Study On Lithium Storage Performance Of Spinel Type Zinc Cobaltate

The lithium ion battery has become a hot topic in the research community due to its high specific capacity,fast charging,and long cycle life.As a key component of the lithium ion battery,electrode material determines the battery performance.Low toxicity,low cost and high theoretical capacity make zinc-cobaltate a highly attractive material intended for anodes;however,low conductivity and large volume variation in the cycling process prevent the application of the material.In this thesis,ZnCo2O4 samples with three dimensional spherical and two dimensional lamellar structures were successfully synthetized for the purpose of improving the capacity and stability.Then,the microstructure and energy storage mechanism of the samples as well as the performance of lithium ion batteries were studied.The specific work is described below:The spherical ZnCo2O4 precursor was prepared by the simple chemical bath method and then coated with carbon in the hydrothermal process to formZnCo2O4@C composite.The structure,morphology and electrochemical properties of the composite were studied through XRD,TG,BET,TEM and electrochemical testing.The discharge capacity of the ZnCo2O4@C composite is 1025.8 mAh·g-1 initially and becomes stable at 426.6 mAh·g-1 after 100 cycles.This means the carbon coating could improve the electrochemical stability of the material.The ZnCo2O4 precursor was prepared by the simple chemical bath method and then coated with a layer of hydroxide and graphene nanosheet by the hydrothermal method,which resulted in ZnCo2O4@ZnCo-LDH@rGO electrode material with a structure rich in mesopores.This special structure provides more active sites,which facilitate the effective contact between the electrolyte and the electrodes.The discharge capacity of ZnCo2O4@ZnCo-LDH@rGO after 100 cycles is increased from 130 mAh·g-1 to 982 mAh·g-1 as a result of the coating,which suggests that graphene coating enhances the electrochemical stability of the material and the overall performance of the lithium ion battery system.On the basis of the ZnCo2O4 precursor prepared by hydrothermal method,the effect of the cooling rate in the sintering process on the morphology and electrochemical properties of the material was studied.The sintering process resulted in ZnCo2O4 nanosheets of different grain sizes.The average grain size of nanosheet(ZCO-S)arising from rapid cooling is 9nm by rapid cooling whereas that of the nanosheet(ZCO-L)from natural cooling is 13 nm.In addition,ZCO-S nanosheets were coated with graphene to fabricate ZCO-S@rGO composite,a structure that is favorable to the rapid penetration of electrolyte and thereby increasing the contact area of electrolyte and electrodes.At the current density of 100 mA·g-1,the discharge capacities of the three materials,i.e.,ZCO-S,ZCO-L,and ZCO-S@rGO after 100 cycles were 150,276,and 1078 mAh·g-1 respectively,an evidence that the electrochemical stability was significantly improved after graphene coating.