Recently, the team of Academician Wu Feng, School of Material Science and Engineering, BIT, has made important breakthroughs in the structural design and performance optimization of sodium-ion battery materials. Efforts are gathered by the whole team, and finally a paper entitled Co-Construction of Sulfur Vacancies and Heterojunctions in Tungsten Disulfide to Induce Fast Electronic/Ionic Diffusion Kinetics for Sodium-Ion Batteries is published in Advanced Materials (IF: 27.398), a top international journal of materials, whose first author is Li Yu, a post-doctor of School of Material Science and Engineering, BIT, and co-corresponding authors are Academician Wu Feng and Prof. Wu Chuan from BIT and Prof. Mai Liqiang from Wuhan University of Technology (Paper link: https://onlinelibrary.wiley.com/doi/10.1002/adma.202005802).

　　With abundant sodium resource reserves and relatively low cost of raw materials, sodium ion batteries have become an ideal choice for a new generation of large-scale energy storage technology. However, the standard electrode potential of sodium is relatively high and the radius of sodium-ions is large, resulting in insufficient energy density of the existing sodium-ion batteries. Therefore, it is necessary to explore advanced electrode materials with high specific capacity and fast ion transport kinetics. TMC is widely used in lithium-ion and sodium-ion batteries due to its open framework structure and good electrochemical performance, whose large interlayer spacing and weak van der Waals interaction can achieve rapid sodium ion transmission. However, the low conductivity of TMC leads to poor specific capacity and rate performance.

Fig. 1 Design of bimetallic sulfide/carbon composites with sulfur vacancy and heterostructure

　　In order to improve the diffusion kinetics, the existing researches mainly focus on the morphology control and modification of electrode materials. However, there are few studies on how to control the crystal structure and improve the ion transport rate. Ion transport in materials includes gap diffusion and vacancy diffusion. The inherent open frame structure of metal sulfide already has the advantage of gap diffusion. Therefore, constructing an appropriate amount of lattice vacancies is expected to introduce vacancy diffusion and further improve the ion transmission rate in metal sulfides. In recent years, Vo effect in metal oxides has attracted widespread attention: (a) excessive electrons are excited around a specific metal atom to form a negative charge center to attract Na+ and promote the rapid transport of Na+; (b) As a charge carrier, the conductivity is greatly improved; (c) additional reactive sites are provided for redox reactions to increase capacitance. In addition, the built-in electric field effect can be formed between nanocrystals with band gap difference. Therefore, the construction of a heterostructure can further enhance the ion transmission rate of the material, and achieve rapid charge transmission and good reaction kinetics.

Fig. 2 Electrochemical performance of sodium-ion battery based on bimetallic sulfide/carbon composite

　　Based on these ideas, the team reported for the first time a bimetallic sulfide/carbon composite with sulfur vacancy and heterostructure, which exhibited fast electrochemical kinetics characteristics and excellent reversible capacity. This method can be called “Three birds in one stone”. By introducing metal organic framework materials, uniform ZIF-8 layers in situ can be grown on the surface of WS2 nanorods. After calcination, a uniform carbon protective layer was formed on the surface of WS2. Besides, due to the difference of electronegativity between Zn and W, Zn and S are more easily combined to form WS2 / ZnS heterostructure in situ; at the same time, abundant sulfur vacancies are formed in WS2. The composite material enjoys the following advantages: (1) the uniform carbon coating promotes the rapid migration of electrons and provides good electrical conductivity. Meanwhile, it inhibits the volume expansion of the material during the cycle, thus ensuring the structural stability of the composite material; (2) the formed WS2/ZnS heterostructure can generate a built-in electric field effect and promote additional charge transfer to enhance the reaction kinetics; (3) the sulfur vacancies generated in the WS2 crystal can not only provide more reactive sites, but also induce excess electrons around W metal atoms to form negative charge centers and accelerate the rapid transfer of Na+. This innovative achievement breaks through the kinetic barrier of two-dimensional transition metal chalcogenide sodium-ion battery, and provides forward-looking theoretical support for the optimal design of energy materials and the construction of high specific energy power batteries.

　　Prof. Wu Chuan, also the team leader, has been engaged in the research of key materials for sodium-ion batteries for a long time and has achieved rich research results. In sodium-ion battery cathode materials, through research ideas such as cation doping, preferential growth of active crystal planes, micro/nano structure control, and flexible electrode design, a Na3V2(PO4)3 cathode with excellent electrochemical performance is prepared, and the electrochemical reaction mechanism of the material is analyzed through theoretical calculation and synchrotron radiation source characterization (Chemistry Materials , 2018, DOI: 10.1021/acs.chemmater.7b03903; Advanced Science , 2017, 10.1002/advs.201600275; Small , 2018, DOI: 10.1002/smll.201702864) . For the negative electrode of sodium ion batteries, a series of high-capacity hard carbon materials are prepared by low-cost biomass (ACS Applied Materials&Interfaces, 2020, DOI: 10.1021/acsami.9b22745; ACS Applied Materials&Interfaces, 2019, DOI: 10.1021/acsami.9b01419; ACS Applied Materials&Interfaces, 2018, DOI: 10.1021/acsami.8b08380); phosphorus functionalized hard carbon materials are prepared by electrospinning technology, and the properties of the materials are greatly improved (Advanced Energy Materials, 2018, DOI: 10.1002/aenm.201702781); by adjusting the reaction parameters, a selenide/graphene composite material with a special morphology is designed, and the transmission X-ray technology is used for the first time to explore the sodium storage mechanism of the material (Advanced Energy Materials, 2018, DOI: 10.1002/aenm.201800927). In the sodium-ion battery electrolyte, the synthesized NaPF6/BMITFSI ionic liquid electrolyte is applied to the sodium ion battery for the first time, and matched with the Na3V2(PO4)3 cathode material, which significantly improves the safety and electrochemical performance of the battery (Nano Energy, 2018, DOI: 10.1016/j.nanoen.2018.07.003). In the aspect of solid electrolyte of sodium-ion battery, hyperbranched comb polyether electrolyte is synthesized. The introduction of short-chain ether side chain inhibits the entanglement of linear long chain, reduces the crystallinity of polymer, enhances the movement ability of chain segment and ion transport, and shows long cycle stability for solid sodium-ion battery (Chemical Engineering Journal, 2020, DOI: 10.1016/j.cej.2020.124885; Chemical Engineering Journal, 2020, DOI: 10.1016/j.cej.2020.126065; ACS Applied Materials&Interfaces, 2020, DOI: 10.1021/acsami.0c04878).

　　The rapid development of sodium-ion batteries aims to solve the contradiction between the major needs of secondary batteries and limited natural resources, and provide new methods and new ideas for the development of large-scale energy storage. With the continuous emergence of high-performance electrode materials, electrolytes, and diaphragms, sodium-ion batteries will play a more far-reaching significance and value in the related field of energy storage.