Porous carbon materials play a key role in rechargeable Li-O2 batteries as the oxygen diffusion media
and the site for reversible electrode reactions. Despite the tremendous efforts in the synthesis of various
porous carbon materials, the influence of carbon materials on cell capacity remains unclear. Based on our
study of eight different carbon electrode materials with various pore size and pore volume in Li-O2
batteries, we found that the initial discharge capacity was hardly affected by the surface area or pore
volume. Instead, it was directly correlated with the pore sizes. To further verify this finding, meso- and
macro-porous carbon materials with pore size in the range of 20 to 100 nm were prepared using spherical
silica as template. The results clearly showed that the cell capacity increases with the increase of pore
size and eventually reached its maximum at 7,169 mAh g-1 at a pore size of 80 nm. A physical model
proposed to illustrate the influence of carbon pore size on cell capacity is the formation of a monolayer
of Li2O2 with a thickness of 7.8 nm inside the carbon pores during the discharge process which limits the
diffusion of incoming oxygen at smaller pore size (< 80 nm).
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This research was supported by the Advanced Energy Storage Research Programme (IMRE/12-2P0503 and IMRE/12-2P0504), Institute of Materials Research and Engineering (IMRE) of the Agency for Science, Technology and Research (A*STAR), Singapore.