Aluminium-doped LiFePO4 single crystals Part II. Ionic conductivity, diffusivity and defect model

Abstract

We report on lithium ion conductivity and diffusivity along major crystallographic directions of Al-doped LiFePO₄ single crystals. Impedance spectroscopy as well as galvanostatic polarization measurements have been carried out on the electronically blocking symmetric cell LiAl/LiI/LiFe(Al)PO₄/LiI/LiAl. Neither ionic conductivity nor lithium diffusivity show anisotropy in the bc planes within the experimental error, but much lower values in the a-direction. Similar features were observed earlier by us for the pure single-crystal and the Si-doped single crystal. On Al-doping the ionic conductivity has increased while the electronic conductivity has decreased compared to undoped LiFePO₄. Not only this donor doping effect but also the temperature dependence of ionic conductivity and of lithium-diffusivity are successfully interpreted in terms of lithium vacancies, holes and associates in the framework of a detailed defect chemical analysis. Ion–electron as well as ion–ion associates play a significant role in this system.