Deep insight into the defect structure, lithium diffusion kinetics and deintercalation study on β-LiVOPO4 cathode material by atomistic simulation method

Abstract

In this work, atomistic simulation methods including first principles density functional theory (DFT) and molecular dynamics (MD) are applied to study the defect structure, the synergetic motion of lithium ions and the deintercalation mechanism in cathode material β-LiVOPO4. Li+ frenkel defect, Li/V antisite and Li+ vacancies are the main intrinsic defects in the lattice. Three Li+ diffusion paths, including 4c mediated diffusion path on the same edge sharing LiO6 octahedra chain along b-axis, diffusion path between two 4b lithium sites on the nearest neighboring LiO6 octahedra chain along b-axis and vanadium site mediated diffusion path are elaborately revealed by molecular dynamics (MD) and bond-valence-energy-landscape mapping (BVEL). The energy barriers of the Li+ hopping are investigated by nudged elastic band (NEB). Lithium mobility is extremely low for the perfect lattice of β-LiVOPO4 where no impurities and defects are present. Upon the deintercalation of Li+, the lithium ions are activated. The state of charge (SOC) dependency of Li+ diffusion coefficient shows a convex profile with a maximum value at x = 0.5. When the deintercalation level falls within 0.5∼0.75, the obtained energy barriers (0.31–0.38 eV) best match the value obtained by vacancy mediated NEB method, 0.34 eV