Molecular dynamics simulation of sodium diffusion in amorphous carbon using optimized reactive force field
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- Institutt for kjemi 
A reactive force fieldhas been optimized to study sodium intercalation and diffusion into graphitic carbon at high temperature. The resulting force field reproduces training data and calculates new data with moderate accuracy. It has been applied in computer simulations of atomistic models representing sodium diffusion in graphitic carbon. In this thesis, the force field is further employed to simulate sodium intrusion in generated amorphous carbon models at room temperature. The objective is to qualitatively evaluate the ability of the force field to describe sodium interaction in relevant carbon structures at lower temperature. Amorphous carbon models have been generated by the liquid-quench routine. The radial distribution functions of these a-C give a good fit to the experimental data. The successive peaks shows that the structures contain largely of sp2 hybridized bonds, suggesting the carbon atoms are fused in rings. Results from the computed angle distribution functions reveal that the structures are mostly comprised of six-membered rings, with small amount of five- and seven-membered rings. Sodium atoms of two different densities are subsequently inserted into the amorphous structures. For the case of low density, sodium atoms enter the structures through the insertion sites of low energy, occupying these sites and preventing further insertion. At higher density, the sodium atoms exert a higher pressure on the carbon structure. More sodium atoms are able to enter the structure through insertion sites of higher energy. These simulation results demonstrate that the force field is able to qualitatively describe sodium diffusion into amorphous carbon.