Alkali Metal Ion Storage Properties of Single-Walled Carbon Nanotube Encapsulation Systems
Authors: Sakamoto Yuki , Canghao Li , Ishii Yosuke , Kawasaki Shinji 
Published: February 2020

Single-walled carbon nanotube (SWCNT) encapsulation systems have been paid much attention since the discovery of C60 peapod in 1998. So far, several kinds of molecules (e.g. coronene, β-carotene, 9,10-dichloroanthracene, water, iodine) have been encapsulated. The eager studies on structural and electronic properties of the encapsulation systems revealed that some of the encapsulated molecules show properties different from their bulk forms. Due to such findings, it has been concluded that the tube interior provides a unique field to the encapsulated molecules to alter their properties. However, there are not many reports describing the applications of encapsulation systems. Lithium ion batteries (LIBs) have been widely used in portable electronic devices such as cell phones and lap-top computers, owing to their high energy density. However, new energy source with higher capacity and better rate performance is required because LIBs are not good enough for larger and higher power machines such as electric vehicles. Recently, metal-air, lithium-sulfur (LIS), sodium-ion batteries and others have been attracting much attention as post-LIB energy devices. LIS battery is one alternative that has higher energy density than LIB has, and is expected to be used for large-scale devices. However, in order for the practical use of LIS battery to be realized, we have to solve the problem that polysulfide ions (Sn−) are easily dissolved in electrolyte. This dissolution problem leads to capacity fading with charge-discharge cycles. Several investigations have been carried out to improve the cycle performance of LIS battery. For example, Kaneko et al. reported the encapsulation of sulfur into the hollow core of single-walled carbon nanotube (SWCNT) [1]. Another post-LIB is sodium-ion battery (SIB) that has received increasing attention in the recent years, because SIB is very advantageous in regards to cost and resources. However, good anode materials having high capacity for SIBs have not been found yet. Graphite, which is used as a negative electrode of LIBs, does not adsorb Na ions. So far, SIB negative electrode properties of several kinds of carbon materials have been studied. Although some kinds of hard-carbons and Na alloys can adsorb and desorb sodium ions, we should explore better anode materials to realize SIBs. Phosphorus is also expected as alternative electrode materials because of its high theoretical capacity (2596 mAh/g) and relatively low volume expansion when adsorbing sodium ions. However, phosphorus electrode still suffers from instability problems, despite the numerous attempts that have been made with different phosphorus polymorphs and/or different kinds of electrode support materials. In the present study, we use the unique environment provided by the hollow core inside SWCNTs to encapsulate both sulfur and phosphorous molecules, in order to address some of the unanswered questions and performance issues of both LIS and SIB. First, we compare the physical stability of sulfur molecules inserted in SWCNTs having different tube diameters, then we report the LIS battery electrode properties of sulfur encapsulated SWCNTs (S@SWCNTs). We also report on the negative electrode properties of phosphorus encapsulated single-walled carbon nanotubes (P@SWCNTs), in which phosphorus atoms are inserted into the pores of SWCNT [2]. Since SWCNTs have good electric conductivity, they should work as a conductive component for the highly resistive phosphorus electrode. In the conference, I will also talk about another types of SWCNT encapsulation systems.
References
[1] T. Fujimori, A. Morelos-Gómez, Z. Zhu, H. Muramatsu, R. Futamura, K. Urita, M. Terrones, T. Hayashi, M. Endo, S. Young Hong, Y. Chul Choi, D. Tománek, and K. Kaneko, Conducting linear chains of sulphur inside carbon nanotubes, Nat. Commun. 4 (2013).
[2] Y. Ishii, Y. Sakamoto, H. Song, K. Tashiro, Y. Nishiwaki, A. Al-zubaidi, S. Kawasaki, Alkali metal ion storage properties of sulphur and phosphorous molecules encapsulated in nanometer size carbon cylindrical pores, AIP Adv. 6, 035112 (2016).