Synthesis And Electrochemical Potassium /Lithium Storage Of Porous Nano-carbon And MnCo₂O₄ Composites

In recent years,with the rapid development of electronic products and new energy electric vehicles,dominated by lithium-ion battery of the alkaline metal ion battery is the main electrochemical power sources,the corresponding performance requirements have also been continuously improved,including cycling life,energy density,power density and thermal safety,etc.However,the widely used as the anode material of graphite cannot meet the needs of the technological development,and the industry and academia urgently need to develop high-performance anode materials.In particular,high-capacity of metal oxide anode materials based on the conversion reaction storage mechanism have received widespread attention.However,it's urgent to improve the volume effect and the dilemma of poor conductivity that are common during the cycle process.At the same time,carbon materials have the advantages of excellent electrical conductivity and cycle stability,but their lower theoretical capacity hinders the improvement of battery performance.Carbon material/metal oxide composite material has been considered as a promising anode material for lithium-ion battery,which inherits the excellent conductivity of carbon material and the high theoretical capacity of metal oxide.Therefore,the design and preparation of high-performance carbon materials/metal oxide composite materials is an important research direction.Although lithium-ion batterie have many advantages,the shortage of lithium resources is a significant shortcoming for its further development.Potassium-ion batteries place great expectations on high expectations as one of the currently vigorously developing alternative technologies.Meanwhile,researchers had discovered that carbon materials represented by graphite can achieve a theoretical capacity of 279 m Ah/g when used as the anode material of potassium-ion batteries.However,the energy density and cycle life of carbon materials reported so far are not ideal.Therefore,the development of carbon anode materials is of great significance for improving the performance of potassium ion batteries and then to their energy power and energy storage applications.Herein,this thesis takes nano-carbon anode materials,transition metal oxide anode materials and metal oxide-carbon composite materials as the research objects,focusing on the microstructure regulation of metal oxide materials,the pore construction of carbon nanomaterials,and the enhancement of conductivity of carbon materials to improve the performance of the lithium/potassium ion battery anodes.The main research contents are listed as follows:(1)Nanocomposites of MWNTs@Mn Co₂O₄@Gs are synthesized via a wet-chemical approach,in which double-layer of Mn Co₂O₄particles evenly attached on MWNTs and further wrapped in graphene.As an anode material for lithium batteries(LIBs),the designed MWNTs@Mn Co₂O₄@Gs anodes deliver LIB performance with superior rate capability and a large reversible capacity of 1094m Ah/g after 300 cycles tested at 200 m A/g.Moreover,the MWNTs@Mn Co₂O₄@Gs electrodes deliver reversible capacities of 752 and 520 m Ah/g after 80 cycles at high current densities of 1.0 and 3.0 A/g,respectively,being much larger than those values of 155 and 125 m Ah/g without Gs wrapping.The results revealed that the reserved space structure constructed by the graphene wrapping and MWNTs effectively alleviates the volume expansion caused by the Li⁺insertion/extraction process,and reduces the pulverization and agglomeration of the material.At the same time,high conductive graphene and MWCNTs enhance the conductivity of the material,promote the ion/electron transport,thus improving battery performance.(2)Using zinc acetate as metal ion source,2-methylimidazole as ligand,polyacrylonitrile as carbon source,and polymethyl methacrylate as pore former,hierarchical pores beaded structure of carbon nanofiber anode material(ZIF-8/P-CNFs)were designed and prepared based on the template pore-forming method and electrospinning technology.Herein,ZIF-8/P-CNFs possess with 701.2m/g large specific surface area and abundant pore structure.Tested as lithium-ion battery anodes,the ZIF-8/P-CNFs showed excellent performance:after 200 cycles at the current density of 0.2 A/g,a high reversible capacity of 692.9 m Ah/g was released;after 660 cycles at the current density of 0.6 A/g,the discharge specific capacity remains at 632.7 m Ah/g,being much higher than that of pure ZIF-8 and P-CNFs.The improvement of electrochemical performance of ZIF-8/P-CNFs is mainly attributed to its hierarchical porous structure,which can not only provide additional lithium insertion sites,but also increase the contact area between electrode and electrolyte,greatly improving ion/electron transport kinetics and performance.(3)Porous nitrogen-doped carbon nanofibers(P-NCNFs)with abundant defects on the surface were designed and synthesized by a simple electrospinning method.Used as anodes for potassium ion batterie(KIB),P-NCNFs delivered good rate performance and excellent KIB performance with the reversible specific capacity of216 and 165 m Ah/g after 3300 and 2000 cycles at 0.05 and 1.0 A/g,respectively,being much better than pure CNF without N-doping and pore-formation treatment.The results revealed that nitrogen doping could induced more defects to provide additional active sites for potassium storage and improve the conductivity of the material.Meanwhile,the internal cross-linked porous structure could effectively shorten the potassium-ion/electron transport path,boost the infiltration of electrolyte in electrode,alleviating the concentration polarization and finally enhancing the potassium ion batterie performance.