Synthesis And Properties Of Olivine-LiCoPO₄/NASICON-li₃M₂?PO₄?₃?M=Fe,V? Composite Cathode Materials

As a kind of cathode material of lithium-ion battery,phosphate material has attracted wide attention due to its high safety,low cost,stable charge-discharge platform and environment-friendly properties.However,the inherent defects of olivine phosphate materials,such as low electron conductivity and lithium ion diffusion rate,seriously impede the marketability of the materials.NASICON polyanionic structural phosphate materials have fast ionic conductor properties.By composing the olivine structure and NASICON structure,addingtheconductiveagent,LiCoPO₄-Li₃Fe₂?PO₄?₃/C?LCP-LF₂P/C?and LiCoPO₄-Li₃V₂?PO₄?₃/C?LCP-LVP/C?composite materials were synthesized to improve the overall electrochemical properties of the materials.The structures,morphologies and electrochemical performances of the composite materials were characterized by X-ray powder diffraction?XRD?,scanning electron microscopy?SEM?,transmission electron microscopy?TEM?and high transmission electron microscopy?HRTEM?,charge and discharge tests,electrochemical impedance spectroscopy?EIS?.The LCP-LF₂P/C composites were synthesized by high temperature solid state reaction method.The effects of the different calcination temperatures on the composites performances were studied.XRD and HRTEM showed that the composite material was the mixed phases of LCP and LF₂P.From the SEM images and particle size distribution curves,particle size with the irregular morphologies became larger with the increasing calcination temperature,carbon was sccessfully mixed with the surface of the material LCP-LF₂P by ball milling,and the particle size became smaller and the distribution was more uniform.XPS results showed that the elemental valence of LCP-LF₂P and LCP-LF₂P/C materials did not change after ball milling.The charge and discharge sequence of composite was studied,the results showed that the electrochemical performance of discharge first on composites calcined at 800?was better than that charge first.And the electrochemical performance of composites calcined at800?was brilliant than LCP/C,LF₂P/C and other composites calcined at other temperatures,in terms of capacity,coulombic efficiency,and cycle performance.The various ratios of xLCP-yLF₂P/C?x:y=3:1,2:1,1:1,1:2,1:3?on the structures,morphologies and electrochemical properties were then studied based on the optimal calcination temperature of 800°C.The results of XRD showed that the composites of xLCP-yLF₂P/C?x:y=3:1,2:1,1:1,1:2,1:3?were successfully synthesized by high temperature solid state phase method.At the same time,it was detected that the diffraction peaks of the LCP phase in composites shifted slightly to the right with the increase of LF₂P ratio,which was speculated that co-doping occured due to the similar size of Co²⁺and Fe³⁺atoms.Electrochemical tests showed that the capacity retention ratios of the composites?x:y=1:1,1:2,1:3?were higher than the composites?x:y=3:1,2:1?,which showed that the LF₂P in the composites played an important role in improving the cycle performance.With the increasing LF₂P content,the discharge capacity and coulombic efficiency of the composites were reduced.The electrochemical performance of the composite with x:y=1:1 was better than other composite materials.LCP-LVP/C composite cathode material was synthesized by rheological phase reaction method.The influences of different carbon content,different sintering temperature and sintering time on the LCP-LVP/C composites were systematically studied.First of all,the effects of different carbon content on the properties of composites were studied to determine the appropriate amount of carbon source.XRD and SEM showed that no heterogeneous Co₂P phase existed in the composites with carbon source content of 20%after sintering at 750°C for 15 h,and the morphologies were more uniform irregular particles.Electrochemical tests showed that the performance of LCP-LVP/C composite cathode material synthesized by rheological phase method was greatly improved.The performance tests showed that the composite systems of LCP-LF₂P and LCP-LVP were successfully synthesized and the desired effects were obtained.It provided a new clue of high energy density cathode material for lithium ion batteries.