CNT Sheet As a Promising Air Electrode Toward High Capacity Li-Oxygen Batteries
Authors: Nomura Akihiro , Ito Kimihiko , Kubo Yoshimi 
Published: February 2020

Lithium-Air Battery (LAB) is believed to be an ultimate secondary battery due to its much high potential capacity, predicted to have 5-10 times larger capacity than that of conventional lithium ion battery (LIB). As the practical properties of LAB cell are dominated largely by air electrode, developing efficient air electrode is a key challenge for realizing commercially viable LAB cell.

At the surface of air electrode, cycle deposition/decomposition of discharge product occurs during discharge/charge cycle. As a result, air electrode in LAB cell experiences great volume expansion and contraction, especially at high capacity cycle beyond LIB. The air electrodes comprised of powdery carbons are often reported they are not tolerate to such large volume change. In contrast, CNT can be a promising material for air electrode toward high capacity LAB cell, maintaining conductive network within the material in the face of volume change at high capacity cycle. In order to bringing out the high energy density nature of LAB, it is important to study the behavior of CNT air electrode during the discharge/charge cycle. Here in this study, we investigated how large cell capacity can be attained in LAB coin cell using CNT sheet air electrode and observed the morphological change after the battery operation.

Two types of CNT sheets, CNT-IPA and CNT-NMP, were prepared in several thicknesses by vacuum filtration of single-walled CNT (e-DIPS, 2 nm diameter) ultrasonically-dispersed in isopropanol (IPA) and N-methylpyrrolidone (NMP), respectively. SEM observation revealed the sheets are composed of CNT bundles of averaged diameter of 52 nm and 145 nm for CNT-IPA and CNT-NMP, respectively. The sheets were evaluated the performance as air electrode of LAB under flowing oxygen gas, using CR2032 coin cell that comprised the layers of lithium metal anode (j16 mm, 0.2 mm thick), GF/A glass-fiber separator, and the CNT sheet cut into j16 mm. The electrolyte was composed of 1 M LiTFSI in tetraethylene glycol dimethyl ether (TEGDME) with water content less than 10 ppm. The cells were disassembled after the discharge/charge test to analyze the morphological change of the CNT sheet and discharge product by electron microscopy (SEM, TEM) and x-ray diffraction (XRD).

Both CNT sheets worked as air electrode, showing 2.65 V discharge voltage and 4.20 V charge voltage at 0.1 mA discharge current in a limited cycle capacity of 1.0 mAh. Figure 1 shows the discharge capacity each cell achieved (2.0 V cut) plotted against the weight of sheets. The graph shows the considerable variation of discharge capacities per cell, however, the highest discharge capacity with CNT-IPA air electrode at each weight increased along with the weight of the sheet (5000 mAh/gCNT), attaining a maximum capacity of 24 mAh per cell with a CNT-IPA sheet of 5 mg (54 μm thick). This achieved capacity corresponds to the capacity per unit area of 12 mAh/cm2, which is ~5 times higher than that of conventional LIB. It was also confirmed that even from such deeply discharged state the cells were able to be fully charged with charge voltage below 4.5 V. Corresponding to the discharge/charge cycle, SEM observation revealed a reversible swelling/contraction behavior of the sheets. Figure 2 shows the XRD patterns of CNT-IPA after discharge test. The sheet increased its intensity of the peaks attributed to the lithium peroxide Li2O2 diffraction along with the discharge capacity, suggesting a deposition of Li2O2 on CNT surface as a discharge product. The discharge product had toroid shape with a diameter of ~100 nm and the toroidal particle increased its number of pieces through the discharge process, as can be seen in Figure 3 that shows SEM images of the CNT-IPA before and after discharge ((a)~(e)). In addition to the toroid deposit, TEM observation of the CNT-IPA after discharge also revealed a presence of thin skin deposit (< 5 nm) around CNT bundles. After the full charge from those discharged states, however, those deposits were disappeared and the sheet almost recovered in terms of the SEM (Figure 3 (f)) and TEM appearances.

Compared to the air electrode behavior of CNT-IPA, the discharge capacity of coin cells with CNT-NMP air electrodes did not increase, showing discharge capacity of 1.0 mAh for almost all of cells regardless of the sheet weight. We will reveal the capacity difference from their nanostructures and discuss the effective design of CNT sheet for developing high performance air electrode.

Figure 1