Controlling the triple phase boundary on Na-O-2 battery cathodes with perfluorinated polymers

Abstract

Sodium-oxygen batteries hold great promise for the transition to a non-fossil fuel economy due to their high theoretical energy density. One of the most important components of these devices is the air-cathode, where the electrons available at the solid electrode, the Na+ ions present in the liquid electrolyte and oxygen gas react to form sodium oxides as discharge products. The kinetics of the discharge/charge reactions depend significantly on the boundary points between the solid-liquid-gas reaction phases, known as triple phase boundary (TPB). The density of TPB points (and therefore the battery efficiency) can be maximized by incorporating perfluorinated polymers on the cathode formulation. Thus, this type of polymers enhance oxygen transport properties which favour the diffusion of gaseous components in detriment to liquid electrolytes on solid electrodes. In this work, polytetrafluoroethylene (PTFE) and fluorinated ethylene propylene (FEP) polymers were added in different weight ratio to commercial graphene nanoplatelets (GNPs) cathodes. The critical physical properties affecting the formation of the TPB have been identified and correlated to sodium-oxygen battery performance. These key properties, which are crucial to modulate the oxygen diffusion within the cathode structure, have been identified for the first time in this work for aprotic metal air devices. This approach is of outmost importance for the development or efficient electrochemical storage devices where oxygen gas is involved.