Modification Of Polypropylene Diaphragm For High-performance Lithium-sulfur Batteries

Compared with traditional lithium-ion batteries,lithium-sulfur batteries have a higher theoretical energy density(2600 Wh kg^-1).At the same time,elemental sulfur is low in cost,environmentally friendly and has a high theoretical specific capacity(1675 m Ah g^-1).Therefore,the lithium-sulfur battery is considered a promising energy storage device.However,due to the poor conductivity of positive sulfur,serious volume expansion(about80%)during charging and discharging,and the"shuttle effect"of lithium polysulfide,an intermediate product of discharge,severely limit the practical application of lithium-sulfur batteries.Aiming at the problem of"shuttle effect"in lithium-sulfur batteries,this thesis uses two-dimensional ultra-thin?-MnO₂ nanosheet materials and Fe-based Prussian blue materials with rich pore structure to modify commercial polypropylene(PP)separators.Effectively improve the"shuttle effect"of polysulfides.The?-MnO₂ nanosheet material was successfully prepared by the liquid phase precipitation method,and the?-MnO₂ bulk phase was exfoliated into ultra-thin nanosheets by the solvent intercalation method.Finally,the carbon nanotubes and the ultra-thin?-MnO₂ nanosheets were suction filtered on the PP membrane by a simple vacuum filtration method to obtain the CNT+?-MnO₂/PP membrane.Because the carbon nanotubes have good electrical conductivity and mechanical properties,and the ultra-thin?-MnO₂ nanosheets have a physical barrier and chemical adsorption to polysulfides,the prepared separator significantly improves the electrochemical performance of the lithium-sulfur battery.The lithium-sulfur battery using CNT+?-MnO₂/PP separator has an initial discharge specific capacity of 934.4 m Ah g^-1 at 1C,and can still maintain a high reversible capacity of 672 m Ah g^-1 after 200 cycles,and the capacity decay rate is only0.136%.In addition,under high sulfur loading(3.8 mg cm^-2),the initial specific capacity can still reach 1055 m Ah g^-1.The Fe-based Prussian blue material was successfully prepared by the hydrothermal method,and the Fe-based Prussian blue was further compounded with ultra-thin Mxene nanosheets by the electrostatic self-assembly method.Finally,the carbon nanotubes and PB@Mxene were suction-filtered on the PP membrane by a simple vacuum filtration method.Utilizing the rich pore structure of Fe-based Prussian Blue,it exhibits strong adsorption characteristics for polysulfides.At the same time,ultra-thin Mxene nanosheets with excellent electrical conductivity can further accelerate the kinetic behavior of the catalytic conversion of polysulfides.The electrochemical test results show that compared with the unmodified PP separator,the lithium-sulfur battery using CNT+PB@Mxene/PP separator exhibits superior electrochemical performance.At 1C,the initial specific capacity is 941.7 m Ah g^-1.After 200 cycles,the high reversible capacity of 674.1 m Ah g^-1can be maintained,and the capacity decay rate is only 0.14%.In addition,when the sulfur loading is as high as 6.1 mg cm^-2,the initial discharge specific capacity can still reach851.5 m Ah g^-1 at 0.1C.