First-Principles Calculations about Elastic and Li^+ Transport Properties of Lithium Superoxides under High Pressure and High Temperature

Lithium superoxides, Li_2O_3, LiO_2, and LiO_4, have been synthesized under high pressure. These materials have potential applications in energy storage devices. Here, we use first-principles calculations to investigate the elastic and Li^+ transport properties of these oxides at high pressure and high temperature. The elastic constants are calculated at 20–80 GPa, and they satisfy the Born stability criteria, indicating the good mechanical stability of these oxides. Their sound velocities calculated with elastic constants are close to each other, but difference in velocity anisotropy is obvious. LiO_2 presents significant shear sound wave anisotropy over 80%. The Li^+ transport properties are investigated using first principles molecular dynamics (FPMD) and climbing-image nudged elastic band methods. The lowest Li^+ migration barrier energies increase from 0.93, 0.86 and 1.22 eV at 20 GPa to 1.43, 1.12 and 1.77 eV at 50 GPa for Li_2O_3, LiO_2, and LiO_4, respectively. The most favorable path for LiO_2 and LiO_4 is along the 001 direction. The FPMD results suggest that these oxides become unstable with increasing temperature up to 2000 K due to O–O dimer clusters in these superoxides. Consequently, a superionic transition is not observed in the simulations.