Aqueous Electrolyte Secondary Battery Using Fast Redox Reactions in Single-Walled Carbon Nanotubes
Authors: Kawasaki Shinji , Takahashi Mikako , Date Remi , Ishii Yosuke 
Published: February 2020

We would like to propose a new concept aqueous electrolyte secondary battery using simultaneous redox reactions of cations and anions in single-walled carbon nanotubes (SWCNTs) that will solve three major problems of Li ion batteries (LIBs): safety, cost and slow-charging problems.

Recently we often hear fire and explosion accidents involving LIBs. The accidents were mainly caused by the use of flammable organic electrolytes. The use of organic electrolytes also leads to another problem: the slow-charging problem. Since ion mobility in organic electrolytes is much lower than that in aqueous electrolytes, it is very hard to obtain fast-charging for LIBs. The time-consuming charging process of LIBs hinders the market growth of electric vehicles (EVs). The high cost of LIBs due to the use of rare metals is another problem for the wider distribution of EVs. Therefore, we have to solve the problems of safety, cost and slow-charging.

The new concept battery we proposed uses mono-valent or di-valent metal halide (e.g. LiI, NaI, ZnI2) aqueous electrolytes instead of the flammable organic electrolytes used in LIBs. The use of aqueous electrolytes is not only much safer than organic electrolytes, but also advantageous to cost and fast-charging, because alkali metal halides such as LiI, NaI, ZnI2 consist only of abundant elements, and the ion mobilities of both metal ions and halogen ions in water are much faster than those in organic electrolytes. We use SWCNTs as negative and positive electrodes. At positive electrode, iodide ions are oxidized inside the empty SWCNTs and caught as a form of diatomic molecule inside SWCNTs. On the other hand, alkali metal ions are caught by quinone organic molecules encapsulated in SWCNTs and di-valent ions are reduced on the surface of SWCNTs.

We used a SWCNT sample purchased from Meijo NanoCarbon Co. The mean tube diameter of the SWCNT sample was determined to be 2.5 nm by XRD analysis. Reagent-grade 9,10-phenanthrenequinone (PhQ) powder sample was also purchased from Wako Pure Chemical Industries. SWCNT and PhQ powder samples were sealed in a vacuum glass tube, and heated at 200ºC for 10 h. The obtained PhQ@SWCNT sample was analyzed by TEM, SEM, Raman and XRD measurements.

The electrochemical measurements were performed in a three electrode cell. PhQ@SWCNT sample (anode) was paired with an empty SWCNT electrode (cathode). For CV measurements, we used Ag/AgCl reference electrode. Aqueous electrolytes of 1 mol/L LiI or NaI were used.

TEM, FT-IR, XRD, XPS measurements confirmed the encapsulation of PhQ molecules in SWCNTs. The content of encapsulated PhQ molecules per SWCNT weight was determined to be 38.0 wt% by TG measurements.

Both in the cases of LiI and NaI aqueous electrolyte cases, PhQ@SWCNT could work as electrode active materials for Li and Na ion charge/discharge. The observed discharge potential for Na ion is about 0.1 V lower than that for Li ion. The cathode reaction of the proposed battery is fixed to the redox reaction of iodide ions, namely to the redox potential of I-/I2 (about +0.45 V vs. Ag/AgCl). Therefore, a lower redox potential of the quinone anode is better to obtain higher EMF from the proposed battery. So, NaI electrolyte is better than LiI electrolyte in terms of the potential. The discharge potential difference between LiI and NaI indicates that PhQ molecule attached to Na ion (PhQ-Na) is chemically more active than PhQ-Li.

On the other hand, iodine electrode (cathode) properties have been reported in our previous papers. The molecular structures and phase transformations of the encapsulated iodine molecules were also fully investigated. Therefore, we were convinced that the iodine electrode should work as cathode in the new concept battery. The full cell performance of the proposed cell was also measured. The cathode potential was kept at iodine redox potential as we expected. So, It was also found that PhQ@SWCNT can work as anode electrode. Therefore, basically we have succeeded in demonstrating that the new concept battery, namely, a safe, low-cost and fast charging secondary battery using aqueous electrolyte can work very nicely. However, the amount of the observed charge capacity is quite different from that of the discharge capacity. We should investigate the reason why such a big irreversible capacity was observed in order to improve the proposed new concept aqueous secondary battery.

We also would like to talk about di-valent metal cases in the conference.