DFT-MD Study of Interface Between Carbon Anode and Amorphous Lithium Carbonate

Introduction Interface between the Solid-Electrolyte-Interphase (SEI) and the anode material plays an important role for Li+ transport during the charging and the discharging processes. Understanding of the structure and Li+ transport behavior at the interface is necessary for further improvement of the Lithium ion batteries. Recent experimental studies such as XPS, NMR, 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 methodology to investigate the structure and the dynamics in the atomic-scale. In this study, we investigate the interfacial structure and Li+ transport process at the interface between the graphite and the inorganic SEI component by the DFT-MD simulation. The effect of the functional groups on the graphite edges is also discussed. Method In this work, the interfacial structures were constructed from the graphite and the amorphous Li2CO3, as the models of the anode and the inorganic SEI, respectively. The composition of graphite anode was set to Li10C240, which corresponds to the dilute stage 1. The edge carbons were terminated by the -H only (clean surface model) or by -H:-COOH:-OH = 4:2:4 (oxidized surface model). Li+ and CO3 2- moieties were located on the graphite surface randomly. To obtain the stable interfacial structure, we performed the DFT-MD simulation during at least 5ps with NVT ensemble at 298K. In order to investigate the Li+ mobility at the interface, the blue-moon ensemble technique was applied to the Li+ insertion reaction into the graphite. For all DFT calculations, we used the PBE exchange correlation functional and Goedecker-type Norm-conserving pseudopotentials. The DFT-MD simulations were carried out in the framework of Car-Parrinello dynamics, by using CPMD code. Results and Discussion Figure 1 shows the obtained interfacial structures of the clean (a) and the oxidized (b) surface models. For the oxidized surface, a distance between the graphite edge and Li2CO3 layer is closer than that for the clean surface. The calculated interface energies were -1.69x10-2 and ­-2.51x10-1 J/m2 for the clean and oxidized surfaces, respectively. The large stabilization for the oxidized surface is caused by the strong interaction between surface functional groups and carbonates. Figure 2 (a) shows an averaged distance (d Li-O SEI) between inserted Li+ and the nearest Oxygen in the SEI during insertion process for the clean surface model. d Li-O SEI remains almost constant until Li+ reaches at graphite edge (z = 0), which indicate that the CO3 2- in the SEI are dragged by the Li+ ion. For the oxidized surface case, a distance between Li+ and nearest Oxygen of the surface functional group (COOH or OH) is constant instead of d Li-O SEI. While CO3 2- is not dragged near the graphite edge in this case, the surface Oxygen supports the Li+ insertion. The free energy profiles of these insertion processes will be also discussed. Figure 1