Improvement of Charge/Discharge Capacity of Lithium-Air Battery Using Porous Carbon Nanotube Electrode
Authors: Matsumoto Masamichi , Eguchi Masato , Ishihara Tatsumi , Fujigaya Tsuyohiko 
Published: February 2020

Recently, renewable energy such as solar and geothermal energy has been attracting great attentions for sustainable society. To minimize the risk of insufficient supply, secondary batteries are promising device to store the part of energy. Recent increase of the amount of renewable energy, increase of the energy density of the secondary batteries is highly required. Lithium air battery is one of the most promising secondary battery due to their larger theoretical energy density (11700 Wh/kg) than the other secondary batteries1, 2. During the discharge reaction of lithium air battery, Li in anode oxidizes to Li+ ions and diffuses through the electrolyte to react with oxygen at the cathode, resulting in the generation of lithium peroxide such as Li2O2. The reverse reaction is proceeded as a charge reaction3.
As the cathode materials, carbon-based materials such as carbon blacks have been studied because of its high electrical conductivity, light weight and low cost4. However, it was pointed out that, diffusion of Li+ ions and oxygen is disturbed owing to pore clogging of Li2O2 during discharge reaction, greatly limiting charge-discharge capacity of the cell4. To solve the problem, single-walled carbon nanotubes (SWNTs) are emerged as the candidate for cathode material since SWNTs form continuous porous structure, which is preferable for the diffusion of oxygen and Li+ ions5, 6. However, it was found that the pore clogging by Li2O2 was occurred and the capacity of the battery was still limited.
In this study, we introduce the internal space in the SWNT network using polystyrene beads (PS beads) as a template (3D network electrode) to avoid the clogging of Li2O2. The advantage of this approach is the good controllability of the pore size by taking advantage of the good size controllability of PS beads. 3D network electrode was prepared by removing PS beads from SWNT/PS beads composite film by heating at 450 ºC for 1 h. The removal of PS beads was confirmed by thermogravimetric analysis and scanning electron microscope observations. Mercury intrusion porosimetry analysis revealed the size of the pore was almost comparable to the size of the PS beads used as the template. In the charging and discharging experiment of the cell, lager capacity were observed (4807 mAh/mg) compared with that of the electrode without using PS beads (2143 mAh/mg). The results is explained by the improved diffusion of Li+ ions and oxygen during the discharge reaction and wider accumulation space for Li2O2 at the cathode.
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