Synthesis Of Cobalt/carbon-based Nanocomposite Materials And Its Application In Lithium-ion Batteries

Lithium-ion batteries(LIB)have the characteristics of high energy density,long cycle life and environmental friendliness,which are one of the most important energy storage devices nowadays.At present,the commonly used anode material for commercial LIB is graphite,but the theoretical capacity of graphite anode is low,which is difficult to meet people’s demand for electric vehicles and portable electronic devices with higher energy and power density.Therefore,it is very important to explore LIB anode materials with higher specific capacity,higher rate,longer cycle life and better security.NiCoP materials have the advantages of high theoretical capacity,low redox potential,and high thermal and chemical stability.Metal Co can be used as a modified conductive agent for nanomaterials to improve the conductivity of composite materials,which can also play a catalytic role in the formation and decomposition of the solid electrolyte boundary(SEI).However,these two materials as LIB anode materials have some defects such as large volume variation during charge and discharge,slow reaction kinetic and inertness during discharge.In order to overcome these shortcomings,in this paper,three novel cobalt-based/carbon-based nanocomposites were prepared to use for LIB through the introduction of different forms of carbon matrix and NiCoP/Co composite and the reasonable design of the morphology and structure of the material composite.These nanocomposites can significantly improve the electrochemical lithium storage performance of LIB.The main research content and research results are summarized as follows:(1)Nitrogen-doped carbon-coated silicon dioxide/cobalt(SiO2/Co@NC)nanocomposite was prepared by an effective stepwise growth strategy.First,using the prepared mesoporous SiO2 sphere as a template,cobalt is grown on the surface of the mesoporous SiO2 through a solvothermal process.And then a nitrogen-doped carbon layer is coated on the surface of the SiO2/Co composite material to obtain the SiO2/Co@NC nanocomposite material.It can be seen that the prepared SiO2/Co@NC nanocomposite has a spherical structure with a size of around 500 nm,and Co nanoparticles with particle size of 15-20 nm are uniformly embedded in the carbon coating lay er.SiO2 has the advantages of high theoretical capacity and low price,which can make up for the defect that metal Co cannot store lithium.Co nanoparticles are conductive,so they can be used as modified conductive agents to effectively improve the overall conductivity of electrode materials and promote the reverse reaction and decomposition of the SEI film during the delithiation process.Nitrogen-doped carbon materials can alleviate the volume expansion effect of SiO2 and further improve the overall conductivity of the composite material.The synergistic effect between the three materials makes the SiO2/Co@NC nanocomposite exhibit excellent lithium storage performance.a current density of 1.0 A g-1,the reversible capacity of SiO2/Co@NC is 415.0 mA h g-1 after 1000 charge and discharge cycles,which is much higher than those of mesoporous SiO2 and SiO2/Co materials.(2)Carbon nanobox encapsulated NiCoP nanoparticle(NiCoP@C NBs)nanocomposite was prepared through a simple and easy temp late strategy.Hollow carbon nanoboxes were first prepared using α-Fe2O3 nanocubes as a template,then NiCo-OH nanosheets were hydrothermally encapsulate in hollow carbon nanoboxes,and finally phosphating treatment was done to prepare NiCoP@C NBs nanocomposites.The morphology of the NiCoP@C NBs nanocomposite is nanocube with a particle size of about 500 nm through Transmission electron microscope(TEM).The composite of NiCoP and the hollow carbon nanobox can not only effectively alleviate the volume expansion effect of NiCoP,but also prevent the agglomeration of NiCoP nanoparticles,thereby increasing its specific surface area,which enables the electrode material to fully con tact the electrolyte.After 100 charge-discharge cycles at a current density of 100 mA g-1,the discharge capacity of NiCoP@C NBs can maintain around 410.9 mA h g-1 with a rising trend,which is better than those of C Nanoboxes(279.5 mA h g-1)and NiCoP(227.9 mA h g-1)electrode materials.The good lithium storage performance of NiCoP@C NBs nanocomposites is mainly due to the synergy between the highly active NiCoP and the conductive carbon nanoboxes.(3)Through pyrolysis and phosphating strategy,NiCoP nan oparticles supported nitrogen-doped carbon nanotubes(NiCoP@NCNTs)nanocomposite was prepared.Using the complex of methyl orange and ferric chloride as the self-template for the reaction,polypyrrole is first grown on its surface to promote the formation of hollow nanotube structure,and then calcined at high temperature under the protection of nitrogen to obtain hollow nitrogen-doped carbon nanotubes(NCNTs).After that,NiCoP nanoparticles are loaded on the surface of NCNTs through hydrothermal treatment and phosphating treatment to prepare NiCoP@NCNTs nanocomposite.The structure of NiCoP@NCNTs nanocomposite is tubular,and NiCoP nanoparticles with a particle size of about 5-10 nm are uniformly wrapped in a carbon matrix,enabling the outer diameter of the NCNTs embedded in the NiCoP nanoparticles increase to about 200 nm.After 800 charge-discharge cycles at a current density of 1 A g-1,the reversible discharge capacity of NiCoP@C NBs is 496.3 mA h g-1.Even at a high current density of 2.0 A g-1,it can still provide a significant capacity of 295.0 mA h g-1,showing excellent cycle stability and rate performance.The excellent lithium storage performance is because that a stable electrode structure can form between NiCoP and NCNTs,effectively reducing the volume expansion effect caused by the crushing of NiCoP.Moreover,N-doped carbon can also increase the electron transfer rate and provide more active site to store Li+.