Theoretical aspects in structural distortion and the electronic properties of lithium peroxide under high pressure

Abstract

The structural phase transition and electronic properties of Li₂O₂ under pressures up to 500 GPa have been investigated using first-principles calculations. Two new structural phase transitions have been proposed at pressures around 75 GPa from the P6₃/mmc structure to the P2₁ structure and around 136 GPa from the P2₁ structure to the P2₁/c structure. The calculated phonon spectra have confirmed the dynamical stability of these structures. The pressure dependence of the lattice dynamics, O–O bond length, and band gaps in Li₂O₂ have also been reported. The band gaps of the P6₃/mmc, P2₁, and P2₁/c structures calculated by PBE and HSE06 have shown increasing trends with increasing pressure. Interestingly, the P6₃/mmc band gap and c/a ratio have significantly decreased with the increasing O–O bond length and ELF value around 11 and 40 GPa. At these pressures, the phonon frequency of the O–O stretching modes has softened. This finding reveals the effects of structural distortion in three phases of Li₂O₂. Our study provides structural understanding and the electronic properties of Li₂O₂ under high pressure, which might be useful for investigating the charge transport through Li₂O₂ in lithium–air batteries and CO₂ capture.