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.