Reciprocal Salt Flux Growth of LiFePO₄ Single Crystals with Controlled Defect Concentrations

Abstract

Improved methods for the flux growth of single crystals of the important battery material LiFePO₄ have been developed, allowing the facile preparation of single crystals up to 1 cm across with well-developed facets at relatively low temperatures. The structural characterization of these samples by both powder X-ray diffraction and single crystal diffraction (X-ray and neutron) indicates that the samples are typically stoichiometric with a very low concentration of Fe defects on the Li site, though crystals with larger concentrations of defects can be specifically grown using Fe-rich fluxes. These defects occur through the formation of a Fe-rich (Li_{1–2<i>x</i>}Fe_<i>x</i>)­FePO₄ partial solid solution, in contrast to the antisite defects more commonly discussed in the literature which would preserve the ideal LiFePO₄ stoichiometry. The LiFePO₄ defects are shown to be sarcopside-like (2 Li⁺ → Fe²⁺ + vacancy) based on compositions refined from single crystal diffraction data, the observed dependence of unit cell parameters on defect concentration, and their observed phase behavior (defects only appear in growths from fluxes which are Fe-rich relative to stoichiometric LiFePO₄). The distribution of defects has been studied by aberration corrected scanning transmission electron microscopy and was found to be highly inhomogenous, suggesting that defect-containing crystals may consist of endotaxial intergrowths of olivine LiFePO₄ and sarcopside Fe₃(PO₄)₂ in a manner that minimizes the detrimental influence of Fe_Li defects on the rate of Li-ion transport within crystallites