polysulfides via electrostatic interactions. Additionally, the catalytic conversion of long-chain polysulfides (Li2Sx, 4 ≤ x ≤ 8) to insoluble Li2S2/Li2S suppresses their dissolution, further enhancing cycling stability.

Cycling stability is a key metric for practical Li-S batteries, and 2D material catalysts contribute significantly to long-term performance. The high catalytic activity of TMDCs and MXenes ensures efficient polysulfide conversion, reducing active material loss. Moreover, their mechanical flexibility accommodates volume changes during cycling, preventing electrode degradation. For example, cells incorporating Ti3C2Tx MXene as a sulfur host demonstrate capacity retention exceeding 80% after 500 cycles at moderate rates. Similar improvements are observed with MoS2-coated separators, which act as a polysulfide barrier while catalyzing their conversion.

Future research directions include optimizing the interfacial engineering between 2D catalysts and sulfur cathodes, as well as exploring new compositions of TMDCs and MXenes for enhanced catalytic activity. Dual-functional catalysts that combine adsorption and electrocatalysis will be critical for achieving high-energy-density Li-S batteries. Scalable synthesis methods and integration into commercial battery designs remain challenges that require further investigation.

In summary, 2D material catalysts, particularly TMDCs and MXenes, play a pivotal role in advancing Li-S battery technology. Their ability to adsorb and catalytically convert polysulfides addresses the shuttle effect and improves cycling stability. Conductive scaffold designs and operando studies provide a deeper understanding of their mechanisms, guiding the development of next-generation energy storage systems. Continued innovation in material design and electrode engineering will be essential for realizing the full potential of Li-S batteries.