The Study Of Functional Interlayers Of Lithium Sulfur Batteries Based On The 3D Structure Of Carbon Nanofibers

Lithium sulfur?Li-S?batteries are considered as one of the most promising systems for next generation batteries,due to their high theoretical specific capacity of 1675 mAh g^-1and energy density of 2600 Wh kg^-1,and sulfur exhibits other advantages such as relative abundance,environment friendliness and low cost.However,there are several critical challenges which influence the electrochemical performance of the Li-S batteries,such as the“shuttle effect”of the dissolved polysulfide,the poor electronic conductivity of the sulfur cathode and the reaction product Li₂S.In this thesis,the functional interlayers of Li-S batteries based on the 3D structure of carbon nanofibers have been successfully synthesized and explored for Li-S batteries to solve the problems and pursue an improved electrochemical performance.A freestanding carbon nanofiber/polyvinylidene fluoride?CNF/PVDF?composite membrane has been successfully synthesized and tested as the functional interlayer of Li-S battery.It turns out that the CNF/PVDF composite membrane has high specific surface area and good mechanical strength,as the interlayer of Li-S batteries it could help confine polysulfide shutting and improve the electronic conductivity of sulfur cathode.The CNF/PVDF composite membrane in Li-S battery delivers a high intitial specific capacity of1739.2 mAh g^-11 at 1 C,and the capacity is above 680 mAh g^-11 after 100 cycles.In addition,the capacity can still maintain 500 mAh g^-11 even at 2 C.Such a good electrochemical performance of the CNF/PVDF demonstrate that CNF/PVDF composite membrane can effectively improve the electronic conductivity and solve the polysufide shutting problem,thus improving the electroperforamnce of Li-S batteries.A separator surface-coated carbon nanofiber/hollow carbon sphere?CNF/HCS?composite membrane has been successfully synthesized and tested as the functional interlayer of Li-S battery.The HCS is synthsised by SiO₂ hard-temple method and mixed with CNF,then coated onto the Celgard separator to obtain CNF/HCS composite membrane.The most appropriate ratio of the CNF and HCS in the CNF/HCS composite membrane has been explored through electrochemical tests.It turns out that the CNF/HCS composite membrane as the interlayer of Li-S battery with 1:1 by weight shows the best electrochemical performance.The CNF/HCS composite membrane in Li-S battery delivers a high intitial specific capacity of 1224 mAh g^-11 at 0.5 C,and the capacity is above 1000mAh g^-11 after 100 cycles.Moverover,an excellent reversible capacity of 600 mAh g^-1achieved even at 3 C.The adsorption test demonstrates that CNF/HCS composite membrane can effectivity solve the polysufide shutting problem and improve the electrochemical perforamnce of the Li-S batteries.