Defect Chemistry of LiFePO[sub 4]

Abstract

Based on experimental results, in particular, obtained on single crystals, the nature of the decisive point defects and charge carriers in LiFePO4LiFePO4 is discussed, and the dependencies of their concentrations on the control parameters Li activity, doping content, and temperature are worked out. In the native regime characterized by Li deficiency δ, lithium vacancies being decisive for ion conduction are compensated by holes as decisive electronic carriers. Very close to the concentration of order (where δ is strictly zero) frozen-in native defects or non-intentional impurities dominate. We typically found lithium vacancies compensated by iron atoms on Li sites (FeLi)(FeLi) . Intentionally introduced donors such as AlFeAlFe or SiFeSiFe , have the same donor effect. The Brouwer diagrams displaying the logarithm of the defect concentration vs log lithium activity or vs log donor/acceptor content are derived. As to the temperature dependence of conductivities, trapping of holes by lithium vacancies is crucial and dominates the effective activation barrier at frozen lithium content. In particular, for the Al-doped samples, ionic association also proves important. Defect reaction and migration enthalpies are derived for electronic and ionic transport. Trapping of holes by lithium vacancies and association of lithium vacancies with impurities also turn out to be key for understanding the temperature dependence of the chemical diffusion coefficient of lithium.