Simple Method to Relate Experimental Pore Size Distribution and Discharge Capacity in Cathodes for Li/O2 Batteries

We analyze in detail the relationship between pore size distribution and discharge capacity for cathodes in ionic liquid-based Li/O-2 batteries at room temperature (RT) and 60 degrees C. We used several porous carbons with similar composition and apparent surface area but with pore distribution peaks in different points of the meso/macroporous region. The porous structure of carbons caused a significant influence on the discharge specific capacity. However, no obvious correlations between specific capacity and surface area or total pore volumes were observed. Carbons with high mesopore volumes and a predominant pore size of 20-40 nm exhibited the highest specific capacities. When temperature rises from room temperature to 60 degrees C, discharge capacity increases by a factor higher than two, with the smallest pores providing the highest increases. A model is introduced to empirically correlate capacity with pore size distribution. This model assumes that during electrochemical discharge the pore walls are uniformly coated in their thickness but that pores below a threshold size value do not participate at all to the capacity. Our model can account for the effects of pore size distribution using a discharge layer thickness of a few nanometers and with threshold values of excluded pore sizes, of 12 nm at RT and 10 nm at 60 degrees C. The model also allowed the estimation of the penetration depth of the discharge reaction on the electrode thickness and indicates that its increase is the main factor justifying the increase of capacity when temperature is increased.