Abstract

Correlations between the ionic conductivity and antisite disorder in low cost cathode materials (1D Li⁺ conducting LiFePO₄ and quasi-1D LiFeSO₄F) and the origin of the experimentally observed drastic conductivity enhancement in nanoscale heterostructures Li_xFePO₄:Li₄P₂O₇ are explored by molecular dynamics (MD) simulations with a novel bond valence (BV) based force-field. Compared to bulk values, ionic conductivity in surface-modified Li_xFePO4 is enhanced by up to 3 orders of magnitude. Details of dynamic ion transport pathways are extracted by our BV transport pathway analysis applied to MD simulation trajectories. Besides heterogeneous doping by the redistribution of mobile ions across the interface, ion mobility varies as quantified via the extension of unoccupied accessible pathway regions. A layer-by-layer analysis indicates a maximum mobility close to the interface, but Li⁺ mobility remains enhanced even at the center of the simulated nanocrystals. Li⁺ diffusion in LiFeSO₄F exhibits a pronounced anisotropy with a “superionic” zig–zag pathway parallel to [111] involving partially occupied Li sites. A notable long range ion diffusion rate can be maintained in macroscopic LiFeSO₄F crystals due to the moderate activation energy for migration perpendicular to the channels.