Lithium air batteries are attracting attention as next generation secondary batteries because powerful secondary batteries are required for an application of electric vehicles. In the lithium air battery, Li₂O₂ is the main discharging products that decomposed into Li⁺ and O₂ during charging, while electrochemically irreversible Li₂CO₃ and Li₂O are also formed. Reaction phenomenon on formation or decomposition of these products is complex and effects significantly the over potential during charging originated from the byproducts decomposition. Therefore, decomposition behavior of discharge products on a cathode electrode during charging is of fundamental of importance in developing high performance lithium air battery. In this study, we carried out first a principle molecular dynamics method (FPMD) considering electron transfer from carbon surface to decomposed species to analyze the relationship between charging condition and decomposition behavior at the atomic level. In particular, the influence of surface structures of negative electrode on the dynamics of decomposition is studied.

In order to simulate the electrochemical reaction during charging, simulation was carried out by first principle molecular dynamics based on density functional theory (DFT) in which the charge of the system was changed corresponding to the transfer of electrons from the cathode materials to products. Calculations were carried out by using models for different surface structures; the carbon (001) surface, the carbon (100) surface, and the carbon surface with step structure models. Calculations was made with NVT ensemble (300 K) using generalized density gradient approximation (GGA) with PBE functional, and total calculated period was 300 fs - 1000 fs. The charging voltage was evaluated from the change of potential energy form FPMD simulations.

From the FPMD simulations, a little change of the Li₂O₂ cluster is stable on the carbon surface with step structure model. However the decomposition of the Li₂O₂ cluster is observed for the model of carbon (001) surface, although the formation of Li ion is not confirmed. In the simulation of a Li₂O₂ cluster on the carbon (001) surface, the Li₂O₂ cluster was decomposed and lithium ion was confirmed. These results suggests that decomposition of Li₂O₂ hardly occurs near a step structure of carbon. From the free energy change calculated from the simulation result of the carbon surface with step structure model, it is estimated that a charging voltage of 5V or higher is necessary for decomposition of Li₂O₂ cluster. Simulation of the Li₂CO₃ cluster on the carbon (001) surface model indicates that decomposition of Li₂CO₃ could be possible if a high voltage is applied to the system.

As a summary from our simulations, it was suggested that Li₂O₂ deposited near step structure of carbon (100) surface is stabilized by strong interaction of terminated groups preventing from the decomposition, resulting in an increase in charging voltage and it would contribute to the reduction in recycling capacity. Details of dynamics of Li2O2 and Li2CO3 and molecular orbital analysis will be presented in the conference.