Molecular Simulation of Decomposition of Li2O2 and Li2CO3 during Charging Process of Li-O2 Battery

Abstract

Much attention has been given to Li-O2 battery due its high energy density compared to Li ion battery. However, charging voltage of Li-O2 battery is sometimes reported to be larger than the voltage during discharge. Therefore, the suppression of charging voltage is the subject for its practical applications. In this study we have applied a first principles molecular dynamics method for the analysis of the dynamics of electrochemical decomposition of Li2O2 and Li2CO3 in Li-O2 battery. Using the first-principles molecular dynamics we also estimated the free energy change for decompositions and the change of overpotential and its relationship with decomposition behaviour were calculated. The simulations of the decomposition reaction for both species have been successfully performed in vacuum and also for the cluster of Li2O2 and Li2CO3 placed on the carbon surface in consideration of the real system. In the case of Li2O2, our simulation results show that all the cases of Li2O2-carbon complexes Li2O2 first decompose into superoxide like LiO2 structure via the removal of lithium atoms. The decomposition pathway from our simulations has a good consistency with experimental findings that lithium superoxide is an intermediate in the formation of Li2O2 in the reduction reaction. The free energy profile of during the decomposition of Li2O2 at the applied potential U = 0 and U = 4 eV were calculated. The free energy change indicates that the system is capable to be charged when applying the potential of 4 eV. Experimentally, charging voltages as high as 4.0-4.5 V have been observed, which validates our simulation results. Our simulation results also show that the charging voltage increase when the interaction of decomposed species becomes large. In summary, our simulation results successfully reveal the decomposition path at the atomistic level as well as the free energy profile which was not clear from the experimental findings. We will discuss the results of decomposition process of Li2CO3 at the conference.