Atomistic modeling of site exchange defects in lithium iron phosphate and iron phosphate

A new set of potentials is presented that allows for modeling of the entire lithium insertion range of the lithium iron phosphate system (LixFePO4, 0 <= x <= 1). By comparing calculated values to experimental crystallographic, spectroscopic and thermodynamic data, the potentials ability to reproduce experimental results consistently and reliably is demonstrated. Calculations of site exchange defect thermodynamics and diffusion barriers for lithium and iron inside the lithium diffusion path suggest that the site exchange defect related capacity loss may be justified exclusively by thermodynamic considerations. Moreover, a low activation barrier for iron transport in the lithium diffusion channel in FePO4 brings into question the significance of the antisite iron ion as an obstacle to lithium diffusion.