First-principles study of reduction mechanism of oxygen molecule using nitrogen doped graphene as cathode material for lithium air batteries

Abstract

Lithium-oxygen battery possesses an extremely high theoretical energy density (<inline-formula><tex-math id="Z-20190605015200-1">\begin{document}$ \approx$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20190181_Z-20190605015200-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20190181_Z-20190605015200-1.png"/></alternatives></inline-formula> 3500 W·h·kg<sup>–1</sup>), and is an ideal next-generation energy storage system. The ideal operation of lithium-oxygen batteries is based on the electrochemical formation (discharge) and decomposition (charge) of lithium peroxide (Li<sub>2</sub>O<sub>2</sub>). At the beginning of the discharge, oxygen is reduced on the electrode, forming an oxygen radical (<inline-formula><tex-math id="Z-20190602062455-1">\begin{document}${\rm O}^{-}_{2} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20190181_Z-20190602062455-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20190181_Z-20190602062455-1.png"/></alternatives></inline-formula>). The <inline-formula><tex-math id="Z-20190602062457-2">\begin{document}$ {\rm O}^{-}_{2}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20190181_Z-20190602062457-2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20190181_Z-20190602062457-2.png"/></alternatives></inline-formula> successively combines with an Li ion, forming the metastable LiO<sub>2</sub>. The LiO<sub>2</sub> may subsequently undergo two different reaction pathways: a chemical disproportionation and a continuous electrochemical reduction, thereby resulting in the formation of Li<sub>2</sub>O<sub>2</sub>. Therefore, the oxygen reduction reaction (ORR) is an important step in the discharge process. Studies have shown that graphene is considered as the most promising cathode material for non-aqueous lithium-oxygen batteries. Moreover, it is found that nitrogen-doped graphene has higher electrocatalytic activity than intrinsic graphene for the ORR. However, up to now, the mechanism of improving the ORR for nitrogen-doped graphene is still unclear, and the effects of different N-doping concentrations on the ORR have not been reported. In this work, on the basis of the first-principles calculations, the reduction mechanism of O<sub>2</sub> molecule by nitrogen-doped graphene with different N concentrations is studied. Results show that after doping N atoms, the adsorption energy of O<sub>2</sub> molecules increases, the O—O bond length is elongated, and the transferred charge increases, which indicates that nitrogen-doped graphene enhances the reduction ability of O<sub>2</sub> molecule. Bader charge analysis shows that both N atom and O<sub>2</sub> molecule obtain charges from C atom, and N atom also provides charges for O<sub>2</sub> molecule, which is consistent with the electronegativity of carbon, nitrogen and oxygen. This charge transfer results in the stronger interaction between the O<sub>2</sub> molecule and the substrate, and can reveal the reason why nitrogen-doped graphene can improve the ORR. In addition, it is found that the reduction ability of O<sub>2</sub> molecule is best when the N-doping ratio is 3.13 at%. It is hoped that this work will play a guiding role in the synthesizing the nitrogen-doped graphene materials, and will be helpful in optimizing the cathode materials of lithium-oxygen batteries.