Molecular dynamics simulations of cathode/glass interface behavior: effect of orientation on phase transformation, Li migration, and interface relaxation

The molecular dynamics (MD) computer simulation technique has been used to study electrolyte/cathode interfaces formed in Li-based thin film oxide solid state ionic devices at the atomistic level. The solid electrolytes are lithium silicate glasses while the cathodes are V2O5 or WO3 crystals. The work presented in this paper will focus on the behavior at the glass/V2O5 interface. The MD simulation technique has been successfully used to simulate a variety of silicate glasses and glass surfaces, with results consistent with a variety of experimental data. The simulations of the vanadia crystal reproduce the experimental crystal structures, vibrational frequency, and the appropriate phase transition of V2O5 as Li ions enter the crystal. The simulations have also shown that Li transport into the crystal is affected by the orientation of the crystal at the interface as well as by surface roughness. While the crystal oriented with the (001) planes parallel to the crystal/glass interface shows the appropriate phase transition to the δ-LiV2O5 phase as Li ions enter the crystal, the work presented here shows that the crystal oriented with the (100) planes parallel to the interface does not transform. The difference is attributed to the effect of interface bonding between the ions in the first crystal layer and those in the glass surface. The simulations show a relaxation occurring in a lithium metasilicate glass electrolyte but not in a lithium disilicate electrolyte. In addition, relaxation at the interface between a roughened glass surface and the crystal creates a distortion in the crystal planes in immediate contact with the glass that creates an induced strain in the crystal.