Free-Standing (Current Collector-Free, Binder-Free and Conductive Additive-Free Binder-Free) Anode Architecture Based on Hierarchical Carbon Nanofiber Networks for Lithium-Ion Batteries

Carbon nanofibers (CNFs) possess exceptional properties such as high electrical conductivity, mechanical strength, and lightweight characteristics, making them ideal for advanced applications. Due to their large surface area and high aspect ratio, CNFs have the potential to significantly enhance energy storage and conversion systems, particularly in fuel cells and lithium-ion batteries. Despite these advantages, their practical application is limited by challenges, including insufficient power density and cycle stability, which need to be addressed for broader adoption.
In secondary batteries, CNFs are promising electrode materials due to their conductivity and durability. However, to compete with conventional materials like graphite, CNFs must demonstrate higher power density and better cycle stability. High conductivity ensures fast charge/discharge rates, while structural stability maintains electrode integrity over repeated cycling. Therefore, improving CNF conductivity and stability is crucial for high-performance, long-lasting batteries.
A promising method to address these issues involves integrating carbon nanotubes (CNTs) into CNFs, forming hybrid CNT-CNF structures. These hybrids offer enhanced crystallinity, improved conductivity, and a larger surface area. CNTs provide extra electron pathways and strengthen CNF structures, making them highly effective for energy storage, fuel cells, and sensors.
However, the use of transition metal catalysts for CNT growth presents challenges, such as unwanted electrochemical side reactions. In lithium-ion batteries, residual metals degrade capacity and efficiency. Post-processing, such as acid washing, is often required to remove these metals, but this increases costs, involves harmful chemicals, and can damage CNFs and CNTs. Consequently, using alkali metals is gaining attention as a more efficient, cost-effective alternative for improving CNF properties without these drawbacks.
In this study, we propose using alkali metals instead of transition metals to grow branched carbon nanofibers (BCNFs) from CNFs. This method enhances the crystallinity, electrical conductivity, and specific surface area of CNF-BCNF hybrids without complex post-processing. Since alkali metals are cheaper than transition metals, the production process becomes more economical, and the absence of residual metals ensures unaltered electrochemical performance.
The growth of BCNFs was controlled by adjusting temperature, pressure, and catalyst concentration. The CNF-BCNF hybrids were characterized using Brunauer–Emmett–Teller (BET) analysis, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and four-point probe measurements. The results showed that the CNF-BCNF hybrid demonstrated approximately 4.05 times higher conductivity than pure CNFs due to the dense branched structure, which offers more electron pathways.
The CNF-BCNF hybrid was also evaluated as a lithium-ion battery anode. The organic connection between CNFs and BCNFs eliminates the need for binders or conductive additives, simplifying electrode fabrication and increasing active material content. This leads to higher energy density and reduced electrode weight. Electrochemical tests showed excellent rate capability and cycle stability, with the CNF-BCNF anode outperforming conventional CNF-based anodes. Minimal capacity degradation was observed after repeated cycles, demonstrating its potential as a high-performance lithium-ion battery material.
Additionally, the alkali metals used in BCNF growth improved initial Coulombic efficiency by reducing the formation of solid-electrolyte interphase (SEI) layers, minimizing capacity losses and further enhancing performance in lithium-ion batteries.