Fabrication of Novel Cobalt Based Anodes and Study on Lithium Ion Storage Performance

Author:Zuo Chun Shuang

Supervisor:chen gang


Degree Year:2019





Lithium ion batteries(LIBs)have been a favorable power source device in electrical vehicles and portable electronics due to high energy density,high working voltage and long cycle life.However,the commercial anode material,graphite,has a low theoretical capacity,which makes it difficult to meet the demands for high energy density in energy storage systems.With the ever-increasing demands of rechargeable power applications,it is urgent to explore alternative anode materials that can endow LIBs with higher energy density and better rate performance.Tansition metal oxides are attracting extensive attention because of their high theoretical capacities.Unfortunately,the practical applications of transition metal oxides are largely hampered due to the low electrical conductivity,low first coulombic efficiency and large volumetric variation during the charg-discharge processes.A step change approach consists of designing the structure of electrodes appropriately,enabling the materials to have desired functions.Meanwhile,the forming mechanisms are analysed to provide the new ideas for understanding and constructing other materials with the noval nanostructure.In this paper,transition metal oxide Co3O4 has been selected as the research object.The anode materials with different nanostructures are fabricated to improve the performance by a series of modification methods.A hybrid consisting of embedded cobalt oxide nanocrystals and encapsulated cobalt nanoparticles on graphitic carbon nanosheet(GCS),denoted as CoOx@GCS(?)Co,was fabricated via a heterogeneous molten salt method.The strong polarizing force of molten salt can induce the forming of bridged bonds.Besides,the forming mechanism of the hybrid is been invesgated.The versatility of as-proposed molten salt method is validated by the fabrication of metal phosphides.The relationship between the component,bridged bonds and the lithium ions storage performance is discussed.The experiments and DFT calculation results show that the introduction of the bridged bonds can effectively facilitate the transport of electron and further improve the rate performance of materials.The CoOx@GCS(?)Co hybrid exhibits superior activity as LIBs anode materials.When the cycling performance is evaluated,the CoOx@GCS(?)Co hybrid delivers a better cycling stability and higher capacity,which is five times that of pure Co3O4.Even at a high current density of 10Ag-1,a large reversible capacity of 713 mAh g-1 is still obtained.A hybrid synthesis procedure by combining electrospinning technique and hydrothermal method is applied.C-doped Co3O4 hollow nanofibers(HNFs)were fabricated by using the electrospun poly(acrylonitrile)nanofibers(PAN NFs)as template and carbon source.C/N co-doped Co3O4 HNFs were also obtained by using NH3 as the calcination atmosphere keeping other conditions unchanged.The element doping and construction of hollow structure are verified by the XPS,FT-IR,TG and TEM characterization.The forming mechanisms of hollow structure and doping are studied by contrast experiments.The DFT calculation results indicate that the introduction of carbon can induce high dispersion of the energy band structure of Co3O4,improving its intrinsic conductivity.When C and N are co-doped into Co3O4,the dispersion in energy band structure of Co3O4 becomes much higher than that of C-doped Co3O4,effectively improving the rate performance of materials.The hollow nature is also favored in LIBs,since it not only reduces the lithium ions diffusion pathway,but also provides the buffering space for the occurred volume change during charge-discharge process,enabling excellent cycling stability.C-doped Co3O4 hollow nanotubes(HNTs)composed of sub-10 nm nanocrystals,were synthesized through topotactic conversion of in situ C2O42-doped CoC2O4·2H2O to C-doped Co3O4 HNTs.The element doping and construction of hollow structure and small sizes are verified by the XPS,FT-IR,XRD and TEM characterization.The forming mechanisms of hollow structure,doping and small size are studied by contrast experiments.The small size of nanoparticles can reduce the lithium ions diffusion pathway and promote the lithium ions transport.The DFT calculation results indicate that the doping will induce two built-in electric fields with opposite directions to accelerate the transport rate of lithium ions during the charge-discharge process.The above structural features can effectively improve the lithium ions storage performance.C-doped Co3O4 HNTs were evaluated by assembling a C-doped Co3O4HNTs//LiCoO2 coin-type full-cell.At a current density of 1Ag-1anode.the specific capacity could be still kept at 765 mAhg-1anode after 1200 cycles.Co3O4 nanosheets exposing the{011}facets with edge dislocations modifying were successfully synthesized by a solid chemical transformation from the Co(OH)2precursors with stacking faults.The edge dislocations and the exposed facet are verified by the TEM characterization.The forming mechanism of edge dislocations is analyzed by contrast experiments.The relationship between structure and performance is also studied.This result indicates the advantage of the present exposed facets and edge dislocations.The introduction of edge dislocations can provide flexible space during charge-discharge process.This flexible space is favorable for buffering volume expansion and channeling lithium ions diffusion,which can improve the rate performance and cycling stability of materials.Co3O4 nanosheets exposing the{011}facets with edge dislocations exhibit superior lithium ions storage performance.At a current density of 1Ag-1,the reversible discharge capacity as high as 1142 mAhg-1 is retained after 200 cycles,which is superior to Co3O4 nanosheets without edge dislocations modifying and Co3O4 nanosheets exposing the{112}facets.Cobalt oxalate ions([CoOx2]2-)intercalated Co(OH)2 nanosheets(I-Co(OH)2NSs)were synthesized via a one-step solvothermal method using cobalt oxalate ion([CoOx2]2-)with conjugated anion dicarboxylate groups as intercalation ions.The I-Co(OH)2 NSs demonstrate enlarged interlayer spacing,abundant oxygen vacancies and nanopores,which are resolved by the XRD,XPS,FT-IR and TEM characterization.These unique structural characteristics can promote the lithium ions transport and provide the buffering space for volume change.The stacked ultrathin Co3O4 nanosheets with surface functionalization(SUCNs-SF)were converted from layered hydroxides with inheritance of included anion groups.Such stacked structure establishes 2D nanofluidic channels,which is verified by the SAXS,AFM,XPS and TEM characterization.The forming mechanism of 2D nanofluidic channels is analyzed by contrast experiments.The generated 2D nanofluidic channels can bring additional lithium ions storage sites,accelerated unipolar lithium ion transport,and suffcient buffer space for volume expansion,which all together result in high lithium ions storage performance.