Scientists prepare a new type of amorphous nanosheet, greatly improving the elec

If we liken a person's growth to the growth of a tree, then the growth of Zhao Jie, a post-90s researcher in the Department of Materials Science at Fudan University, is an excellent combination of "seeds, air, and sunlight."

She graduated with her undergraduate and doctoral degrees from Zhejiang University and Stanford University in the United States, respectively, and her doctoral supervisor was Cui Yi, a member of the American Academy of Sciences.

After graduation, she continued her postdoctoral research in the research group of John A. Rogers at Northwestern University in the United States.

Later, she was selected for the national-level overseas talent introduction program (youth) and joined the Department of Materials Science at Fudan University in 2020. Currently, she is mainly committed to conducting research related to high-capacity and high-energy-density batteries.

Not long ago, she, together with Professor Zhou Guangmin from Tsinghua University Shenzhen International Graduate School and Professor Jia Binbin from China Three Gorges University, prepared an amorphous two-dimensional iron-tin oxide nanosheet material and used it as a functional modification layer for polyolefin separators in room-temperature sodium-sulfur batteries.Electrochemical performance testing and density functional theory calculations have demonstrated that the introduction of tin elements increases the concentration of oxygen vacancies in amorphous oxide nanosheets.

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At the same time, it can significantly enhance the binding energy between the material and polysulfides, reduce the energy barrier for the polysulfide conversion reaction, and thereby improve the overall reaction kinetics.

For common two-dimensional materials, they often form a dense stack on the separator surface, which hinders the normal migration of sodium ions.

However, the amorphous iron-tin oxide nanosheets prepared in this study have a large number of nano-perforations, which not only help sodium ions to achieve effective migration during the battery operation process but also regulate the uniform deposition of sodium ions.

At present, room temperature sodium-sulfur batteries usually use glass fiber separators, which are generally more than 300 micrometers in thickness and require a large amount of electrolyte to be absorbed.This study employs a commercially available polypropylene separator that has been modified, with a modification layer that is only 200 nanometers thick. This thin layer can greatly improve the electrochemical performance of room-temperature sodium-sulfur batteries, thereby enhancing the energy density and power density of such batteries, and achieving stable operation of the batteries.

At the same time, by introducing a small amount of crystalline transition metal carbide materials and combining them with amorphous iron-tin oxide materials, they have successfully constructed an amorphous/crystalline biomimetic interface. This interface greatly improves the mechanical properties of the separator as a whole. Therefore, its Young's modulus is as high as 4.9 GPa, which can effectively suppress dendrite growth.

It can be seen that room-temperature sodium-sulfur batteries using multifunctional separators have excellent electrochemical performance and can bring about practical prospects for commercial application.

Room-temperature sodium-sulfur batteries have a high theoretical energy density, and sodium and sulfur resources are abundant and raw material prices are low, so they are expected to be used in large-scale energy storage systems.

How to achieve high-performance room-temperature sodium-sulfur batteries?There are many topics in the field of battery materials. Why did this team choose this direction?

It is reported that the current reserves of lithium resources on Earth cannot meet the future large-scale energy storage needs, so some new feasible energy storage systems are needed.

Sodium and sulfur elements are abundant in reserves, and the cost of raw materials is also relatively low. Moreover, the energy density of room temperature sodium-sulfur batteries is 3-5 times that of commercial lithium-ion batteries, making them very suitable for large-scale energy storage systems.

The research dilemma of room temperature sodium-sulfur batteries has some similarities with lithium-sulfur batteries, but the problems are more prominent. For room temperature sodium-sulfur batteries, slow sulfur reduction reaction kinetics, polysulfide shuttling, and dendrite growth of metal anodes are the main problems they face.

Although a large number of research results have emerged in recent years, previous methods have mainly focused on the structure of sulfur cathodes and the design of catalysts, neglecting other problems existing in the system.The diaphragm, as a component in the battery that can directly contact both the anode and cathode, has a significant impact on the electrochemical performance of the battery.

Currently, in lithium-sulfur batteries, the common polyolefin diaphragms cannot be directly applied to room temperature sodium-sulfur battery systems due to their poor wettability with ester-based electrolytes.

In the existing reports on room temperature sodium-sulfur batteries, researchers mostly use glass fiber diaphragms. Previously, a few studies have modified the glass fiber diaphragms or modified functional materials to speed up the sulfur reduction reaction rate, thereby improving the overall performance of the room temperature sodium-sulfur battery.

However, the thickness of the glass fiber diaphragm is about 300 micrometers, the pore size is also in the micrometer range, and it has a very high demand for electrolyte, which seriously reduces the actual mass and volume energy density of the room temperature sodium-sulfur battery and increases the risk of dendrite growth inducing battery short circuits. Moreover, the poor mechanical properties of the glass fiber diaphragm are also not conducive to the large-scale production and application of the battery.

Therefore, some industry insiders are focused on exploring the application of polyolefin diaphragms after modification and decoration. And the decoration materials are mainly focused on transition metal carbides or transition metal nitrides. Although some progress has been made, the thickness of these functional modification layers is usually between 3-10 micrometers, which seriously hinders the migration of ions.Therefore, there is an urgent need to develop new materials that have both high adsorption and high catalytic properties for polysulfides. Only in this way can high performance of room temperature sodium-sulfur batteries be achieved by modifying them on the surface of the separator.

How to solve the interference problems between the positive and negative electrodes during the battery operation through a simple means is also the starting point of this research.

Why are amorphous two-dimensional materials so "excellent"?

To achieve the above goals, amorphous two-dimensional materials are an ideal choice.The reason lies in the fact that, with a high specific surface area and aspect ratio, two-dimensional materials can effectively cover the separator.

In addition, many past studies have shown that introducing defects and unsaturated sites in crystalline materials can accelerate the conversion reaction of polysulfides. Amorphization, on the other hand, is a powerful means of introducing defects. The inherent disorder of amorphous materials brings a large number of defects and catalytically active sites, which can not only improve the electrical conductivity of the material, but also effectively adsorb and catalyze polysulfides.

At the same time, amorphous materials have no grain boundaries and dislocation defects inside, which can significantly improve the mechanical properties of the material and resist catastrophic deformation damage caused by external forces. When used as a functional modification layer, it can effectively improve the overall mechanical properties of the separator and suppress the growth of sodium dendrites.

Based on this, amorphous two-dimensional materials are very suitable as separator modification materials for room temperature sodium-sulfur batteries.

In the preliminary research and trial and error process of this topic, the different physical and chemical properties of lithium and sodium make similar systems based on the two exhibit many different characteristics. Feedback to the electrochemical level leads to a wide range of performance differences.In order to clarify the differences between the two and to verify the solutions from lithium-sulfur batteries in room-temperature sodium-sulfur batteries, they embarked on a three-year attempt, during which they once fell into a state of stagnation, and the participating students were also very disheartened.

However, through continuous trial and error and adjustments, they finally clarified the many differences in sodium-sulfur batteries.

Recently, the relevant paper was published in Angewandte Chemie International Edition[1] with the title "Amorphous FeSnOx Nanosheets with Hierarchical Vacancies for Room-Temperature Sodium-Sulfur Batteries."

Sun Wu and Hou Junyu from Fudan University are the first authors, Zhao Jie, Professor Zhou Guangmin from Tsinghua University Shenzhen International Graduate School, and Professor Jia Binbin from China Three Gorges University serve as co-corresponding authors.

Subsequently, they will continue to carry out related research on room-temperature sodium-sulfur batteries, starting from the many important components inside the battery system, such as the cathode, electrolyte, and separator, for modification research.Integrate these methods within the system to explore potential interference issues between different modification methods, with the aim of commercializing room-temperature sodium-sulfur batteries.

We plan to use AI for high-throughput computing to help screen materials with efficient catalytic capabilities for polysulfides, which will then be used to design more suitable catalysts.

Zhao Jie added: "Our research group has established a mature electrochemical and chemical synthesis experimental platform, which can smoothly carry out new energy-related research and also meet the requirements for material characterization and device fabrication."

"The group has ample funding, and has successfully supported postdoctoral fellows in our group to apply for and obtain various levels of super postdoctoral projects, as well as special funding from the National Postdoctoral Science Foundation and general postdoctoral projects. We have also established close ties with well-known domestic and international teams," she continued.

"Therefore, we are very willing to provide excellent postdoctoral fellows and students with opportunities for collaboration or further study. After leaving the station, we will also fully support outstanding postdoctoral fellows to apply for project-based researchers at Fudan University, young (associate) researchers, or other university teaching positions," she concluded.