Lithium diffusion in the spinel phase Li4Ti5O12 and in the rocksalt phase Li7Ti5O12 of lithium titanate from first principles
Lithium titanate (LTO) is a promising candidate as an anode material in future generations of lithium ion batteries due to its high intrinsic safety and stability. In this work, we investigate the diffusion barriers for lithium ions in two different crystal structures of LTO using the density functional theory. Our calculations show that the activation barriers vary between 0.30-0.48 eV for the spinel phase Li4Ti5O12 and between 0.20-0.51 eV in the lithiated rocksalt phase Li7Ti5O12. The origins of the rather broad ranges of activation energies are related to different chemical environments of the diffusion channels due to mixed occupancies of some sites in LTO. Our results reveal that the determination of lithium diffusion constants in LTO can not be carried out by using a single activation barrier. Instead, the local environment of the diffusion paths must be considered to correctly capture the variety of activation barriers. Moreover, we find the sites which have mixed occupation in LTO to trap lithium vacancies in the spinel phase. This effect is not observed in the rocksalt phase. This behavior explains the low lithium diffusivity found in experiments for lithium concentrations in the vicinity of the spinel phase.