Synchrotron crystal and local structures, microstructure, and electrical characterization of Cu-doped LiFePO4/C via dissolution method with ironstone as Fe source

Powders of lithium iron phosphate (LFP) with Cu doping and carbon coating were prepared by a dissolution method using Fe sourced from natural ironstone. Two dopant amounts were used, 2 and 3 at.% while the carbon coating used carbonization with 9 wt.% citric acid. Synchrotron X-ray diffraction (XRD), Fe K-edge X-ray absorption spectroscopy (XAS), and scanning electron microscopy (SEM) were used to reveal the crystal and local structures and grain morphology of the formed phase. Electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and charge-discharge (CD) measurements were carried out to determine the electrical properties of the samples. According to the XRD and XAS data, LFP was the main phase in all samples with Fe coordination number 6 and an oxidation number of 2+. Small amounts of hematite were detected in the doped and carbonized samples. SEM images of the 2 and 3 at.% Cu-doped samples showed a spherical morphology with clear grain boundaries, whereas carbonization resulted in smaller grain sizes. XAS analysis showed that Cu doping increased the distance between Fe as the absorbing atom and its nearest-neighbor atoms, while carbon coating reduced it. The EIS and CD tests showed that Cu doping and carbonization increased the conductivity up to 10 times and the specific capacity up to 50 times for the undoped and uncarbonized samples. The CV curves showed that Cu doping and carbonization provided better intercalation, deintercalation, and reversibility properties of LFP as shown by the smallest potential difference at a value as low as 0.2 V. The changes in the electrical properties are explained in terms of the LFP structures.

Automated summary

In this study, powders of lithium iron phosphate (LFP) with Cu doping and carbon coating were prepared using a dissolution method. Various analysis techniques were employed to investigate the crystal and local structures, grain morphology, and electrical properties of the samples. The results showed that Cu doping and carbon coating significantly enhanced the conductivity and specific capacity of the samples, improving the intercalation, deintercalation, and reversibility properties of LFP.