Influence of Cathode Surface Structure on Decomposition Behavior of Li2O2 and Li2CO3 in Lithium Air Battery Investigated By First Principles Molecular Dynamics
Authors: Soeno Mashiro 
Published: February 2020

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.