Due to their high theoretical energy density, lithium-air batteries (LABs) are expected to become the standard secondary battery system of the next generation. However, for practical usage, their cycle performance needs to be improved. Thus far, few studies have investigated how the structure of carbon cathode materials a ects their cycle performance. In this study, meso-to macro-porous carbon gels (CGs) heat-treated at di erent temperatures were prepared, and were used as model materials to discuss the relationship between carbon structure and cycle performance. Nitrogen adsorption experiments, X-ray di raction analysis, Raman spectroscopy analysis, and thermal gravimetric analysis were conducted to derive the pore structure, crystal structure, carbon bonding state, and oxidation resistance of CGs, respectively. In addition to standard analysis methods, temperature program desorption measurements were conducted under vacuum to aid the analysis of surface oxygen-containing functional groups. Charge-discharge measurements indicated that cycle performance improves when the carbons were heat-treated at higher temperatures. Functional groups on the carbon surface were found to promote side reactions and reduce reversible capacity. Defects in the carbon are thought to promote uneven deposition of deposits which tend to cause the sudden "death" of LAB.

In lithium–air batteries (LABs), controlling the characteristics of the Li2O2 deposited during discharging can lead to the reduction of the large overpotential required for charging. The large overpotential is one of the most significant problems that needs to be solved to improve the cycle performance of LABs. Here, we focused on the effects of functional groups in the cathode carbon on the characteristics of the Li2O2 deposited during discharging and the cathode performance of LABs. In this study, 4 types of carbon gels (CGs) were prepared using different treatment methods to modify their surface properties. The types and amounts of oxygen-containing functional groups (OCFGs) existing within the CGs were clarified along with the number of edge H’s by a high-sensitivity temperature-programmed desorption (TPD) technique. The results of N2 adsorption analysis of discharged CGs suggested that, by increasing the number of OCFGs from 0.40 to 1.80 mmol g–1 through acid treatment, the ratio of Li2O2 deposited within the mesopores of the porous carbon particles can be increased from 1% to 60%. This significant change in the manner of Li2O2 deposition led to the reduction of the charging overpotential. Side reactions that are thought to deteriorate cycle performance tended to proceed in CGs having a large number of OCFGs. This negative effect could be reduced by removing carboxyl groups in the CGs through simple heat treatment at 300 °C in an inert atmosphere. Our study clarified the critical roles of OCFGs in the cathode during the discharging and charging of LABs. The obtained knowledge can be utilized for the development of a high-performance cathode for LABs.

Lithium−oxygen batteries (LOBs) are promising nextgeneration rechargeable batteries due to their high theoretical energy densities. The optimization of the porous carbon-based positive electrode is a crucial challenge in the practical implementation of LOB technologies. Although numerous studies have been conducted regarding the relationships between LOB performance and the physicochemical properties of carbon electrodes, most of these studies evaluate the performances of the electrodes under unrealistic conditions with inappropriate technological parameters. In this study, we prepared carbon gel-based self-standing membranes as positive electrodes and evaluated their performances in LOBs under lean-electrolyte, high-areal-capacity conditions. We clarified the following three crucial points: (1) The nanometer-sized pores exhibited limited effects in improving the cycle performance, although they contributed in enhancing the discharge capacity. (2) The macro-sized pores displayed positive effects in enhancing the discharge capacity. (3) The crystallinity and/or surface functional groups influence the discharge potential and cycle life. The results of this study suggest the significance of controlling the physicochemical properties of a porous carbon-based positive electrode in preparing a LOB with a practically high energy density and an extended cycle life.

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The OTA method helps improve the porous properties of activated carbon providing higher surface area and mesopore volume than the conventional activation method.