Lithium diffusion pathways and vacancy formation in the Pmmn-Li1−xFeO2electrode material

Abstract

Models for Li⁺ ion mobility were developed and investigated in the ‘corrugated layer’ orthorhombic phase of Li_1−xFeO₂, an attractive possible electrode material for reversible lithium ion batteries. The ground-state crystal energy was computed by first-principles DFT (Density-Functional-Theory) methods, based on the use of the hybrid B3LYP functional with localized Gaussian-type basis sets. Appropriate supercells were devised as needed, with full least-energy structure optimization. In the defect-free case (x = 0), ion diffusion was found to take place cooperatively inside a fraction of active lithium layers separated by inert ones, so as to reduce lattice strain; intermediate bottleneck states of Li are either in tetrahedral (energy barrier ΔE_a = 0.410 eV) or linear (ΔE_a = 0.468 eV) coordination. For the Li_0.75FeO₂ deintercalated material a number of low energy vacancy configurations were considered, investigating also the vacancy influence on electron density of states and atomic charge distribution. The most favourable ion transport mechanisms (ΔE_a = 0.292 and 0.304 eV) imply a linear Li bottleneck state, with all lithium layers active and a quite small lattice strain. Accordingly, in the defective material the predicted ionic conductivity at room temperature rises from 10⁻⁵–10⁻⁶ (LiFeO₂) to 4 × 10⁻⁴ ohm⁻¹ cm⁻¹ (Li_0.75FeO₂).