Based on experimental results, in particular, obtained on single crystals, the nature of the decisive point defects and charge carriers in LiFePO4 was considered, and the dependencies of their concentrations on the control parameters Li activity, doping content, and temperature were worked out. In the native regime characterized by Li deficiency δ, Li vacancies being decisive for ion conduction were compensated by holes as decisive electronic carriers. Very close to the concentration of order (where δ was strictly zero) frozen-in native defects or non-intentional impurities dominate. It was typically found that Li vacancies were compensated by Fe atoms on Li sites (FeLi). Intentionally introduced donors such as AlFe or SiFe, had the same donor effect. Brouwer diagrams displaying the logarithm of the defect concentration versus log Li activity or versus log donor/acceptor content were derived. In the case of the temperature-dependence of conductivity, trapping of holes by Li vacancies was crucial and dominated the effective activation barrier at frozen Li content. In particular, for Al-doped samples, ionic association also proved important. Defect reaction and migration enthalpies were derived for electronic and ionic transport. Trapping of holes by Li vacancies and the association of Li vacancies with impurities were also found to be key factors in understanding the temperature dependence of the chemical diffusion coefficient of Li.

Defect Chemistry of LiFePO4. J.Maier, R.Amin: Journal of the Electrochemical Society, 2008, 155[4], A339-44