LiCoPO4 and Li2CoSiO4 Nanoparticles Synthesis by Supercritical Fluid Process for Lithium Ion Battery Application

In recent years, transition metal polyanionic compounds such as phosphates and silicates are considered as promising positive electrode materials for lithium-ion batteries. These members of the lithium metal polyanion family have been a major focus of research for reasons of high safety, low cost and low environmental impact. Among, metal phosphate cathode material, LiCoPO₄ electrode materials have received considerable attention for the application in lithium secondary batteries due to its high stability, non toxicity, good charge/discharge capabilities and cheap cost. Recently, polyanion-containing frameworks are of great research interest in the area of lithium ion batteries, because, these materials can reversibly insert/extract two lithium ions per formula unit would help to increase the capacity and energy density. Regarding this, silicates with the general formula Li₂MSiO₄ (M=Mn, Fe and Co) have been considered as potential lithium insertion/ extraction cathodes. Among silicates, Li₂CoSiO₄ is an attractive cathode due to its high operating voltage. In this work, we have used supercritical fluid process for  the synthesis of LiCoPO₄ and Li₂CoSiO₄ nanoparticles and are investigated for lithium ion battery applications. LiCoPO₄ and Li₂CoSiO₄ nanoparticles were synthesized from LiOH.H₂O/Lithum acetyl acetonate, CoCl₃.6H₂O/C₄H₆CoO₄.4H₂O and H₃PO₄/TEOS in molar ratio of 1:1:1 for LiCoPO₄ and 4:1:1 for Li₂CoSiO₄ nanoparticles via supercritical fluid process. Cheap solvents such as water, ethanol and water-ethanol mixed solvents were used to prepare precursor solutions and reducing agents were also used in the synthesis. Supercritical synthesis was carried out at 400-450^oC for about 5-10 min of reaction time. Then, the products were washed thoroughly in water and ethanol followed by vacuum dry for overnight. The as-synthesized LiCoPO₄ and Li₂CoSiO₄ nanoparticles  showed 20-150 nm in diameter,  which was analysed by transmission electron microscopy (TEM). Further, LiCoPO₄ and Li₂CoSiO₄ nanoparticles were systematically characterized using powder X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), energy dispersive spectroscopy (EDS) and charge-discharge measurements. The discharge capacities of 80-110 mAhg^-1 for LiCoPO₄ nanoparticles and 40-110 mAhg^-1 for Li₂CoSiO₄ nanoparticles were observed. The electrochemical property was measured by galvanostatic discharge method using 1M LiPF₆ mixed in EC:DEC as electrolyte. The electrode was prepared using 10 wt% PTFE as binder. The weight ratio of active material:AB:PTFE are 80:10:10.