Room-temperature sodium-sulfur batteries, with their low cost, abundant resource reserves, and extremely high theoretical energy density, are widely regarded as a key technological solution for future large-scale energy storage systems. In current research, to overcome the polysulfide shuttle effect, researchers often employ porous carbon frameworks for physical confinement or use polar host materials to introduce chemical interactions, thereby enhancing the affinity for polysulfides and promoting their catalytic conversion. However, these strategies still fundamentally rely on the dissolution and re-precipitation conversion pathway involving soluble long-chain polysulfides. Due to the relatively weak interaction between carbon hosts and active materials, the shuttle effect is often not fully suppressed, and sluggish redox kinetics still limit the rate performance of the battery, leading to rapid capacity decay during cycling.

The serious challenge in this field is how to effectively address the sluggish sulfur redox kinetics and the negative impact of long-chain sodium polysulfide shuttling, which cause electrolyte side reactions and degradation of the sodium anode. Although the quasi-solid-state redox pathway, as a highly promising strategy, can improve cycling stability by eliminating soluble long-chain intermediates, it often suffers from low sulfur loading in practical applications, making it difficult to meet the demand for high energy density. More challengingly, under high current densities, the system exhibits severe polarization due to hindered sodium ion transport, insufficient catalytic activity, and weak electronic coupling, leading to incomplete sulfur conversion and rapid capacity failure. Currently, there is a lack of thorough understanding of the deep catalytic mechanisms driving quasi-solid-state conversion and the laws governing ion transport kinetics, which directly constrains the further development of high-performance sodium-sulfur batteries.

To address the above issues, a team from the University of Shanghai for Science and Technology, Tianjin University, and other institutions conducted systematic research using ZEPTOOLS' in-situ TEM measurement system. By proposing an unsaturated coordination chemistry strategy and loading unsaturated molybdenum disulfide catalysts onto cross-stacked carbon microspheres, they successfully activated a fast-kinetics quasi-solid-state redox pathway in room-temperature sodium-sulfur batteries.

Title: Undercoordinated Molybdenum Catalysts Enable Ultrafast Quasi-Solid Sulfur Chemistry in Sodium-Sulfur Batteries
Journal: Advanced Materials
URL: doi.org/10.1002/adma.2024

This study proposed an innovative unsaturated coordination chemistry strategy to address the issues of sluggish kinetics and polysulfide shuttling in room-temperature sodium-sulfur batteries. The research team uniformly anchored unsaturated molybdenum disulfide nanosheets onto cross-stacked micron-scale carbon spheres using low-temperature pyrolysis, successfully constructing a novel multifunctional sulfur host. In this structure, the cross-stacked carbon spheres physically confine sulfur species through spatial confinement, effectively mitigating volume expansion during charge-discharge cycles and limiting the loss of active material. Meanwhile, because the molybdenum atoms in molybdenum disulfide are not fully coordinated, these unsaturated sites exhibit higher chemical reactivity, acting as Lewis acid centers to generate strong chemical interactions with sulfur intermediates.

Through electronic structure analysis, the study further revealed the reshaping effect of unsaturated molybdenum sites on the redox pathway. Experimental data and theoretical calculations jointly confirmed that unsaturated molybdenum sites significantly reduce the Gibbs free energy barrier in the sulfur redox process and induce a transition of the reaction from the traditional dissolution-precipitation mechanism to a quasi-solid-state redox pathway. Under this mechanism, the reaction bypasses the easily soluble long-chain sodium polysulfide stage directly, generating low-solubility short-chain intermediates that ultimately convert into insoluble products. This transformation not only fundamentally suppresses the polysulfide shuttle effect but also significantly accelerates the interfacial charge transfer rate by enhancing electronic coupling, laying a solid physicochemical foundation for achieving high rate performance.

To intuitively analyze the dynamic evolution inside the battery at the atomic scale, the research team used the ZEPTOOLS in-situ TEM sample holder to construct a high-performance nanobattery model. Driven by the precise manipulation of the in-situ sample holder, the researchers were able to monitor the nanoscale sodiation and desodiation kinetics in real time and successfully capture the transport behavior of ultrafast sodium ion flux. By introducing two quantitative descriptors—sodium migration distance and local sodium content—the study accurately assessed the ion diffusion kinetics within the active material. This high-resolution in-situ characterization confirmed the significant promotion of sodium ion transport by unsaturated molybdenum catalytic centers and demonstrated the excellent mechanical stability and structural reversibility of the electrode material under extreme voltage bias.

Benefiting from the high conversion efficiency and structural stability offered by the quasi-solid-state pathway, this sulfur host exhibited highly competitive electrochemical performance. Experimental results showed that the electrode maintained high capacity after 150 cycles at low current density, and under extremely high current intensity, it achieved an ultra-long cycle life of up to 30,000 cycles with a capacity retention rate far exceeding that reported in comparable studies. To verify its practical application potential, the team also assembled a pouch cell with high sulfur loading, which successfully powered a red light-emitting diode after continuous cycling, demonstrating good flexibility and promising commercial application prospects. This series of achievements provides important technical support and a theoretical blueprint for designing high-energy-density, long-life large-scale energy storage systems.

ZEPTOOLS, as a Chinese high-end precision instrument company, is a leading supplier of in-situ electron microscopy characterization solutions. The PicoFemto series of in-situ transmission electron microscopy characterization solutions have successively provided technical support for major research achievements of users both domestically and internationally. The figure below shows the ZEPTOOLS in-situ TEM sample holder product used in this research achievement: