As a positive electrode material, sulfur has a high theoretical specific capacity (one order of magnitude higher than the capacity of existing commercial cathode materials), and has the advantages of low cost, abundant reserves and environmental friendliness. Therefore, lithium-sulfur batteries are considered to be electrochemical. One of the most promising new generation batteries in energy storage. However, in the process of practical application of lithium-sulfur batteries, there are still many problems to be solved, such as the low conductivity of sulfur and lithium sulfide products, the shuttle effect of soluble polysulfides formed between the positive and negative electrodes during charge and discharge. , will significantly affect the rate performance and cycle life of the battery. In order to solve these problems, a multi-component composite electrode can be formed by introducing a guest material (such as a carbon material, a metal oxide, a nitride, etc.) into an electrode material, utilizing the high conductivity of the guest material and the adsorption of polysulfide, Limiting action to suppress the shuttle effect, thereby improving the performance of the lithium-sulfur battery.
Recently, the Research Institute of Energy Storage Materials and Devices of the Advanced Carbon Materials Research Department of the National Institute of Materials Science of the Institute of Metal Research of the Chinese Academy of Sciences has carried out a series of research work on high-performance multi-component composite sulfur electrode materials. They first calculated by density functional theory and used nitrogen-doped graphene as a model. It was found that the pyridinium nitrogen clusters can strongly adsorb polysulfide molecules in different nitrogen-doped forms, and proposed components and polysulfide. The energy judgment relationship of the object interaction. On this basis, nitrogen-doped graphene with excellent electrochemical performance and high nitrogen content was obtained by high temperature treatment of graphene oxide by ammonia gas. In order to further improve the limiting effect on polysulfides, the physical limitation of carbon nanotubes on polysulfides is combined with the chemical confinement of carbon-sulfur bonds in organic sulfur polymers to polysulfides, and organic sulfur polymers are packed. An organic sulfur polymer/carbon nanotube composite material was prepared from carbon nanotubes synthesized from anodized aluminum as a template (Fig. 1). The composite material physically limits the polysulfide dissolution through the lumen of the carbon nanotubes, and simultaneously uses the carbon-sulfur bond in the organic sulfur polymer to chemically fix the sulfur, synergistically inhibiting the shuttle effect of the polysulfide.
However, for a non-polar carbon material, even if it is treated by doping or the like, its effective adsorption to the polar polysulfide cannot be further improved, so that it is difficult to completely suppress the shuttle effect. The theoretical calculations show that the effect of chemically adsorbing polysulfides and inhibiting the shuttle effect by using polar oxides is significantly better than that of carbon materials. However, the insulating oxide hinders the transmission of electrons and lithium ions, reducing sulfur utilization and rate performance. How to combine the characteristics of the two, to find highly conductive polar adsorption materials has become the core of research. To this end, the researchers proposed the idea of â€‹â€‹constructing carbon-based composite electrodes with chemically anchored polysulfides, combining carbon nanomaterials with highly conductive metal nitrides with chemically anchored polysulfide functions, using one-step hydrothermal The vanadium nitride nanobelt is supported on a three-dimensional graphene-based body, and lithium polysulfide is used as an active material to fill the three-dimensional pores of the graphene and vanadium nitride composite current collector (as shown in FIG. 2). The composite positive electrode structure not only utilizes the three-dimensional framework and pore structure of graphene, but also combines the highly conductive polar vanadium nitride to promote the chemical adsorption and transformation of polysulfides, effectively solving the "shuttle effect" band. The resulting capacity degradation and low coulombic efficiency have resulted in excellent electrochemical performance. Compared with a single graphene electrode, the vanadium nitride/graphene composite electrode has less polarization and faster redox reaction kinetics, showing better rate and cycle performance, which may be used in the application of high-energy lithium-sulfur batteries. Has great potential. At the same time, metal nitrides are a large family with high conductivity and chemical polarity that provide new options for related electrochemical applications.
Relevant papers based on the above findings are published in Nano Energy 2016 (25, 203-210), Carbon (Carbon 2016, 108, 120-126), Advanced Materials DOI: 10.1002/ Adma.201503835) and Nature Communication DOI: 10.1038/ncomms14627. The above work has been funded by the National Key Research and Development Program, the National Natural Science Foundation Project, the Chinese Academy of Sciences Pilot Project, the Chinese Academy of Sciences Youth Innovation Promotion Association and the Metals Innovation Fund Project.
Fig.1 Schematic diagram of synthesis of organic sulfur polymer (a) and organic sulfur polymer/carbon nanotube composite (b); scanning electron micrograph of organic sulfur polymer/carbon nanotube composite (c) and membrane electrode (d) Comparison of rate performance (e) and cycle performance (f) of physical and chemically constrained organic sulfur polymer/carbon nanotubes with a single physically limited sulfur/carbon nanotube composite.
Figure 2 (a) Preparation of vanadium nitride/graphene composite and schematic diagram of lithium-sulfur battery assembly; scanning electron micrograph (b) and scanning transmission electron micrograph (c) of vanadium nitride/graphene composite; (d) nitrogen The adsorption performance of vanadium/graphene composites for polysulfides; the theoretical calculation model for the interaction of nitrogen-doped graphene (e) with vanadium nitride (f) and polysulfides.
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