Alkali Metal-Derived Cup-Stacked Type Carbon Nanotubes—Synthesis and Evaluation for Conductive Additive

The high electrical conductivity of carbon nanotubes (CNTs) allows them to be used in small amounts as conductive additives in lithium-ion batteries (LIBs), enabling the addition of more active material to the electrodes and thus increasing the reversible capacity of the LIBs. When CNTs are used as conductive additives in the cathode, their high electron mobility allows the amount of conductive material to be reduced to one-fifth. This reduction makes it possible to add more active material in the space previously occupied by conductive additives like carbon black, thereby increasing battery capacity. Therefore, reducing the use of conductive additives to increase the proportion of active material is key to enhancing the capacity of lithium-ion batteries.
Typically, carbon nanotubes (CNTs) are synthesized in high-temperature reactors using transition metal catalysts such as Fe, Co, or Ni nanoparticles. However, after synthesis, the metal catalysts remain as impurities in the CNTs, requiring additional purification processes. If CNTs are used in LIBs without undergoing these purification steps, various side effects can occur, making purification essential. Moreover, the current purification method, which involves treatment in high-temperature inert gases, is not environmentally friendly, is costly, and can significantly damage the CNTs. As a result, there is a growing need for a new synthetic method that is both environmentally friendly and prevents damage to CNTs at a reasonable cost. In this study, we successfully synthesized CNTs in powder form using Li alkali metal as a catalyst instead of transition metal. These CNTs are easily soluble in water, and the precursor can be removed without the need for additional acid treatment. Moreover, we successfully synthesized cup-stacked CNTs(CSCNTs) through new chemical vapor deposition(CVD) using a lithium catalyst at 550 degrees, showing that the catalyst precursor can be removed at a relatively lower temperature compared to using a transition metal catalyst. This demonstrates the environmental friendliness of the entire process.
Furthermore, in contrast to previous studies, the powder-form CNTs facilitated electrochemical analysis and applications in secondary batteries. Morphological characterization conducted through FE-SEM and HR-TEM, along with structural analysis using HR-XRD and Raman spectroscopy, confirmed the synthesis of CNTs. Elemental analysis performed with XPS and TOF-SIMS indicated that the synthesized CNTs did not grow in conjunction with the transition metal. In addition, traces of lithium used as a catalyst were observed. Morphologically, it is noteworthy that the synthesized CNTs have numerous edges, unlike conventional cylindrical CNTs, and grow in a cup-stack arrangement with a length of more than 5 µm and a diameter of 10–20 nm. This unique structure resulted in enhanced electrochemical performance in various electrochemical tests. Hence, we named them cup-stack CNTs, and this unique structure exhibited enhanced electrochemical performances in various electrochemical tests. Unlike conventional cylindrical CNTs, the cup-stacked CNTs had numerous lateral planes, which provided sufficient paths for lithium-ion transport, thereby improving the electrochemical properties of the electrode. These crystallographic features resulted in approximately 10% higher discharge capacity at all C-rates and ~2% higher rate capacity compared to commercial CNTs.
Cup-stacked CNTs were grown at a low temperature (550°C) using lithium catalysts, confirming that the alkali metal catalysts can be removed with water or at temperatures below 600°C. This study proposes a cost-effective and environmentally friendly synthesis method that can be utilized to advance the secondary battery industry. The synthesis method is not limited to lithium and can be applied to other alkali metals. This approach will provide deeper insights into the use of alkali metals for CNT synthesis.