Research On Synthesis And Electrochemical Performances Of Several Composite Anode Materials For Lithium-ion Battery

In this paper,three kinds of composite material:Ge/Graphene composite aerogel,ZnO/Zn₂GeO₄/Graphene aerogel and Ni O/Ni MoO₄ were considered as the anode of the lithium ion batteries.And their synthesis,structure,electrochemical performance and mechanism were studied separately.The results were as followed:?1?The graphene oxide was dispersed in the deionized water,and the polyvinylpyrrolidone which has the molecular weight as 1,300,000 was added as the drivers of the reaction.The active groups on the surface of graphene oxide were linked with the polyvinylpyrrolidone and formed hydrogen bonds,which was considered as the principle of self-assembly.Then GeCl₄ was taken as the germanium source.After hydrolytic action,the GeO₂/PVP/Graphene composite aerogel was obtained,and after being annealed at H₂/Ar atmosphere in different temperature,the Ge/Graphene composite aerogel was prepared.The samples were characterized by X-ray diffraction?XRD?techniques,and results showed that the GeO₂ was transformed into Ge,and the carbon was amorphous so that no peake in the X-ray diffraction pattern were detected.Meanwhile,the samples which were respectively annealed at 500?and 650?were characterized by scanning electron microscopy,and the results showed that after annealing at higher temperature,the germanium particles were reunion seriously due to the decomposition of polyvinylpyrrolidone.When the sample annealed at lower temperature,the graphene sheets were linked with each other and the germanium particles distributed in the graphene composite aerogel network uniformly.The electrochemical properties of the two samples were investigated as followed,it showed that GG500 samples maintained capacity as 900 m Ah g^-1 after 140 cycles at a constant current density of 1 A g^-1 in the voltage range of 0.01-1.5 V.And the materials can maintain electrical proformance at the high rate of 20 A g^-1.The sample annealed at 500?showed lower charge transfer resistance and higher Li⁺diffusion coefficient.Such process might own to these reasons:First of all,the network provided by graphene had shown Li⁺transform channels,which could be taken advantage of lithium ion diffusion,and enhanced the diffusion coefficient.Furthermore the polyvinylpyrrolidone which was carbonized but not resolved could ease the volume expansion of Ge nanoparictles during charge and discharge to some extent,and it also can hold back agglomeration during the annealing.Besides,the introduction of N element also could enhance the capacity of cycling and rates,and atomic hybridization brought by N element could ensure the high Li⁺diffusion coefficient.?2?Zn?AC?₂ was used as the zinc source and the Na₂GeO₃ prepared by high temperature solid state reaction method was used as the germanium source.And ion exchange was taken to load the ZnO and Zn₂GeO₄ on the graphene oxide.Furthermore,ascorbic acid was considered as the reducing agent and drivers of the self-assembly reaction during the low-temperature hydrothermal method.The phase of samples were examined by XRD,and it had shown that the Zn₂GeO₄ was amorphous and the ZnO was highly crystalline.That may caused by the preferential crystallization of Zinc which was led by the active group from the graphene oxide.The microstructures of the as-prepared composite sample were examined by the field-emission scanning electron microscopy techniques.graphene sheets were linked with each other and formed stable three-dimension structure,and the core-shell ZnO-Zn₂GeO₄nanoparticles with the size of 20 nm were distributed on the graphene sheet uniformly,due to the docking interactions led by the hydroxyl and carboxyl on the graphene oxide.The electrode was tested at 500 m A g^-1 for 250 cycles in the voltage range of 0.01-2.8 V.After 250cycles,the composite electrode still can exhibited a high reversible capacity of 905 mA h g^-1.Both cycling and rate performance were much better than that of comparison samples.The superior performance of the ZnO/Zn₂GeO₄/Graphene aerogel suggested high-efficiency synergistic effect resulted from four parts in its structure.First,the three-dimension porous network ensure stability of electrode to decrease pulverization of it during charge and discharge precess.Second,the graphene sheets can slow down the volume expansion.The core-shell structure consisted of the amorphous and crystalline composite can maintain a lower volume expansion during lithiation and delithiation process.Third,the relatively low bandgap provided by the amorphous structure can be benefit for Li⁺diffusion,and enhanced the diffusion coefficient.At last,hierarchical structures of core-shell ZnO-Zn₂GeO₄ and synergies effect contribute to charge/discharge process at high currents.?3?Ni?AC?₂ and Na₂MoO₄ power at the molar rate of 2:1 were dissolved in 25mL DI water,keeping stirring for 30 min at room temperature.The mixture was transferred into a45mL Teflonlined autoclave and heated at 180?for 12 h.The collected green precipitations were cleaned by DI water and ethanol for several times and dried at 80?for 6 h.The NiO/Ni MoO₄ precursor was prepared.During the hydrothermal process,Ni?AC?₂ was acted as both nickel source and surfactant.After annealing at 550?,the NiO/Ni MoO₄ was prepared.As a contrast,Ni Mo O₄ nanorod and NiO of micro-nano structure were synthesized by the hydrothermal reaction,and both of them were annealed at 550?.The phase and the structure were characterized and analyzed by XRD and field-emission scanning electron microscopy techniques,respectively.The NiO and Ni MoO₄ characteristic peaks were showed obviously in the XRD patterns.And the NiO/Ni MoO₄ hollow spheres were made up of nanosheets with the thickness of 10-20 nm.The diameter of the hollow spheres was about 1?m.The NiMoO₄ nanorod with the diameter of about 20 nm had length of 1-2?m.And the NiO micro-sphere with the diameter of 3?m was assembly of nanowires with the diameter of about 30 nm.After investigation of electrochemical testing,all the indexes had showed that the composite material NiO/Ni MoO₄ was better than other two materials.The reasons should divided into four parts:The first one was that the two-dimension basic element was more favorable to the transport of the Li⁺ion.Furthermore,the hollow sphere structure had shown more stability than other two.Besides,the synergistic effect of the transition metal oxide composite material had played an important part in enhancing the capacity during the charge and discharge.Fianlly,the two-dimension micro-nano materials could be beneficial for lithium ions diffusion and decrease of polarization during lithiation and delithiation process.