LiFePO4: from single crystals to nanostructures

Batteries based on the light, small and very electropositive element lithium are undoubtedly significant devices for our technology-based society. Among the materials of interest, FePO4 is one of the most important materials in this context. On discharging it transforms to LiFePO4 which can be reversibly delithiated on charging. Unlike electronic devices not only electronic carriers are relevant, but also ionic carriers as to enable mass transport and electrochemical reaction. In spite of the importance, examples of systematically studying the charge carrier chemistry in battery-relevant materials are extremely rare. In our work that essentially started with the growth of large LiFePO4 single crystals we were able to perform such studies on both LiFePO4, and more recently FePO4 [1]. As we found, the former intrinsically conducts electrons by a holeand lithium ions by a vacancy-mechanism whereas excess electrons and lithium interstitials are predominant in FePO4. These thorough defect-chemical studies based on pure, Al, Siand Na-doped materials enabled a quantitative interpretation of the ionic and electronic conductivities as well as of chemical Li-diffusion coefficients as a function of lithium activity (cell voltage), temperature and acceptor or donor doping content. Figure 1 gives the dopantdependence of both LiFePO4 and FePO4 that is in full accordance with our experimental results.