Dynamics of Deposited Li2O2 on Cathode Surface in Li-O2 Battery by First-Principles Molecular Dynamics

In recent years, Li-O₂ battery has received a great attention as a battery with high capacity and energy density. However, further studies are needed for the use of practical application to solve the issues must be overcome, such as lithium dendrites, side reactions of electrolyte, a blocking effect of diffusion path of oxygen and a corrosion of carbon. In this study we focus on, among from these issues, a blocking effect of diffusion path of oxygen in the cathode, which is the most important issue to realized the high capacity. Discharge products would inhibit oxygen diffusion in a cathode. Main discharge product is Li₂O₂. In this study we also focus on a dynamics of Li₂O₂. Li₂O₂ is generated from electrochemical reaction and will cover with the surface of the cathode resulting in the termination of further electrochemical reaction. Thus, the elucidation of dynamics of Li₂O₂ cluster on cathode surface is a key problem for the better understand of the life time of battery and the theoretical design of more improved Li-O₂ battery.

The electrical conductivity of Li₂O₂ is also influenced on the life time of the battery. There has been reported that a hole forming by the defect of Li palys a important role in the electrical conductivity of Li₂O₂. In this study, we model the formation of deposited Li₂O₂ layer as well as the forming process of the crystal defect, from the point of view of diffusion of Li₂O₂ cluster on the cathode surface. We calculate the sintering and diffusion processes of the discharge product Li₂O₂ on the carbon surface using a first-principles molecular dynamics method. We also examine how Li₂O₂ is deposited on the carbon surface. As a result, it deposited on the cluster shape up to the thickness of 3 " at 300 K. It is observed that the band gap of the Li₂O₂ is related to the thickness and it rapidly increases when it exceeds the 8 ". Diffusivity decreases in proportion to the increase in the size of Li₂O₂ cluster. Calculated diffusion coefficient of Li₂O₂ is revealed to the effect on the electrical conductivity of Li₂O₂. From these results, we investigate the effect of electrical conductivity of Li₂O₂ on the discharge curve analyzing by a numerical simulation. Conductivity shows little effect on the discharge curve. Therefore, It is concluded that a pore blocking is major factor of capacity of Li-O₂ battery. This is the first study will be able to provide a breakthrough for the development of better Li-O₂ battery.