Nature Communications Volume 10, Article number: 3917 (2019)
Long-term stability and high-rate capability have been the major challenges of sodium-ion batteries. Layered electroactive materials with mechanically robust, chemically stable, electrically and ironically conductive networks can effectively address these issues.
Herein we have successfully directed carbon nanofibers to vertically penetrate through graphene sheets, constructing robust carbon nanofiber interpenetrated graphene architecture. Molybdenum disulfide nanoflakes are then grown in situ alongside the entire framework, yielding molybdenum disulfide carbon nanofiber interpenetrated graphene structure. In such a design, carbon nanofibers prevent the restacking of graphene sheets and provide ample space between graphene sheets, enabling a strong structure that maintains exceptional mechanical integrity and excellent electrical conductivity.
The as-prepared sodium ion battery delivers outstanding electrochemical performance and ultrahigh stability, achieving a remarkable specific capacity of 598 mAh g−1, long-term cycling stability up to 1000 cycles, and an excellent rate performance even at a high current density up to 10 A g−1.
Sodium ion batteries (SIBs), as one of the most promising candidates among next-generation energy storage systems, have attracted tremendous interest due to sodium’s natural abundance and ready accessibility. However, compared to lithium ions (0.59 Å), the larger diameter (0.99 Å) of sodium ions (Na+) limits the number of suitable electroactive materials and hinders the electrochemical interfacial reaction kinetics. As such, owing to the sluggish Na+insertion/extraction efficiency, the poor rate performance of SIBs has been well recognized as an inherent challenge In the last decade, much effort has been devoted to developing promising 2D structural anode materials, such as phosphorus, carbonaceous materials, metallic alloys, and two-dimensional carbides (MXenes), to improve the electrochemical performances of SIBs and promote their practical application.
Among the investigated electrode materials, 2D molybdenum disulfide (MoS2), a layered transition-metal-dichalcogenide (TMD) material with S–Mo–S motifs stacked together by Van der Waals forces, is considered one of the most promising anode materials for SIBs. MoS2 materials can be further modified as intercalation-type anode materials with expanded d-spacing to improve the electrochemical performances of state-of-art anodes. However, MoS2-based electrodes exhibit poor rate capability and fast capacity fading upon cycling due to low electrical conductivity and the huge volume variations during charge/discharge process. Incorporation of MoS2 nanomaterials into highly conductive carbonaceous matrices was suggested as an effective way to address this problem. To date, several MoS2-carbon hybrid materials have been developed, such as MoS2-graphene composites, MoS2-CNT hybrids, and MoS2-carbon spheres.
The electrochemical performance, in terms of specific capacity, has been significantly improved due to the excellent electrical conductivity offered by the carbon matrices ensuring rapid electron transfer in the charge/discharge processes. However, there is still much room for improvement in terms of rate capability and stability of these anode materials. Thus, development of MoS2/carbon hybrids with resilient porous structure for rapid ionic transport and storage is urgently needed and of great importance.
Graphene is considered a most promising carbon material due to its inherent advantages, including large surface area, high conductivity and exceptional mechanical strength. However, such advantages would vanish if the graphene sheets restack. Carbon nanotubes (CNTs) and carbon nanofibers (CNFs) are used to prevent the restacking of graphene sheets but the improvement is very limited. Such simple hybrids offer limited surface area enhancement and limited channels for ionic transfer due to the fact that the CNTs and CNFs are in parallel with the graphene plane. It is extremely challenging to steer the CNFs to vertically penetrate through the graphene plane. To the best of our knowledge, this vertical penetration has not been achieved in the literature.
In this work, inspired by the floors-and-pillars concept in construction (see below), we design and develop a robust 3D conductive CNFs interpenetrated graphene (CNFIG) architecture by directing CNFs to penetrate through the graphene sheets. MoS2 nanoflakes are then in situ deposited on the surface of the CNFIG framework, producing a MoS2 CNFIG hybrid. It is envisaged that the MoS2 CNFIG hybrid possess several important advantages due to its unique structural characteristics, including: (i) excellent transportation channels can be integrally preserved during the rapid penetration of electrolyte and rapid transfer of ions for long-term cycles; greatly contributing to the high rate performance of the assembled batteries; (ii) the CNFs can simultaneously act as supporting pillars between different carbon layers and play an important role in rapid transfer of electrons; and (iii) due to their homogeneous deposition, all the active sites of MoS2 nanosheets can be thoroughly exposed to the electrolyte and Na+, which produces high energy density for the MoS2 CNFIG hybrid. Furthermore, the MoS2 CNFIG hybrid in this work could inspire more electrode designs with stable inner structures with high rate performance and long-term cycling stability.