MnO2 Coated with Polyaniline As Active Air Electrode for Li-O2 Reversible Battery
Authors: Eguchi Masato , Jang Il-Chan , Ida Shintaro , Ishihara Tatsumi 
Published: February 2020

Lithium air battery is now attracting much interest as the large capacity battery. For the lithium air batteries, increase in cycle stability is strongly required. For this purpose, reversibility of air electrode should be increased. In this study, we synthesized a-MnO2 coated with polyaniline, which is conductive polymer, and the air electrode property of prepared a-MnO2 coated with polyaniline was studied. Polyaniline coating is also studied for negative electrode of Li-S battery, however, in case of Li-O2 battery, number of studies on coating effects of conductive polymer is still limited. However, by increasing the conductivity in air electrode, it is expected that deposited Li2O2 could be removed effectively and so coating oxide catalyst with conducting polymer will be positive effect on cycle stability of Li-O2 battery.

Synthesis of polyaniline coated a-MnO2 is performed by using aniline and a-MnO2 as the starting material, and the synthesis was carried out by cooling in a mixture of acid solution and an organic solvent. Resulting samples were subjected to SEM observation, it was confirmed that the polyaniline is deposited because the plate like precipitates observed around a-MnO2. From TG measurement, coating amount of polyaniline was around 25wt%. BET specific surface area of polyaniline/a-MnO2 is as large as 176m2g-1. Li-O2 battery was fabricated by using hand-made cells. Mixture of tetraethylene glycol dimethyl ether (TEGDME) and ethylene carbonate (EC) at 3:1 ratio was used as an electrolyte and Lithium bis (trifluoro methanesulfonyl) imide (LiTFSI) at 1M was dissolved as a supporting salt. For air electrode, Mixture of Ketjen Black (ECP-600JD, KB), polymer coated MnO2, Pd and polytetrafluoroethylene and (PTFE) at 40: 65:15:10 weight ratio were used and metallic Li was used for negative electrode, and charge and discharge performance was evaluated at constant current of 0.1mA / cm2 and potential window of 1.5-4.5V

Since a-MnO2 shows reasonable reversibility for air electrode reaction, we used a-MnO2 coated with various conducting polymer. Among the examined conducting polymer, it was found that the discharge capacity of Li-O2 battery using polyaniline coated MnO2 air electrode was exhibited around 500 mAh/g-carbon, however, much higher cycle performance was observed. In case of the cell using the non-coated a-MnO2, discharge capacity was significantly decreased after 7 cycles, however, stable discharge capacity was sustained up to 20 cycles when polyaniline/a-MnO2 was used for air electrode. From SEM observation, superior capacity can be achieved by removing Li2O2 effectively. Therefore, coating a-MnO2 with polyaniline is effective for increasing cycle stability by increased conductivity in air electrode.

Effects of polyaniline coating amount are further studied and it became clear that cycle stability was significantly decreased when coating amount was around 45 wt%. Therefore, excess amount of polyaniline was decreased cycle stability because of low activity to air electrode activity. This study revealed that a-MnO2 coated with polyaniline was effective for increasing cycle property of Li-O2 battery.