Adsorption mechanisms of lithium oxides (LixO2) on N-doped graphene: a density functional theory study with implications for lithium-air batteries

Abstract

We utilized density functional theory (DFT) study to understand the adsorption mechanism of lithium oxides (LixO2) onto N-doped graphene during oxygen reduction reaction (ORR) for lithium-air batteries. We systematically proposed two possible ORR pathways and examined various adsorption configurations in each system, including for the O-2 and Li ORR reactants and the LiO2 and Li2O2 ORR products. The doping of the N atom into graphene was calculated to enhance the adsorption of O-2, but to attenuate the adsorption of Li, because of the repulsion between the electron-rich N-doped graphene and the electron-donating Li atom, and the attraction of this N-doped graphene for electronegative O-2. Nevertheless, since the adsorption of Li onto N-doped graphene (-1.001 to -0.503 eV) was still stronger than the adsorption of O-2 (-0.280 to -0.215 eV), Li should bind N-doped graphene first. Moreover, N-doped graphene was calculated to bind LiO2 (-0.588 eV) more strongly than was pristine graphene (-0.450 eV). Additionally, the Li2O2 configuration that yielded the most stable adsorption on N-doped graphene was calculated to yield an adsorption energy of -0.642 eV, which is more favorable than that for pristine graphene (-0.630 eV). Overall, N-doped graphene was found to strengthen the adsorption of lithium oxides (LixO2) and increase charge transfer to substantial levels.