- Nano Express
- Open Access
Carbon and Binder-Free Air Electrodes Composed of Co3O4 Nanofibers for Li-Air Batteries with Enhanced Cyclic Performance
© Lee and Park. 2015
- Received: 9 July 2015
- Accepted: 29 July 2015
- Published: 12 August 2015
In this study, to fabricate a carbon free (C-free) air electrode, Co3O4 nanofibers were grown directly on a Ni mesh to obtain Co3O4 with a high surface area and good contact with the current collector (the Ni mesh). In Li-air cells, any C present in the air electrode promotes unwanted side reactions. Therefore, the air electrode composed of only Co3O4 nanofibers (i.e., C-free) was expected to suppress these side reactions, such as the decomposition of the electrolyte and formation of Li2CO3, which would in turn enhance the cyclic performance of the cell. As predicted, the Co3O4-nanofiber electrode successfully reduced the accumulation of reaction products during cycling, which was achieved through the suppression of unwanted side reactions. In addition, the cyclic performance of the Li-air cell was superior to that of a standard electrode composed of carbonaceous material.
- Lithium air battery
- Air electrode
- Nano fiber
- Cyclic performance
Recently, Li-air batteries have attracted much attention because of their potential as the next generation of battery systems; they provide higher energy densities than state-of-the-art Li-ion batteries [1–8]. However, the electrochemical performance of Li-air batteries is currently far from satisfactory for their commercialization to be viable. One of the major barriers to enhancing the performance of Li-air batteries is developing an air electrode that can offer a high capacity, low overpotential, and good cyclic performance. In non-aqueous Li-air cells, the basic reactions during the discharging and charging processes are the formation and decomposition of Li2O2, respectively, on the surface of the air electrode [9–15]. To obtain a reversible and sufficient capacity, the solid Li2O2 must be formed and stored on a conducting matrix with a high surface area. Hence, porous carbon, which has a high conductivity and surface area, has been recognized as one of the most attractive matrix materials for air electrodes. However, C promotes electrolyte decomposition during cycling, and it readily reacts with Li2O2 to form Li2CO3 [16–19]. These side reactions caused by the presence of C generate unwanted reaction products, such as Li2CO3 and organic materials, which are attributed to the decomposition of the electrolyte. While Li2O2, the ideal reaction product, is efficiently decomposed during the charging process, dissociating the unwanted reaction products is difficult, so they can be easily accumulated on the surface of the air electrode. This results in a high overpotential and limited cyclic performance [20, 21].
The use of C-free matrices in air electrodes is a possible solution for suppressing the formation of unwanted reaction products. Several research groups have already investigated C-free electrodes by using inorganic materials, such as TiC and Co3O4, which can also act as catalysts [22–24]. However, while these C-free electrodes exhibited enhanced cyclic performances, their capacities were relatively small (approximately 500 mAh⋅gelectrode −1) because inorganic matrices are heavy and have low surface areas. Therefore, to obtain C-free electrodes with high capacities, an optimum nanostructure with a high surface area must be fabricated.
In this study, we investigated Co3O4 nanofibers grown directly on the surface of a Ni mesh (the current-collector matrix) as a potential C- and binder-free air electrode. Co3O4 is considered a promising catalyst material for Li-air batteries [25–29], as well as a high-capacity anode material for Li-ion batteries [30–33]. The Co3O4 nanofibers, which acted as electron pathways, were strongly attached to the Ni mesh because they were grown directly on it. In addition, they had a high surface area, which offered sufficient space for the storage of Li2O2 and resulted in a high capacity of the air electrode. Moreover, the C- and binder-free structures were expected to suppress the unwanted side reactions related to the presence of C, which should enhance the electrochemical performance of the air electrode by increasing the cyclic performance.
A Ni mesh was used as the current collector and substrate. For the Co3O4 nanofiber seed solution, cobalt nitrate (Co(NO3)2⋅6H2O), ammonium fluoride (NH4F), and urea (CO(NH2)2) were dissolved in deionized water under stirring. The solution was then transferred to an autoclave. Polyimide tape was attached to the back of the Ni mesh to ensure the Co3O4 nanofibers only grew on the front of the mesh. The etched Ni mesh was then put into the seed solution. The hydrothermal reaction was performed at 95 °C for 8 h inside the autoclave. After the hydrothermal reaction, the sample was washed with deionized water and heat-treated at 350 °C for 2 h in an air atmosphere. To check the crystallinity of the Co3O4 nanofibers, the X-ray diffraction (XRD) pattern of the air electrode was obtained with a Rigaku X-ray diffractometer equipped with a monochromatized Cu-Kα radiation source (λ = 1.5406 Å).
The Co3O4 nanofibers grown on the Ni mesh were then tested as the air electrode of a Li-air cell. For comparison purposes, an air electrode composed of Ketjen black (90 wt.%) and polyvinylidene fluoride (PVDF, 10 wt.%) was prepared and tested, which will be referred to as the “standard electrode.” The loading mass of the Co3O4 nanofibers, and Ketjen black + PVDF was adjusted to be 0.5 ± 0.05 mg in both electrodes. Li metal and a glass fiber filter (GF/F, Whatman) were used as the anode and separator, respectively. A 1 M solution of lithium bis(trifluoromethane)sulfonimide (LiTFSI) in tetraethylene glycol dimethyl ether (TEGDME) was used as the electrolyte. The cells were assembled in an Ar-filled glove box. The electrochemical measurements were performed with Swagelok-type cells and a WonATech battery cycler (WBCs 3000) under an O2 atmosphere (1 atm) at 30 °C. Scanning electron microscopy (SEM, AP Tech TECNAI G2 F30 STwin) was employed to observe the surface morphology of the electrodes during the cycling tests. Fourier transform infrared (FT-IR) spectra of the electrodes were collected with a JASCO FT-IR-4200 to ascertain the reaction products that accumulated on the electrodes during the cycling tests.
On the other hand, Fig. 6d shows that the surface of the Co3O4-nanofiber electrode is also covered with a film of reaction products after the first discharging process. After the first charging process, the reaction products have clearly dissociated (Fig. 6e). Moreover, the surface of the Co3O4-nanofiber electrode after 50 cycles is very clear (Fig. 6f). The surface morphology of the standard electrode is very different after the 1st and 50th cycles, as shown in Fig. 6b, c. In contrast, the surface morphology of the Co3O4-nanofiber electrode after 50 cycles is very similar to that after the first cycle, as shown in Fig. 6e, f. These results clearly confirm that the C-free Co3O4-nanofiber electrode successfully suppresses the accumulation of unwanted reaction products during cycling.
In this study, Co3O4 nanofibers were successfully grown on the surface of a Ni mesh and they were tested as the air electrode of a Li-air cell. The Co3O4 nanofibers were strongly attached to the Ni mesh and provided a high surface area for the storage of reaction products. While the Co3O4-nanofiber electrode exhibited a smaller discharge capacity than that of a standard electrode composed of Ketjen black, it demonstrated a superior cyclic performance. Compared to the standard electrode, the Co3O4-nanofiber electrode effectively reduced the accumulation of unwanted reaction products during cycling, as confirmed with both SEM and FT-IR analyses. The Co3O4-nanofiber electrode did not contain any carbonaceous materials that could promote side reactions, such as the decomposition of the electrolyte and formation of Li2CO3. Therefore, Co3O4-nanofiber electrodes can limit the unwanted side reactions during cycling, which improves the cyclic performance of such electrodes.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (No. 2014R1A2A2A01003542) and by the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (No. 20112020100110/KIER KIER B5-2592).
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