2023 年 2 巻 1 号 p. 60-66
The surface properties of carbons significantly influence their performance when they are used as lithium-air battery cathodes. This study used carbon nanofiber sheets derived from polyacrylonitrile (PAN) nanofiber sheets and prepared by electrospinning (ES-CNF) as a model cathode to investigate the influence of surface properties. The sheet structure enabled sufficient access of O2 to the carbon surface. For this investigation, five types of ES-CNF sheets having different surface properties were prepared by varying the heat-treatment temperature from 800 to 2200 °C. A portion of the heat-treated samples were further treated with nitric acid. The surface properties of the obtained samples were analyzed in detail by temperature programmed desorption analysis conducted under vacuum in the temperature range 100–1800 °C. The results indicated that discharge and charge voltages depend on the number of edge-H and the type and number of carbon functional groups. Oxygen-containing functional groups on the carbon promoted not only the surface-route deposition of Li2O2, which can be charged (decomposed) at voltages below 3.7 V, but also undesired side reactions, which can increase the charge voltage up to over 4.0 V. Furthermore, it was also found that carboxyl or phenol/ether groups effectively promote surface-route reactions much more efficiently than N-containing functional groups (NCFGs) and carbonyl groups. The knowledge obtained in this study can be utilized as a new guideline to design cathode carbon for lithium-air batteries.
