Lithium-based batteries have found new applications in technologies such as electric vehicles, plug-in hybrid electric vehicles, robots, and electric power storing systems. However, commercial lithium-ion batteries still do not offer enough energy density for these high power consumption devices although extensive research has been conducted to increase charge-storage capability 1234. The energy storage of lithium-based batteries can be remarkably enhanced by a new approach, lithium-air [Li/air] batteries 5678. A Li/air battery consists of a Li metal anode and an air electrode containing a catalyst. Oxygen accessed from the environment is reduced catalytically on the air electrode surface to form anions, which then react with lithium cations supplied by the anode on the air electrode surface during the discharge process 910. Owing to light and unlimited cathode active material (oxygen), Li/air batteries have a much larger theoretical specific energy (11,400 Wh·kg^-1 excluding oxygen) than any other rechargeable battery system including lithium-ion batteries. In this work, carbon nanotubes [CNTs] and nanosized Co₃O₄ were successfully composited to catalyze the anion formation in the air electrode of Li/air batteries. The CNT serves to support the catalyst and provides a surface for the redox reaction to occur. Co₃O₄ has generated extensive interest as a promising catalyst in various fields 1112. Co₃O₄ nanoparticles composited with CNT are expected to show excellent catalytic activity owing to their nanoscale size and large surface area.

The shape and morphology of the CNT/Co₃O₄ composite was observed using scanning electron microscopy [SEM] and transmission electron microscopy [TEM], as shown in Figure 1. In the SEM images, it was observed that Co₃O₄ nanoparticles with spherical shapes were attached to the sidewalls of the CNT. As shown in the TEM image in Figure 1c, the distribution of the Co₃O₄ nanoparticles was not perfectly uniform, but virtually no free nanoparticles were found in any of the images. The size of the Co₃O₄ nanoparticles was 20 to 30 nm. The formation of the CNT/Co₃O₄ composite was also confirmed by XRD analysis. The patterns from which are shown in Figure 2 (top). The crystalline peaks can be clearly indexed to a typical Co₃O₄ crystalline phase with a spinel structure. The broad peak located at approximately 2θ = 23.5° is characteristic of a CNT. The weight ratio of the CNT to the Co₃O₄ crystalline phase was 60:40, as determined by inductively coupled plasma atomic emission spectroscopy analysis.

To investigate the electrochemical properties of the composite as a catalyst for the air electrode, the test cell containing the composite was characterized. An air electrode containing only CNT without Co₃O₄, which had the same catalyst (CNT) to carbon to binder ratio, was also prepared and characterized for comparison of the electrochemical properties to those of the composite. Figure 3 shows the initial voltage profile of the electrodes at constant current densities of 0.2, 0.4, and 0.6 mA·cm^-2 in the voltage range of 4.35 to 2.35 V (30°C). The capacity shown in Figure 3 was based on the total electrode mass (CNT/Co₃O₄ composite + carbon + binder), which may be reasonable to present the storage ability of energy as rechargeable batteries. The average charge voltage of the electrode containing the composite was around 4.1 to 4.2 V at current densities of 0.2 and 0.4 mA·cm^-2. In contrast, the charge voltage of the electrode containing only the CNT showed over 4.3 V at a current density of 0.4 mA·cm^-2. The high charge voltage indicates high overvoltage, so the composite electrode appeared to have lower overvoltage than the electrode containing only the CNT. This implies that the CNT/Co₃O₄ composite is a good catalyst for reducing the overvoltage during the charge process. As expected, the average charge voltage increased as the current density increased to 0.6 mA·cm^-2, which is still lower than that of the CNT electrode.

The discharge capacity of the air electrode containing the composite catalyst reached approximately 1,450 mAh·g^-1 at a current density of 0.2 mA·cm^-2. As the current density was increased to 0.4 and 0.6 mA·cm^-2, the discharge capacity of the electrode decreased to approximately 950 and 450 mAh·g^-1, respectively. This discharge capacity of the air electrode is much higher than that of typical cathode materials used in lithium-ion batteries. In general, the specific discharge capacity of the cathode, which is composed of intercalation oxide, carbon, and binder, for lithium-ion cells is just 120 to 170 mAh·g^-1. Furthermore, the air electrode containing the CNT/Co₃O₄ composite demonstrates superior discharge capacity compared to those previously reported (600 to 800 mAh·g^-1 based on the total electrode mass) for the air electrode containing oxide catalysts 56. This may be attributed to the large reaction surface area provided by the CNT matrix supporting the nanosized Co₃O₄ particles. The discharge process of the air electrode is terminated when the total catalytic active sites are blocked by reaction products 10. The Co₃O₄ particles distributed in the walls of the CNT may provide abundant catalytic active sites, which could extend both the discharge process and the capacity. Moreover, the stable contact between Co₃O₄ and the CNT will facilitate electron conduction during reaction. The air electrode containing only the CNT showed a discharge capacity that is considerable but smaller than that of the composite electrode. In addition, the charge capacity of the CNT electrode was very small, indicating poor reversibility.

Figures 4a to c show the discharge profiles of the air electrode containing the CNT/Co₃O₄ composite for the initial three cycles at constant current densities of 0.2, 0.4, and 0.6 mA·cm^-2 in the voltage range of 4.35 to 2.35 V. The discharge capacity and the voltage profile changed significantly from their initial values, indicating that the reaction was not fully reversible. As shown in Figure 4d, the cyclic performance of the composite electrode was not sufficient for a rechargeable electrode although it was much more enhanced than that of the CNT electrode. Actually, capacity fading has been a typical feature of all previous results about the air electrode 6910. It has been established that insoluble reaction products are formed during the discharge process, which accumulate during cycling, block the active catalytic sites, and decrease the capacity of the air electrode 910. The reaction products may be composed of lithium oxides (Li₂O₂ and/or Li₂O) formed from the reaction between Li ions and oxygen. Carbon products such as C₃H₆(LiOCO₂)₂, Li₂CO₃, LiHCO₂, and LiCH₃CO₂ are produced from the decomposition of the carbonate electrolyte and reaction with Li ions during cycling 1314. To avoid capacity fading and obtain more stable cyclic performance of the air electrode, it may be necessary to control reaction products and decomposition of the electrolyte.

A CNT/Co₃O₄ composite was successfully fabricated for use in the air electrodes of Li/air batteries. Nanosized Co₃O₄ particles (20 to 30 nm) were attached to the outer surface of the CNT. The air electrode containing the CNT/Co₃O₄ composite exhibited a high discharge capacity and low overvoltage during the charge-discharge process, which indicates that the composite is potentially a good catalyst for the air electrode.

CNT: carbon nanotubes; EG: ethylene glycol; MWCNTs: multiwall carbon nanotubes; PSS: poly sodium 4-styrenesulfonate; SEM: scanning electron microscopy; TEM: transmission electron microscopy; XRD: X-ray diffraction.

The authors declare that they have no competing interests.

TH performed the synthesis and characterization in this study. YJ gave the advice and guided the experiment. Both authors read and approved the final manuscript.