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Advanced materials: building sulfur vacancies and heterostructures to improve the transport kinetics of metal sulfides in sodium ion batteries

sodium ion battery is a potential choice for a new generation of large-scale energy storage technology due to its abundant sodium resources relatively low cost of raw materials. Due to the high stard electrode potential large radius of sodium ion it means that the energy density of sodium ion battery is low. Therefore it is urgent to explore advanced electrode materials with high specific capacity fast ion transport kinetics. Two dimensional transition metal chalcogenides (TMC) are widely used in lithium-ion sodium ion batteries due to their open framework structure excellent electrochemical performance. WS2 as a typical two-dimensional TMC material has many advantages such as large interlayer spacing weak van der Waals interaction which can achieve rapid sodium ion transport when used as anode material; however its low conductivity leads to poor specific capacity rate performance. At present in order to improve the diffusion kinetics of

most researches focus on the morphology control modification of electrode materials. However there are few studies on how to adjust the internal crystal structure of materials to further accelerate the ion transfer rate. It is well known that there are two forms of ion transport in materials: interstitial diffusion vacancy diffusion. For the gap diffusion the inherent open frame structure of metal sulfide has advantages. Therefore it is expected that the ion transport rate in metal sulfides can be greatly improved by constructing an appropriate amount of vacancies in the crystal structure to provide effective vacancy diffusion. In recent years the introduction of oxygen vacancies (VO) in metal oxides has attracted extensive attention: a) VO can excite excessive electrons around specific metal atoms thus forming negative charge centers to attract Na promote the rapid transport of Na ; b) VO can be used as a charge carrier to greatly improve the conductivity; ; c) VO can provide additional active sites for redox reaction to increase capacitance. In addition the heterostructure can enhance the ion transfer rate of the material the two nanocrystals with different b gaps can form the built-in electric field effect so as to achieve fast charge transfer good reaction kinetics.

based on the above ideas Academician Wu Feng Professor Wu Chuan of Beijing University of technology Professor Mai Liqiang of Wuhan University of Technology reported an ingenious method to synthesize bimetallic sulfide / carbon composites with sulfur vacancy heterostructure which showed rapid electrochemical kinetics characteristics excellent reversible capacity. This method can be called "one stone three birds". By simply introducing metal organic framework materials uniform zif-8 layers were grown on the surface of WS2 nanorods. After calcination uniform carbon protection layer was formed on the surface of WS2 nanorods. In addition because the electronegativity of Zn is stronger than that of W it is easier for Zn to combine with s to form WS2 / ZnS heterostructure form abundant sulfur vacancies in WS2.

the composite has the following advantages: 1) the uniform carbon coating promotes the rapid electron migration provides good conductivity inhibits the volume expansion of the material during the cycling process so as to ensure the structural stability of the composite; 2) the WS2 / ZnS heterostructure can produce the built-in electric field effect promote the additional charge transfer to enhance the reaction kinetics In addition the formation of more active sites in the WS2 crystal can accelerate the reaction.

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