Effect of Carbon Surface Area on First Discharge Capacity of Li-O₂Cathodes and Cycle-Life Behavior in Ether-Based Electrolytes

In this study we report on the direct proportionality between cathode surface area and first discharge capacity of non-aqueous Li-O2 cells using ether-based electrolytes. Seven different highly structured carbon blacks, characterized by different surface areas and porosities, were used to prepare non-catalyzed cathodes. Surface measurements and porosity analyzes were carried out on both raw materials and electrodes in order to estimate the fraction of cathode surface accessible by the electrolyte. The first specific discharge capacity of different cathodes was then normalized over its specific surface and a strong correlation between the two quantities was found. This result strongly supports a discharge mechanism for Li-O2 batteries wherein the main factor limiting the capacity is the formation of a passivating layer of products on the surface of the cathode material, impeding ORR at carbon active sites. Also the cyclability of the cells was considered, demonstrating the effect of electrolyte degradation on the increased capacity upon cycling of ethereal electrolytes (e.g. DEGDME). The first discharge specific capacity and cyclability using TEGDME, commonly used in Li-air research, was found similar to reacting or degraded electrolytes, suggesting a much higher reactivity toward superoxide ion radical in comparison to its lighter homologues DEGDME and DME.