Atomic-scale studies of garnet-type Mg3Fe2Si3O12: Defect chemistry, diffusion and dopant properties

Abstract

Materials with low cost, environmentally benign, high structural stability and high Mg content are of considerable interest for the construction of rechargeable Mg-ion batteries. In the present study, atomistic simulations are used to provide insights into defect and diffusion properties of garnet type Mg 3 Fe 2 Si 3 O 12 . Calculations reveal that the Mg–Fe anti-site defect cluster (0.44 eV/defect) is the lowest energy intrinsic defect process. Three dimensional Mg-ion migration pathway with the activation energy of 2.19 eV suggests that Mg-ion diffusion in this material is slow. Favourable isovalent dopants are found to be Mn 2+ , Ga 3+ and Ge 4+ on the Mg, Fe and Si sites respectively. While the formation of Mg interstitials required for the capacity is facilitated by Al doping on the Si site, Mg vacancies needed for the vacancy assisted Mg-ion diffusion are enhanced by Ge doping on the Fe site. The electronic structures of favourable dopants are calculated and discussed using density functional theory. • Defect processes and Mg transport in Mg 3 Fe 2 Si 3 O 12 are studied theoretically. • The energetically favourable intrinsic defect type is the Mg–Fe anti-site. • Activation energy of migration of Mg ions via the vacancy mechanism is 2.19 eV. • Subvalent doping by Al on the Si site can eenhance the capacity.