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 LiFePO 4 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 1−2x Fe x)FePO 4 partial solid solution, in contrast to the antisite defects more commonly discussed in the literature which would preserve the ideal LiFePO 4 stoichiometry. The LiFePO 4 defects are shown to be sarcopside-like (2 Li + → Fe 2+ + 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 Ferich relative to stoichiometric LiFePO 4). 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 LiFePO 4 and sarcopside Fe 3 (PO 4) 2 in a manner that minimizes the detrimental influence of Fe Li defects on the rate of Li-ion transport within crystallites.

Effects of laser energy and wavelength on the analysis of LiFePO4 using laser assisted atom probe tomography

“…Needle-shaped APT specimens of LFP were fabricated by focused ion beam liftout and were mounted onto commercial silicon flat-top micro-post arrays [20]. The LFP liftouts were extracted from a single crystal grown by a flux growth method reported elsewhere [21]. All specimens were analyzed in the [100] crystal orientation, although no crystallographic features were apparent in the subsequent APT reconstructions.…”

Advanced analytical electron microscopy for lithium-ion batteries

“…9,10 With the aid of this technique, Chung et al 33 discovered the localized aggregation behavior of Fe Li . 34 On the basis of their observations, a vacancy-driven mechanism was proposed for the formation of the localized defect segregation (Figure 3a). 35 In addition to unraveling the defect distribution, electron microscopy has also been used for the detailed investigation of the intercalation mechanisms in LFP.…”

Atomic‐Scale Structure Evolution in a Quasi‐Equilibrated Electrochemical Process of Electrode Materials for Rechargeable Batteries

“…Shao‐Horn et al indirectly observed lithium and oxygen in LiCoO 2 cathode through exit wave reconstruction from focal series of high resolution TEM images, but this method is operationally complicated and computationally expensive, and cannot be routinely applied to characterize a wide range of electrode materials. High‐angle annular dark‐field (HAADF) imaging based on aberration‐corrected STEM has enabled direct imaging of the atomic structure of electrode materials, for example, the antisite defect (i.e., Fe ions on Li sites) in LiFePO 4 that has been demonstrated to impede lithium diffusion . However, the contrast in HAADF image is proportional to Z 1.7 ( Z refers to atomic number) .…”