Silica Nanoparticles as Structural Promoters for Oxygen Cathodes of Lithium–Oxygen Batteries

Abstract

In lithium/oxygen batteries with liquid nonaqueous electrolytes, metal or metal oxide electrocatalysts are generally used in the carbon cathode to decrease the cell overvoltage and increase the capacity. In this study, a composite carbon cathode composed of Ketjen black carbon in a mixture with catalytically inactive fumed silica with a mean particle size of about 7 nm has been employed. The addition of 2 wt % silica nanoparticles into an otherwise pure carbon cathode led to 50% increase of the discharge capacity. Moreover, the charge overvoltage was decreased by 0.2 V, and a higher rate capability was achieved. We assume that the silica nanoparticles partially block the carbon surface, enforcing the formation of only nanosize Li2O2 deposits during discharge, compared to micrometer size Li2O2 deposits without nanosilica resulting in the effect of pore clogging. The silica nanoparticles kept the porous structure open for oxygen to diffuse deeper into the cathode causing the increase of capacity and rate capability. Additionally, the nanosize Li2O2 discharge product appears to show higher electrical conductance lowering the charge overvoltage. By controlling the morphology of the Li2O2 discharge product, the electrochemically inert nanosilica acts as a structural promoter and shows similar behavior as reported for catalytically active particles in lithium/oxygen batteries.