Intrinsic Conductivity in Sodium-Air Battery Discharge Phases: Sodium Superoxide vs Sodium Peroxide

The primary discharge product in sodium air batteries has been reported in some experiments to be sodium peroxide, Na2O2, while in others sodium superoxide, NaO2, is observed. Importantly, cells that discharge to NaO2 exhibit low charging overpotentials, while those that discharge to Na2O2 do not. These differences could arise from a higher conductivity within the superoxide; however, this explanation remains speculative given that charge transport in superoxides is relatively unexplored. Here, density functional and quasi-particle GW methods are used to comparatively assess the conductivities of Na-O-2 discharge phases by calculating the concentrations and mobilities of intrinsic charge carriers in Na2O2 and NaO2. Both compounds are predicted to be electrical insulators, with bandgaps in excess of 5 eV. In the case of sodium peroxide, the transport properties are similar to those reported previously for lithium peroxide, suggesting low bulk conductivity. Transport in the superoxide has some features in common with the peroxide but also differs in important ways. Similar to Na2O2, NaO2 is predicted to be a poor electrical conductor, wherein transport is limited by sluggish charge hopping between O-2 dimers. Different from Na2O2, in NaO2 this transport is mediated by a combination of electron and hole polarons, An additional distinguishing feature of the Superoxide is its ionic conductivity, which is 10 orders of magnitude larger than the electronic component. The ionic component is comprised primarily of p-type contributions from (surprisingly mobile) oxygen dimer vacancies, and from n-type contributions from negative sodium vacancies. In the context of sodium air batteries, the low electronic conductivity afforded by NaO2 suggests that enhanced bulk transport within this phase is unlikely to account for the low overpotentials associated with its decomposition. Rather, the enhanced efficiency of NaO2-based cells should be attributed to other factors, such as a reduced tendency for electrolyte decomposition.