LiCoPO4 synthesis via hydrothermal and solvothermal routes for application in Li-ion batteries

In this communication we present our recent advancements in the synthesis of LCP materials by hydrothermal and solvothermal routes. LCP has been synthesized in water and mixtures of water and glycols in order to drive the crystal growth. A systematic study of the synthesis conditions has been carried out by varying the annealing temperature, reaction time, pH, glycol/water ratio and glycol chain length. X-ray diffraction, scanning electron microscopy, N2 adsorption, Fourier transform infrared spectroscopy and inductively coupled plasma atomic adsorption have been extensively used to characterize the reaction products and intermediates. A general picture of the reaction mechanism has been sketched. In all hydrothermal and solvothermal synthesis the olivine LCP structure is formed as final product of a complex reaction sequence that involves various intermediate compounds. The morphology of the final materials is strongly affected by the adopted hydro/solvothermal conditions. The Rietveld refinements of the XRD patterns showed that all the synthesized materials suffer of 0.5-1% crystal cell volume expansion that can be put into correlation with the high cationic disorder (Li/Co site substitution) also refined. Apparently in acidic synthesis conditions this effect can be mitigated. Furthermore although large Li+ excess were used no contamination due to Li3PO4 has been observed. The synthesized LCP materials have been tested in lithium cells using a PC/LiPF6 electrolyte. The material prepared in hydrothermal bath showed very poor performances in galvanostatic tests whereas the solvothermal materials were able to cycle lithium. The capability to reversibly de-insert/insert lithium can be enhanced by reducing the crystallite size, increasing the surface area and minimizing of the Li/Co cationic disorder.