Preparation And Electrochemical Properties Of Co/Co₃O₄/Nitrogen Doped Carbon Composite

As anode electrode material of lithium ion batteries,Co₃O₄ has the advantages of high theoretical capacity(896 mAh g^-1),stable chemical properties,and good safet y performance.However,Co₃O₄ has also the disadvantages of poor conductivity,large irreversible capacity,and serious volume effect.The research indicated that the electrochemical performance of Co₃O₄ can be improved by means of nanotechnology,carbon composite and morphology control.Carbon composite is the most promising method.Carbon composite can not only promote the transmission of electrons,but also relieve the crushing and shedding phenomena by the volume effect caused,showing excellent uniformity,electrochemical performance,and kinetic performance.Co/NDC,Co₃O₄/NDC,Co/Co₃O₄/NDC and other series of composites material coated with nitrogen-doped carbon?NDC?were prepared using ionic liquids as carbon source.Using XRD,Raman,SEM,TEM and other technologies characterized its structure and morphology,the materials of electrochemical performance was test by constant current charging and discharge,CV,and AC impedance.The relationship between the structure and morphology of the material and electrochemical performance was also discussed.The main research contents are as follows:?1?The Co/NDC composite material was prepared by one-step method using1-butyl-3-methylimidazolium dicyanamide as carbon source.TEM tests showed that cobalt nanoparticles were uniformly embedded in NDC material and formed a core-shell structure.The results of the constant current charge and discharge test showed that the reversible capacity of Co/NDC composite material is 657 mAh g^-1after 60 cycles at 0.1 C,which is significantly higher than that the NDC material of393 mAh g^-1.The AC impedance test showed that the Co/NDC composite material has the smaller charge transfer impedance than the NDC material.The excellent electrochemical performance of Co/NDC composite material is due to the fact that cobalt nanoparticles enhance the transfer of electrons and the transport of lithium ions during the charging and discharge process.The Co/NDC composite material wa s oxidized in the air and obtained the Co₃O₄/NDC composite material.The SEM and TEM tests showed that the NDC material was coated on the surface of Co₃O₄particles.The constant current charge and discharge test results show ed that the Co₃O₄/NDC composite has a reversible capacity of 1088 mAh g^-1 after 100 cycles at0.1 C.After 100 cycles,the SEM and AC impedance of Co₃O₄/NDC composite material and the pure Co₃O₄ were tested.The results showed that the NDC material not only reduced the Co₃O₄ charge transfer impedance,but also reduced the structural damage by the volume effect caused during the charging and discharging process.?2?The precursor was prepared by urea homogeneous precipitation and the Co/NDC composite material was prepared by high temperature reduction precursor using 1-hexyl-3-methylimidazolium dicyanamide as the carbon source.TEM and SEM showed that cobalt nanoparticles are evenly dispersed in NDC materials and that a layer of nitrogen-doped carbon material with onion morphology is formed around cobalt nanoparticles.The constant current charge and discharge test results showed that the Co/NDC composite material has a reversible capacity of 1322 mAh g^-1 after850 cycles at 0.5 C,and a reversible capacity of 436 mAh g^-1 after 2400 cycles at 5 C.The AC impedance test showed that the Co/NDC composite change rule of the SEI film impedance under different cycles.The excellent electrochemical performance of Co/NDC composite material is attributed to the fact that the cobalt nanoparticles not only enhance conductivity,but also have catalytic activity.The Co/NDC composite material is oxidized in the air and obtained the Co/Co₃O₄/NDC composite material by controled the oxidation conditions.TEM tests showed that the nitrogen-doped carbon material of the onion morphology wrapped Co/Co₃O₄ nanoparticles to form two core-shell structures.The composite material of this structure has the following advantages:?1?the nitrogen-doped carbon of the onion morphology can relieve the volume effect,?2?the metal cobalt can further enhance the conductivity,?3?the formed two core-shells can further stabilize the structure,?4?The electrolyte has good contact with the active material,thereby improving the transmission efficiency of lithium ions.The constant current charge and discharge test results showed that the Co/Co₃O₄/NDC composite material exhibits excellent rate performance(1500 cycles at 5 C and 10 C,reversible capacity of 790 mAh g^-1 and 304 mAh g^-1,respectively).?3?The Co/NDC composite material prepared with 1-hexyl-3-methylimidazolium dicyanamide was dissolved in hydrochloric acid solution to remove the metal core and obtained a hollow structure of nitrogen-doped onion carbon?NDPOC?material.SEM and TEM tests showed that the nitrogen-doped carbon material is a hollow structure,and there is a layer of nitrogen-doped carbon material with onion morphology around pore.The nitrogen-doped carbon material of this structure has the following advantages:?1?The hollow structure can shorten the trans mission distance of lithium ions,?2?the morphology of the onion can reduce the volume effect and reduce the rupture of the pore.The constant current charge and discharge test showed that the NDPOC material maintained a reversible capacity of 805 mAh g^-1 50 cycles at 0.1 C.After 300 cycles at 5 C and 500 cycles at 10 C,the reversible capacity was 306 mAh g^-1 and 267 mAh g^-1,respectively.The AC impedance test not only shows that the NDPOC material has a small charge transfer impedance,but also further verifies the excellent cyclic stability of NDPOC material?after 1 cycle,after 5 cycles,after 10cycles,the charge transfer impedance is basically the same?.?4?The Co₃O₄/NDC composite material was prepared by solvothermal method using 1-hexyl-3-methylimidazolium dicyanamide as the carbon source.TEM tests showed that amorphous nitrogen-doped carbon material is evenly distributed around Co₃O₄ nanoparticles.These amorphous carbon forms a conduct ive network around Co₃O₄,which not only improves the electron transfer between particles but also relieves the volume effect caused by the charging and discharging process.Thus improving the electrochemical performance.The constant current charge and discharge was test at 0.1 C,the first discharge specific capacity was 1101 mAh g^-1,the first charge specific capacity was 879 mAh g^-1,and the first coulombic efficiency was74.86%.The reversible capacity was 1056 mAh g^-1 after 50 cycles.The results of AC impedance test showed that the carbon composite can effectively reduce the charge transfer impedance.The excellent electrochemical performance of Co₃O₄/NDC composite material is attributed to the formation of coating between the NDC material and the Co₃O₄ particle,which is conducive to improving the conductivity of Co ₃O₄and reduce the volume effect caused during charging and discharging process.