Effects of Oxygen Cathode Substrates with Various Carbon Catalysts on the Discharge Capacity of Lithium-Oxygen Battery

Abstract

Lithium-oxygen batteries have been considered strong candidates to supersede current rechargeable batteries especially lithium-ion batteries. Despite having tremendously high theoretical energy density, the practically achievable energy density is far from commercial application till date. The oxygen electrode is one of the major bottlenecks in maximizing the battery capacity. It is comprised of a porous structure (mainly carbon) with high surface area and sufficient reaction sites. High porosity facilitates lithium ion and oxygen mass transfer and results in high storage capacity for the insoluble discharge product during discharging. The cathode electrode is fabricated by coating a catalyst layer on a porous carbon substrate. The substrate, which contacts with oxygen channels, plays a vital role in mass transfer of oxygen from outside to the reaction sites. Therefore, it is important to select a substrate that does not clog with deposition of Li2O2 particles immediately after discharging begins and allows oxygen to reach the catalyst side for longer period of time. This study investigates the effects of oxygen electrode substrates with various coating materials on the discharge capacity of lithium-oxygen battery. For this purpose, we have compared the discharge capacity of plain carbon cloth, plain carbon paper and carbon cloth or carbon paper coated with carbon catalysts including Vulcan XC, Super P and Graphene Oxide. 1.5mg/cm2 (+/-0.3mg/ cm2) is used as catalyst loading. The electrolyte used in this study is 1M LiTFSI in TEGDME. PTFE (30wt%) is used as a binder. All experiments are conducted at room temperature with pure oxygen supply. With carbon cloth as a substrate, the discharge capacities obtained were 3 to 6 times higher (depending on the carbon catalysts coated) than the capacities obtained with carbon paper. The bare carbon cloth, Vulcan XC, Super P and Graphene oxide with carbon cloth as a substrate have discharge capacities of 0.11 mAh/mg, 13 mAh/mg, 6.8 mAh/mg, 1.5 mAh/mg respectively. On the other hand, the discharge capacities achieved with bare carbon paper, Vulcan XC, Super P and graphene oxide with carbon paper as substrate were 0.02 mAh/mg, 2.4 mAh/mg, 2.7 mAh/mg, 0.28 mAh/mg respectively. The results clearly indicate that the cathode with carbon cloth as a substrate yields significantly higher discharge capacity with all the coating materials as compared with the carbon paper. The different performances of two substrates can be attributed to their structure and pore size distribution. Carbon cloth is composed of flexible woven threads with longer lengths ( ̴ 30µm) and broader pore size distribution (5-200 µm) with peaks at 10, 70 and 100 µm. This type of structure is favorable in reducing tortuosity, enhancing mass transport of oxygen and lithium ion and improving ionic conductivity. On the other hand, the structure of carbon paper comprises of non-woven short carbon fibers ( ̴ 15µm) and smaller pore size distribution range (10-70 µm) with a peak at 50 µm. This renders poor transport of oxygen from outside to the reaction sites leading to lower discharge capacity. The primary function of coating materials is to increase surface area of cathode thereby providing more reaction sites and creating tri-phase boundaries for reaction to take place. These catalysts also have different pore size distributions and resistance to mass transfer. Therefore, various carbon catalysts used yield different discharge capacity with same substrate.