2, Japan Science and Technology Agency, Kawaguchi, Saitama, Japan
3, Kyoto University, Kyoto, Kyoto, Japan
Interface between solid electrolyte interphase (SEI) and the anode material plays an important role for Li+ ion transport during the charging and discharging processes. Understanding of the structure and Li+ ion transport behavior at the interface is crucial for further improvement of the Lithium ion batteries (LIBs). Recent experimental studies such as XPS and TOF-SIMS have revealed the chemical components and their spatial distribution in the SEI layer. However, it is still difficult to characterize the interfacial structure experimentally. DFT-MD simulation technique is a powerful tool to investigate the structure and the dynamics in the atomic-scale. In this study, we investigate the interfacial structure and Li+ ion transport process at the interfaces between Li-intercalated graphite and amorphous Li2CO3, a model inorganic SEI, by the DFT-MD simulation. Effect of the functional groups at the graphite edges is also examined.
The Li-intercalated graphite was set to Li10C240, corresponding to the dilute stage 1. The edge carbons were terminated by the -H only (reduced surface model) or by -H:-COOH:-OH = 4:2:4 (oxidized surface model). Li+ and CO32- were located on the graphite surface randomly. To obtain the stable interfacial structure, we performed the DFT-MD simulation for at least 5ps with NVT ensemble at 298K. In order to investigate the Li+ ion migration at the interface, the blue-moon ensemble technique was applied to the Li+ ion position. For all DFT-MD calculations, we used the PBE exchange correlation functional.
Regarding the equilibrium structures, it is demonstrated that the the oxidized surface termination is more close to the Li2CO3 SEI than the hydrogen-terminated reduced surface. The large stabilization of the oxidized surface is attributed to strong interaction between the surface functional groups and the carbonates.
In the investigation of Li+ ion transport, we applied constrained DFT-MD for the Li+ ion position. For the reduced surface, Li+ ion drags the CO32- unit to around the graphite edge and then releases it to move into the anode. On the other hand, the oxidized surface shows correlation between the inserted Li+ ion and the nearest Oxygen of the surface functional group (COOH or OH) instead of the CO32- unit. Therefore, Oxygen in the termination is likely to support the Li+ ion insertion. However, the free energy profiles in these two cases do not look so different. In the presentation, we will propose possible mechanisms of these observations with the detailed analysis.