Study On The Fabrication, Structure And Electrochemical Properties Of ZnO/C Composite Electrode Materials

Zinc-nickel battery,which uses ZnO/Zn as the negative electrode material and nickel hydrate as the positive electrode material,has the advantages of high working voltage,high energy density,and high power density,wide work temperature,free of environment pollution in produce and application,etc.It is an attractive secondary rechargeable battery of high power.However,its shortcomings of Zn electrode shape deformation,Zn dendrite formation,Zn densification and self-discharging damage the properties of zinc-nickel battery and prevent its wide application in the industry.On the basis of a systematic review of the investigations of zinc-nickel battery and the zinc electrode materials,the present thesis aims at the fabrication of new zinc electrode material to improve the properties of the zinc-nickel battery.In the thesis,Zn(CH₃COO)₂·2H₂O and citric acid are used as raw materials to synthesize ZnO/C composite by a sol-gel method.The compositions and the structures of ZnO/C composite electrode materials were modified by varying the ratio of Zn(CH₃COO)₂·2H₂O and citric acid and the sintering temperature.Structure analysis methods of XRD,SEM,TEM,Laman spectrum,element analysis,etc.were used for the analysis of the structures of the ZnO/C composites.The electrochemical properties of the different ZnO/C composites electrode including discharge capacity, cycle property,the behavior of the voltage-current of the cycling,mid-discharge voltage,ect.,were measured and investigated.The relationship of the electrochemical properties of the structure of the ZnO/C electrode material is discussed.In addition, the oxidation and reduction reaction characteristics of the ZnO/C electrode during cycling and the dissolution of ZnO in the lye and the re-precipitation of Zn-ion from the electrolyte onto the electrode were discussed also.The results of the present investigations show that,at the synthesis condition of the mole ratio of Zn(CH₃COO)₂·2H₂O to citric acid ranging 1:0.1 to 1:1.5,and under the sintering parameter of 600℃×6h in N₂ atmosphere for the dry gol ZnO/C composites were obtained.The content of carbon in the ZnO/C composites ranges 7-13wt%,depending on the addition of citric acid.The carbon content increases with increasing the amount of the citric acid in the sol.Carbon mainly existed as disordered,however,there still some order arrange for the atoms of the carbon,and the degree of the order of the carbon atoms increased with increasing the sintering temperature.ZnO existed as crystal structure,the crystal is in nanometer order.The de-agglomerate and homogeneity of the dry gel is very important for the homogenous of the composition and the microstructure of the ZnO/C composites.When less amount of citric acid was used,the microstructure of ZnO/C displayed as the ZnO particles dispersed in the matrix of carbon,and when more amount of citric acid was used,the microstructure of ZnO/C displayed as the ZnO particles being coated by carbon film.For the structure of ZnO dispersed in the carbon matrix or ZnO was coated with carbon film,the ZnO/C composite showed better cycle ability than the bare commercial ZnO.However,since carbon is not the active electrode material for the zinc-nickel battery,two much high carbon low the capacity of the electrode.ZnO/C prepared under sintering temperature of 600℃,the ZnO/C composites electrode with carbon content of 7wt%and 9wt.%show a maximum discharge capacity of 455mAh/g and 445mAh/g under the discharge current density of 100mA/g,while the maximum discharge capacity decreased to 415mAh/g when the carbon content increased to 13wt.%.Moreover,with the increase of the sintering temperature,the discharge capacity only showed a limited decrease.The ZnO/C composites electrode showed a good rate capacibility.The discharge capacity of the ZnO/C composites only showed a slight decrease when the discharge current density increased from 100mA/g to 200mA/g.Carbon coating of ZnO reduced the polarization of the electrode.Further,with increasing the sintering temperature,the polarization of the ZnO/C electrode increased.The ZnO content in the KOH electrolyte played great affect on the capacity and cycle property of the ZnO/C electrode.Compared with the KOH electrolyte of saturated ZnO,the electrolyte of over-saturated ZnO illustrated higher capacity and better cycle performance for the ZnO/C electrode.Since the ZnO particles in the over-saturated ZnO electrolyte precipitated onto the electrode and participated the charge/discharge cycle.In this case,modification the content of ZnO of the electrolyte is key important for the electrochemical properties of the ZnO/C electrode.In addition, it is found that the dissolved ZnO in the electrolyte was not precipitated on the original place when Zn-ion occurred as Zn reduced from the electrolyte.Zn preferred to nucleation at the edge of the electrode and nickel belt.The re-precipitated Zn participated the cycling and generated and also broke off and fall to the electrolyte, and thus led to the capacity loss and decreased the efficiency of the active material of ZnO.