Design Of Key Materials For High Specific Energy Lithium-sulfur/Oxygen Battery And Research On Charge/Discharge Mechanism

In recent years,with the rapid development and application of green energy technology and various energy storage equipment,low-cost,high-safety and environmentally friendly energy storage systems have become the current research focus.Among many energy storage systems,lithium metal batteries are considered to be the most promising lithium secondary battery system due to their ultra-high energy density(2567 Wh kg^-1 for lithium-sulfur batteries and 3458 Wh kg^-1 for lithium-oxygen batteries).Moreover,the cathodes(sulfur,oxygen)of these two systems have the advantages of low cost,non-toxic and environmental friendliness,etc.,which have received extensive attention and research.However,their commercial applications are facing to numerous challenges.In lithium-sulfur batteries,the poor conductivity of sulfur and lithium sulfide,the"shuttle effect"of lithium polysulfide,and the dendrites of the lithium negative electrode leads to low active material utilization,poor battery cycle stability,and may even cause security risks.In the lithium-oxygen battery,it also faces a series of problems such as high over-potential and many side reactions due to the poor conductivity of the discharge product lithium peroxide,which seriously affects the cycle life of the lithium-oxygen battery.In this paper,the design of the sulfur cathode materials and optimization of the separator are mainly used to improve the utilization of sulfur,alleviate the shuttle effect of polysulfides,and improve the cycle stability of the battery.Through the research on the discharge products and charge-discharge mechanism of the lithium-oxygen battery,a strategy of tuning the charge-discharge mode is proposed to essentially optimize the charge transfer process of the lithium-oxygen battery.The main research contents of this paper are as follows:(1)The self-upright porous CoO nanosheets arrays are anchored on the three-dimensional cross-linked conductive carbon fibers,which is applied to the positive electrode material of lithium-sulfur battery with high sulfur loading.The three-dimensional cross-linked conductive fiber modified by graphene oxide can provide a good electron and ion transmission channel for the sulfur cathode,reducing the interface resistance.At the same time,the upright porous CoO nanosheets arrays anchored on the carbon fibers have a good chemical adsorption effect on polysulfides in the electrolyte,and can further catalyze the conversion of polysulfides and improve the shuttle effect of polysulfides.In addition,the self-supporting three-dimensional structure can provide a higher sulfur loading.The electrochemical test results show that the material still maintains a capacity of 940 mAh g^-1 when the sulfur load is 2.44 mg cm^-2 and at a current density of 0.5 C after 300 cycles,even when the sulfur load is as high as 9.76 mg cm^-2,it can also show good cycle stability.(2)MoS₂/CoS₂ heterojunction nanorods and graphene oxide composites serve as a strong polysulfide anchor in cathode materials for lithium-sulfur batteries.In this structure,graphene oxide can provide enough storage space for the sulfur anode and ensure good conductivity.The nanorod structure composed of MoS₂/CoS₂ heterojunction nanosheets can provide a large amount of exposed active surface,which is very effective for polysulfides adsorption and catalytic effects help to accelerate the chemical kinetics of polysulfide reactions.In the battery charge-discharge cycle test,it was cycled for more than 600 cycles at a current density of 1C,and it still maintained a capacity of more than 620 mAh g^-1,showing good rate performance and cycle stability.(3)The CoS-CoS₂ heterojunction nanoparticles were successfully prepared by the one-step mixed solvent method,and the membrane was modified after being compounded with graphene oxide and applied to lithium–sulfur batteries.In separator of lithium-sulfur battery,the strongly charged regions in the CoS-CoS₂ heterojunction structure can enhance the chemical adsorption of polysulfides,and the built-in electric field generated by the heterojunction structure also helps to accelerate the charge transfer in the catalytic reaction process,to speed up the kinetics of chemical reactions.In the battery performance test,at a current density of 0.5C,the charge/discharge cycle exceeded 300 cycles,and the reversible capacity of 804 mAh g^-1 was still maintained.And in the cycle test of Li symmetric battery,it is found that the modified separator has a lower overpotential,which can prolong the stable cycle life more than double compared to the PP separator.(4)By tuning the charge and discharge mode of the lithium-oxygen battery,the charge transfer characteristics of the discharge product lithium peroxide are essentially optimized.Due to the extremely poor conductivity of lithium peroxide,the discharge product of lithium-oxygen battery,the over-potential during charging is seriously increased,and the internal mass transfer of the battery is blocked,side reactions increase,reaction kinetics is slow,and the cycle performance of the battery is reduced.A simple and effective strategy was proposed to control the morphology and size of the discharged product lithium peroxide by adjusting the charge/discharge mode,and form lithium vacancies,optimize the charge transfer performance,reduce the overpotential,and improve the cycle performance of the lithium-oxygen battery.