Preparation Of MXene And Its Application In Lithium-sulfur Battery Separator

Lithium-sulfur（Li-S）battery is considered to be the most promising secondary battery to replace lithium-ion batteries due to its abundance in raw materials and low price,as well as the high theoretical energy density(1675 mA h g^-1)and energy density(2600 Wh kg^-1).However,some challenging issues in Li-S batteries such as the poor conductivity of positive electode sulfur,large volume changes during charge/discharge and shuttle effect lead to poor cycling stability and rate capability.Therefore,the commercial applications of Li-S batteries are prevented.In order to improve the performances of Li-S batteries,this work aims to find a conductive material that can effectively hinder lithium polysulfide shuttle to the negative electrode which can be used to modify the separators to inhibit the shuttle effect and increase the utilization of active sulfur.As a result,the preformance of the Li-S batteries are significantly improved.The research contents are as followed:1.The MXene（Ti₃C₂）nanosheets were synthesized by two methods of HF etching and in situ stripping by fluoride salt and acid.The obtained Ti₃C₂ nanosheets are characterized by varies methods.The MXene nanosheets obtained by HF etching are smaller,thicker and not suitable for film formation.And the whole process is complicated with low yield.By comparison,the in situ stripping method shows more advantages including the facile preparation,low toxicity,and the high yield beyond 75%.Furthermore,the single layer MXene nanosheets with micron-sized width obtained from this method are beneficial for film formation.At the same time,there are abundant-OH,=O and other groups on the surface of the obtained MXene nanosheets,and the Ti,C,F,O and other elements on the nanosheets are uniformly distributed.2.A thin MXene layer with thickness of only 350 nm was loaded on a commercial PP by vacuum filtration and the loading capacity is 0.075 mg cm^-2.The MXene@PP separator not only has good stability to improve the wettability of the separator,but also can inhibit the shuttle effect and increase the utilization of active sulfur.In addition,the modified separator shows comparable lithium ion conductivity to PP which indicates that the thin modification layer is not detrimental to the lithium ion transport.The Li-S cells with MXene@PP delivered an initial discharge specific capacity of 1146 mA h g^-1 at 0.2 C,and 554 m A h g^-1 can still be retained after 100 cycles.Even at a higher rate of 1 C,a high initial discharge capacity of 844mA h g^-1 can be achived and 510 mA h g^-1 can be retained after 500 cycles corresponding to an average decay rate per circle of relative low 0.079%.In the self-discharge test,the discharge specific capacity decay rate of the Li-S cells with MXene@PP was only 8.3%after72 hours which was less than the ones with PP（13.65%）.