Evolution of Discharge Products on Carbon Nanotube Cathodes in Li–O2 Batteries Unraveled by Molecular Dynamics and Density Functional Theory

Abstract

The performance of Li–O2 batteries (LOBs), such as capacity and overpotential, is closely related to the morphology of the discharge product. Here, the relationship between the growth behavior of Li2O2 on the surface of the carbon nanotube (CNT) cathodes and the cycle performance of LOBs was innovatively revealed based on molecular dynamics (MD) and density functional theory (DFT). Our results demonstrated that the growth of (Li2O2)n on the CNT surface mainly undergoes three stages: adhesion, branching, and connection. The stable gap between (Li2O2)n and the CNT surface was determined to be approximately 2.47 Å. Interestingly, the dense deposition thickness is positively correlated with the number of Li2O2 monomers. In addition, the formation of free Li2O2 directly induces the instability of (Li2O2)n and capacity loss. Moreover, armchair-type CNTs with larger diameters, especially single-walled CNTs and multi-walled CNTs with an odd number of tube walls were found to be more conducive to the stable growth of discharge products. Notably, (Li2O2)n is mainly composed of internal stable parts with low conductivity and amorphous components distributed on the surface with p-type semiconductor characteristics. Therefore, the regulation of the CNT structure and the preparation of catalysts to promote the conversion of Li2O2 from an ordered state to an amorphous structure play a vital role in breaking the technical bottleneck of LOBs. Our results identify the long-term controversial evolution mechanism of the product morphology, and the unique calculation ideas are also applicable to the intuitive exploration of the microscopic growth behavior of discharge products in other metal–air batteries.