Preparation Of Spinel Oxide/Carbon Composite And Its Application In Lithium-Sulfur Battery

Along with the emergence of intensive society and continuous evolution of human society,people's demand for energy is becoming increasingly pressing and the requirements of energy storage techniques move towards the trend of low cost and high energy density.Various energy storage technologies have noticeable impact on the form of energy storage and the utilization of energy.Among the numerous energy storage systems,electrochemical energy storage,especially the electrochemical battery,has gradually become a vital branch.Lithium sulfur battery,as a member of the rechargeable battery field,has attracted the attention of researchers due to its high specific energy(2600 Wh kg-1)and high theoretical specific capacity(1675 mAh g-1).Furthermore,lithium sulfur battery also plays an unparallel role compared to traditional lithium ion battery.In addition,sulfur has become an outstanding candidate in masses of active substances in lithium batteries due to its special strength in reserves,safety and environmental protection,which constitutes the cathode together with lithium as the anode.However,not everything is going smoothly.There are still some challenges that can not be neglected in the commercialization of lithium sulfur batteries including the low conductivity of sulfur(5 × 10-30 S cm-1)and Li2S(3.6 ×10-7 S cm-1).Moreover,the intermediates of long-chain polysulfides shuttle between the anode and the cathode;the volume expansion of the electrode material happens due to the imparity of the densities of sulfur(density:2.03 g cm-3)and Li2S(density:1.67 g cm-3).To solve the above problems,this dissertation mainly focuses on the research of cathode materials for lithium-sulfur batteries.The purpose of this paper is to design and synthesize electrode materials with unique structural advantages,and to test the electrochemical properties of materials which were loading the active sulfur.The main research contents of this paper are as follows:(1)In the first place,the hybrid of ZnCo2O4@N-RGO was prepared via the combined procedures of refluxing and hydrothermal treatment.Due to the relatively high specific surface area of the reduced graphene,it is beneficial to wide distribution of ZnCo2O4 and sulfur.The strong ion bonds formed by metal-oxygen ensure the high polarity of the spinel oxide ZnCo2O4,and the combined advantages with the synergistic and interface effect between different metal ions make that ZnCo2O4@N-RGO(I)exhibits strong adsorption effects on polysulfides.The theoretical calculations based on the density functional theory show that the strong binding energy between ZnCo2O4 and Li2S4 is as high as 3.1eV,and the adsorption experiment of ZnCo2O4@N-RGO(I)to Li2S6 also further confirms its strong adsorption to polysulfides.In this work,various testing techniques,such as FESEM,TEM,HRTEM,HAADF-STEM and XRD,have been intensively employed to characterize and analyze the morphology,elemental distribution and composition of materials.When evaluated as the scaffold of sulfur cathode,the rate and cycle performance of ZnCo2O4@N-RGO were analyzed in depth.ZnCo2O4@N-RGO incorporated with as high as 71%and even 82%sulfur still can work well and offered 645 mAh g-1 recovering 71%of the initial capacity at 1600 mA g-1 even after 200 cycles.As the areal sulfur mass loading of ZnCo2O4@N-RGO(I)/S71 was increased to 2.12 mg cm-2,the discharge capacity can still reach 624 mAh g-1 at a current density of 800 mA g-1 after 200 cycles.(2)By calcining a mixture of chitosan and urea in a certain proportion at high temperature,a thin N-doped carbon nanosheet(NCN)was obtained.Urea is unstable to heat and plays a crucial role in the synthesis of carbon nanosheets.First,as a gas release agent,it can effectively control the morphology in the synthetic process.Second,N contained in urea can be used as a heterogeneous atom doped into the carbon material.A thin layer of carbon doped with heterogeneous atom not only supports sulfur,but also physically confines and chemically anchors polysulfides during cycling.The NCNs were used as sulfur holder and were assembled into coin cells for electrochemical testing.In the case of 72%sulfur content loaded,the reversible capacity remains at 612 mAh g-1 after 270 cycles at a current density of 800 mA g-1,it can be seen that heterogeneous atom-doped carbon also plays an unique role in the cathode materials of lithium sulfur batteries.