MOLECULAR DYNAMICS SIMULATION OF LI-ION COORDINATION STRUCTURE AND TRANSPORT PROPERTIES IN LIB ELECTROLYTES: INFLUENCE OF MOLECULAR SIZE

摘要

Lithium-ion batteries (LIBs) are currently attracting much attention because electric vehicles and large storage batteries are becoming widespread. For lightweight and efficient usability, LIB performance for safety, capacity, charge/discharge cycle characteristics, and electric current must be improved. Development of high performance LIB relies mostly on progress of materials used for electrolytes and electrodes. In particular, electrolytes are an important factor because they play a role in carrying charged substances, i.e. ions, between separated electrodes. The ion-carrying performance depends on the choice of electrolyte, and thus, in this study, we discuss the relationship between the geometric shape of molecules used as electrolytes and their physical properties, using molecular dynamics (MD) simulations. We showed that, as the smaller solvent is used, ionic conductivity of the system is further enhanced. MD simulations showed that there are two main reasons for this behavior. First, a smaller radius allows the solvent molecule to diffuse easily. Consequently, when solvent molecules frequently surround a lithium ion, and the diffusion coefficient of the lithium ion becomes larger. Second, because small solvent molecules naturally come close to ions, salt (composed of cations and anions) is forced to be dissociated because of the solvent molecules, and the degree of salt dissociation increases.