Hierarchical FeS<sub>2</sub>@FeS<sub>2</sub>@N,S-C hollow nanocubes for lithium storage

Abstract

<p indent="0mm">With the increasingly prominent problems of energy shortage and environmental pollution, the requirement for high-efficiency energy storage devices is becoming more urgent. Lithium-ion batteries (LIBs) are commonly used in portable electronic devices, such as laptops, mobile phones. However, their limited performances cannot meet the demands of ultrahigh energy density and safety in the fields of new energy vehicles and energy storage systems. The development and application of novel anode materials can increase the capacity of LIBs. Transition metal sulfides (TMSs) with ultrahigh theoretical capacity and chemical stability present bright application prospects. Among them, iron sulfide (FeS_{<italic>x</italic>}) is regarded as one of the most promising anode materials due to its abundant reserves and environmental friendliness. However, the shortcomings of large volume expansion during cycling severely hinder the application of FeS_{<italic>x</italic>}<italic> </italic>and other TMSs. Designing TMS anode materials with special structures and combining them with carbon-based materials can effectively improve their electrochemical performance. Constructing nanosized TMSs with a hierarchical hollow structural property is a common strategy to increase their cycling stability. The abundant free volume inside the hollow structure can serve as a buffer space for TMSs with severe volume expansion, greatly reducing the structural fracture phenomenon. The internal free volume is also conducive to the electrolytic penetration into the composites. In addition, the dense nanosheets on the surface enable the active materials to fully contact the electrolyte, and subsequently, more reaction sites are generated for the electrochemical reactions. Combining amorphous carbon with TMSs can increase the dispersity of composites and reduce the presence of agglomerated structures. The amorphous carbon layer can serve as a stable interface for electrochemical reactions, which is beneficial for maintaining high coulombic efficiency and structure retention during cycling. The disordered structures and defects inside the amorphous carbon layer can accelerate the transport of the electrolyte and Li⁺ ions. Moreover, the amorphous carbon layers can also provide a structural support network and electronic conduction paths for TMSs, thereby improving the electrochemical performance of composites. Polydopamine (PDA) demonstrates special “viscosity,” which can coat almost any material’s surface under a weak alkaline condition. After carbonization, PDA is carbonized into an amorphous carbon layer with nitrogen doping, and the increased carbon interlayer spacing and defects can serve as additional active sites for lithium storage. In this study, the hierarchical FeS₂@FeS₂@N,S-C hollow nanocubes are successfully synthesized by a series of solvothermal, PDA <italic>in-situ</italic> polymerization, and high-temperature vulcanization processes. The inner hierarchical hollow FeS₂ is encapsulated in the heteroatom-doped carbon layer and in LIBs; the fabricated composites with a special structural property demonstrate improved electrode reaction kinetics. The results show that the discharge capacities of FeS₂@FeS₂@N,S-C are 505.6, 472.2, and 360 mAh/g at 0.2, 0.5, and <sc>2 A/g,</sc> respectively. Moreover, the reversible capacity of 492.1 mAh/g is maintained after 500 cycles at <sc>1 A/g,</sc> and the structure remains intact after cycling. The excellent electrochemical performance of FeS₂@FeS₂@N,S-C can be attributed to the heteroatom-doped carbon layer coating the hierarchical hollow structure and the synergistic effect of multiple components.