Preparation and lithium storage properties of ZIF-derived hollow NiS₂/CoS₂@HNC

With the advent of new energy-powered vehicles and the frequent iteration of various technological devices, developing lithium-ion batteries with bigger capacity, improved safety, and longer durability is critical. The anode material is an essential component of a lithium-ion battery, and optimizing its performance is an efficient way to create a superior lithium-ion battery. However, the theoretical specific capacity of modern graphite electrodes is constrained, limiting future lithium-ion battery performance improvements. Transition metal sulfides (TMSs) are an excellent option for graphite anode materials because of high reversible capacity, easy availability of raw materials, and environmental protection. However, TMSs suffer from volume expansion and structural instability, resulting in irreversible capacity loss. The common problem of poor electrical conductivity also limits its performance improvement. To solve these defects, researchers have improved their electrical conductivity and stability through special structural design and composite with other materials. The hollow structure is a typical form of structural design which can effectively buffer the volume expansion effect during the charge and discharge process. A large specific surface area may increase the contact area with the electrolyte, enhancing the electrochemical process. Self-formwork is a common method for constructing hollow buildings. Its template materials not only support the construction of hollow structures similar to hard templates, but also participate in the subsequent formation process, simplifying the reaction phases. Zeolitic imidazolate framework (ZIF) is formed by the coordination of transition metal ions (Zn2+, Co2+, etc.) with imidazole or imidazole derivatives which has good thermal and chemical stability. Materials generated from it as the self-template are frequently employed in the field of electrochemistry due to its various and customizable structure and numerous pores. Polydopamine is an excellent coating material because it can be oxidized and self-polymerized in an alkaline environment without complicated reaction conditions and can be produced on a wide range of materials. Nitrogen-doped carbon compounds were produced after calcination. Nitrogen doping encourages the production of more active sites, and the formation of additional micropores in the structure during the preparation process, which enhance the embedding and removal of Li+ and improve the electrochemical performance of the composite materials. Furthermore, the carbon layer could effectively improve the conductivity and address the inherent flaws of TMSs. Therefore, we synthesized the hollow dodecahedral structure NiS2/CoS2@HNC composites with Ni/Co-ZIF-67 as self-templates via polydopamine coating and high-temperature calcination. The hollow nitrogen-carbon layer contained NiS2 and CoS2 nanoparticles. When compared with NiS2/CoS2@NC generated by direct vulcanization of Ni/Co-ZIF-67, the electrochemical performance of NiS2/CoS2@HNC is significantly improved. The capacity of NiS2/CoS2@HNC can reach 828 mA h g(-1) at 0.2 A g(-1) current density after 100 cycles, demonstrating excellent cycling stability. The reversible capacity remains 905 mA h g(-1) after the current density gets back to 0.1 A g(-1), which is due to the contribution of the pseudo capacitor. Additionally, the NiS2/CoS2@HNC structure remains intact after the cycle which is related to that the hollow nitrogen carbon layer buffers the volume expansion effect during the cycle, enhancing the stability of the structure.