Bifunctional Effects of Cation Additive on Na-O2 Batteries.

Aprotic Na-O2 batteries have attracted growing interest owing to their low overpotentials and high energy density. Their cycling stability and Coulombic efficiency are limited, however, by Na dendrite formation and superoxide (O2 - ) degradation. Here, we present a bifunctional cation additive, the tetrabutylammonium cation (TBA+ ), to simultaneously protect the Na anode and stabilize the superoxide. The adsorption of TBA+ on the Na anode suppresses the dendrite formation in the Na plating and ensures stable anode cycling at high current density in both Ar and oxygen atmospheres. Aprotic Na-O2 batteries with TBA+ show increased Coulombic efficiency as well as good rate capability. These are rewarded by the fast desolvation kinetics of O2 - and suppression of the disproportionation reaction of O2 - with TBA+ , as confirmed by molecular dynamics simulation and DFT calculations. This bifunctional effect of this cation additive paves a new avenue for the development of Na-O2 batteries.