Carbon nanotube/Co3O4 composite for air electrode of lithium-air battery
© Yoon and Park; licensee Springer. 2012
Received: 8 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
A carbon nanotube [CNT]/Co3O4 composite is introduced as a catalyst for the air electrode of lithium-air [Li/air] batteries. Co3O4 nanoparticles are successfully attached to the sidewall of the CNT by a hydrothermal method. A high discharge capacity and a low overvoltage indicate that the CNT/Co3O4 composite is a very promising catalyst for the air electrode of Li/air batteries.
Keywordscomposites nanostructures chemical synthesis electrochemical properties
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 [1–4]. The energy storage of lithium-based batteries can be remarkably enhanced by a new approach, lithium-air [Li/air] batteries [5–8]. 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 [9, 10]. 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 [11, 12]. Co3O4 nanoparticles composited with CNT are expected to show excellent catalytic activity owing to their nanoscale size and large surface area.
First, 0.5 g of purified multiwall CNTs [MWCNTs] was dispersed in 50 ml of 1 wt.% cetyltrimethylammonium bromide aqueous solution for 30 min, which was followed by centrifuging and washing. Then, the MWCNTs were mixed with 50 ml of a 1 wt.% aqueous solution of poly sodium 4-styrenesulfonate [PSS] and stored for 12 h. After removing excess PSS, the MWCNTs were dispersed in 40 ml ethylene glycol [EG] by sonication for 30 min. Then, 0.5 g of Co3O4 nanoparticles (Sigma-Aldrich, St. Louis, MO, USA) was dispersed in 40 ml of functionalized MWCNT EG solution. Next, 3.0 g of NaAc (C2H4NaO2) and 1.0 g of polyethylene glycol were added with constant stirring for 30 min. The solution was then transferred to a Teflon-lined stainless steel autoclave with 100 ml capacity and kept at 200°C for 14 h. The black product was washed and dried at 90°C. X-ray diffraction [XRD] patterns of the powder were measured using a Rigaku X-ray diffractometer (Rigaku Corporation, Tokyo, Japan). The microstructure of the powder was observed by field-emission scanning electron microscopy (JEOL-JSM 6500F; JEOL, Ltd., Akishima, Tokyo, Japan) and field-emission transmission electron microscopy (JEOL-JEM 2100F; JEOL, Ltd., Akishima, Tokyo, Japan). The electrochemical performance of the air electrode containing CNT/Co3O4 composite was examined using a modified Swagelok cell (Swagelok Company, Solon, OH, US) consisting of a cathode, metallic lithium anode, glass fiber separator, and an electrolyte of 1 M LiTFSi in EC/PC (1:1 vol.%). The cathode contained carbon (Ketjen black), catalyst (CNT/Co3O4 composite), and binder (polyvinylidene fluoride). The weight ratio of the CNT/Co3O4 composite to carbon was adjusted to 80:20, and 10 wt.% of the binder for the total electrode was used. The cells were subjected to galvanostatic cycling using a WonAtech (WBCS 3000; WonAtech, Seoul, South Korea) charge-discharge system. Experiments were carried out in 1 atm of O2 using an air chamber.
Results and discussion
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 [5, 6]. 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 . 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.
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.
multiwall carbon nanotubes
poly sodium 4-styrenesulfonate
scanning electron microscopy
transmission electron microscopy
This work was supported by a grant from the National Research Foundation of Korea funded by the Korean Government (MEST) (NRF-2009-C1AAA001-0094219).
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