Preparation,Modification And Lithium Storage Performance Of Co₉S₈/Graphene Self-Standing Anode Materials

Lithium-ion batteries are one of the most widely used energy storage devices due to the advantages of high energy density and long cycle life.However,the commercial graphite anode is hard to meet the demands of new generation of energy storage systems for high-energy density batteries,because of its low theoretical specific capacity,there is an urgent need to develop novel high-capacity anode materials.Co₉S₈ is one of the research hotspots of lithium ion battery anode materials due to the merits of high theoretical specific capacity,stable crystal structure and high thermal stability.Unfortunately,the practical applications of Co ₉S₈ are largely hampered due to the problems including the slow diffusion of lithium ions in the bulk phase,and a relatively large volumetric variation（～83.2%）during the charge-discharge processes,which lead to inferior specific capacity,rate performance and cycling performance of the prepared lithium-ion batteries.In this thesis,through the integrated research on the structure design,preparation,modification and electrochemical performance of Co₉S₈ anode material,a Co₉S₈-based/graphene self-standing anode with good electrochemical performance was obtained,which provided an important reference for the development of new anode materials for lithium-ion batteries.The Co₉S₈/rGO self-standing anode was prepared by combination of high-energy ball milling and vacuum filtration methods.The influence of the raw material ratio and ball milling speed on structure and electrochemical performance of the Co₉S₈/rGO self-standing anode was studied.SEM,TEM,XRD,Raman,IR,XPS,BET,Nano tensile tester and electrochemical workstation were used to analyze the micromorphology,crystal structure,specific surface area,mechanical properties and electrochemical peformance of the Co₉S₈/rGO self-standing anode.And the relationship between the composition,structure and electrochemical performance of the Co₉S₈/rGO self-standing anode was discussed.The results show that when the mass ratio of Co₉S₈ to rGO is 2:1 and the ball milling speed is 700 rpm,the obtained Co₉S₈/rGO self-standing anode has better mechanical properties and higher specific surface area.The specific capacity of the Co₉S₈/rGO self-standing anode can still maintain 412.3 mAh g_Co9S8^-1(274.87 mAh g_electrode^-1)after cycled 800 times at a current density of 1.0 A g^-1.The formation of S-C chemical bond between rGO and Co₉S₈ by high-energy ball milling,which accelerates the electron transfer rate between Co₉S₈ and rGO interface,and further improves the ability of graphene to alleviate the volume change of Co₉S₈,thereby improving the cycle stability and rate capacity of the Co₉S₈/rGO self-standing anode.ZnO was deposited on the surface of Co₉S₈ by atomic layer deposition to obtain ZnO-Co₉S₈ anode material,which was then modified by rGO to prepare ZnO-Co₉S₈/rGO self-standing anode.Through the analysis and characterization of the structure and electrochemical performance of the Co₉S₈/rGO self-standing anode before and after ZnO modified,the modification mechanism of ZnO on the Co₉S₈/rGO self-standing anode was discussed.The results show that when the number of atomic layer deposition cycles is 100,the mass ratio of ZnO-Co₉S₈ to rGO is 2:1,and the ball milling speed is 700 rpm,the discharge capacity of ZnO-Co₉S₈/rGO self-standing anode still maintain 1020.45 mAh g^-1_ZnO-Co9S8(680.3 mAh g_electrode^-1)after 1000 cycles at a current density of 1.0 A g^-1,and the capacity at 10.0A g^-1 is 1511.4 mAh g^-1_ZnO-Co9S8(1007.6 mAh g_electrode^-1),which is much higher than that of Co₉S₈/rGO.This is mainly due to the discontinuous deposition of ZnO on the Co₉S₈ surface,which effectively increased the specific surface area of the Co₉S₈/rGO self-standing anode and prevented the formation of large Co₉S₈ particles during charge and discharge.In addition,the S-O-Zn bond formed between ZnO and Co₉S₈not only shortened the transmission path of Li⁺and electrons,but also maintained the structural stability during charging and discharging process.Using first principles calculation,electrochemical impedance technology and cyclic voltammetry,the adsorption,diffusion and capacitance behavior of lithium ions at the electrode/electrolyte interface were studied,and the modification mechanism of ZnO on electrochemical performance of Co₉S₈ was discussed.The results show increased adsorption energy of Co₉S₈/rGO for lithium ions from-63.6to-64.7 kcal/mol;The charge transfer resistance R_ct after 500 cycles at a rate of 0.2A g^-1 decreased from 48.52 to 16.78Ω;The proportion of pseudocapacitance at a sweep rate of 1.0 mV s^-1 increased from 82.07%to 87.4%;The energy barrier of lithium ion migration was reduced from 203.74 kcal/mol of Co₉S₈ to 18.24 kcal/mol of ZnO-Co₉S₈.In summary,ZnO can effectively improve the adsorption capacity of electrode materials for Li⁺,reduce the charge transfer resistance R_ct,increase the diffusion coefficient of Li⁺in the bulk phase and the ratio of pseudocapacitance behavior in electrochemical reactions,and enhance the electrode/electrolyte interface electrochemical reaction kinetics,thereby improve the specific capacity,cycle performance and rate performance of the electrode material.