1 DFT+U Study of Polaronic Conduction in Li2O2 and Li2CO3: 2 Implications

7 ABSTRACT: The main discharge products formed at the 8 cathode of nonaqueous Li−air batteries are known to be Li2O2 9 and residual Li2CO3. Recent experiments indicate that the 10 charge transport through these materials is the main limiting 11 factor for the battery performance. It has been also shown that 12 the performance of the battery decreases drastically when the 13 amount of Li2CO3 at the cathode increases with respect to 14 Li2O2. In this work, we study the formation and transport of 15 hole and electron polarons in Li2O2 and Li2CO3 using density 16 functional theory (DFT) within the PBE+U approximation. 17 For both materials, we find that the formation of polarons (both hole and electron) is stabilized with respect to the delocalized 18 states for all physically relevant values of U. We find a much higher mobility for hole polarons than for the electron polarons, and 19 we show that the poor charge transport in Li2CO3 compared to Li2O2 can be understood through a polaronic model for the 20 conduction. Furthermore, the hole polaronic model in Li2O2 provides a possible explanation for the experimentally observed 21 preferential growth direction of the films. Our results also suggest that doping is unlikely to be a viable route for improving the 22 transport properties of Li2O2 or Li2CO3. 1