Study On The Technology And Improvement Of Graphene Based Composite For Pouch Lithium-sulfur Cell

Lithium-sulfur batteries have high theoretical energy density and theoretical discharge specific capacity,and elemental sulfur has the advantages of rich material sources and low cost.Pouch cells have the characteristics of light weight,simple process,high safety,and so on.The study of the advantages of pouch lithium-sulfur cells is of great significance.At present,the research and application of anode,separators,and battery cases for pouch cells are relatively sufficient,but cathode materials and battery technologies of pouch lithium-sulfur cells still need further exploration and development.Therefore,this paper focuses on pouch lithium-sulfur cells,and explores the cathode materials and battery technology,and further studies its electrochemical performance.The main contents are as follows:Firstly,porous graphene MPG,KPG and APG were prepared by KMnO4 etching,KOH activation,and graphene self-assembly,respectively and KPG/S and APG/S composites are obtained after loading with sulfur.Compared to KPG with mesopores structure,the microporosity structure of APG makes partial sulfur and conductive materials unable to fully contact,which leads to lower utilization of sulfur.So the electrochemical performance of KPG/S is better than that of APG/S.At 0.1C rate,KPG/S has an initial discharge specific capacity of 1016 mA·h/g,and can maintain 708.5 mA·h/g after 30 cycles,while the inital discharge specific capacity of APG/S is 923.6 mA-h/g and drops to 485.6 mA-h/g after 30 cycles.Secondly,According to different proportions of composite conductive agents,four kinds of pouch lithium-sulfur cell were prepared.Through XRD and SEM analysis,the positive electrodes sheets before and after cycling have good consistency.The electrochemical performance of CA-2 is better.Its initial discharge specific capacity is 643.4 mA·h/g at 0.1 C,and the discharge specific capacity is 510.1 mA·h/g after 30 cycles.The capacity retention rate is 79.3%.According to the electrochemical impedance spectroscopy,the CA-2 pouch cell has less impedance before and after cycling.The reason is that,when the ratio of graphene and Super-P in the composite conductive agent is 10:10,(1)the porosity of the positive electrode sheet is smaller and the degree of agglomeration is lighter,(2)the electrode sheet has better wettability,and(3)the Li+diffusion rate is improved.Six kinds of pouch lithium-sulfur cells are assembled with positive electrode sheets of different areal densities,which were prepared by different coating methods.According to XRD and SEM analysis,it can be seen that the positive electrode sheets before the charge-discharge cycles have a good consistency.Among them,the electrochemical performance of SD-5 is better,and its initial discharge specific capacity displays as 1550.5 mA·h/g at 0.1 C,and has a high energy density.After 30 cycles,the discharge specific capacity of SD-5 is 885.2 mA·h/g,which shows good cycle stability and rate performance at 1 C.The possible reason is that the internal resistance,impedance,and energy density of SD-5 pouch lithium-sulfur cell with the area density as 3.99 mg/cm2 have a good balance,which improves the overall performance of the pouch lithium-sulfur cell.