Carbon Nanofoam-Based Cathodes for Li–O₂Batteries: Correlation of Pore–Solid Architecture and Electrochemical Performance

Abstract

Freestanding, binder-free carbon nanofoam papers afford the opportunity to gauge the influence of pore size on the discharge capacity of Li–O2 cells. Four sets of carbon nanofoam papers were synthesized from resorcinol–formaldehyde sols, with pore size distributions in pyrolyzed forms ranging from mesopores (5–50 nm) to a size regime not represented in the literature for Li-O2 cathodes—small macropores (50–200 nm). The first-cycle discharge capacity in cells containing 0.1 M LiClO4 in dipropylene glycol dimethyl ether tracks the average pore size distribution in the carbon nanofoam cathode, rather than the specific surface area of the nanoscale carbon network or its total pore volume. The macroporous nanofoams yield cathode specific capacity of 1000–1250 mA h g−1 at –0.1 mA cm−2 discharge rate, approximately twice that of the mesoporous nanofoams (∼580–670 mA h g−1), even though the macroporous foams have lower specific surface areas (270 and 375 vs. >400 m2 g−1). The specific capacity of the cathode decreases as the thickness of macroporous carbon nanofoam paper is increased from 180- to 530-μm, which indicates that the interior pore volume is underutilized, particularly with thicker nanofoams. For the four pore–solid nanofoam architectures studied, the specific capacity is limited by pore occlusion arising from solid Li2O2 product that is electrogenerated near the outer boundaries of the nanofoams.