Study On The Application Of Transition Metal Compound Nanoparticles/Porous Carbon Composites In Separator Modification Of Lithium-Sulfur Batteries

Lithium-sulfur battery（Li-S battery）possesses an ultra-high theoretical capacity(1675 mAh g^-1)and many merits such as low cost and non-toxic.However,its development is seriously hindered by the electrical insulation characteristics of sulfur and lithium sulfide,the"shuttle effect"of lithium polysulfides（LPS）and slow reaction kinetics.To address those challenges,it is essential to design and synthesize some separator modification materials that combined excellent conductive networks and the powerful ability to bond with LPS and accelerate their conversion.Porous carbon materials are selected as one of the"puzzles"of those kinds of functional materials because they not only possess good conductivity but also can be provided with sufficient space that can alleviate the volume expansion of sulfur during the charging and discharging effectively.Although metal compounds do not conduct well,sufficient metal active sites can be supplied to trap the LPS and accelerate the kinetics of the reaction.Therefore,metal compounds are selected as another indispensable"puzzle"of the functional separator modification material in this paper.On this basis,our works are based on the reasonable combination of those two"jigsaw puzzles",and have made some considerable achievements in improving the long-term cycle stability and the rate performance of the batteries,and also have a certain extent understanding of the relationship between the electrochemical reaction of the Li-S battery and the physical properties of the separator modification materials.The specific research contents and conclusions are as follows:（a）Sulfur,Nitrogen and Phosphorus co-doped porous carbon material decorated with Cobalt Selenide（Co Se）nanoparticles（SNP-PC@MPC@Co Se）are synthesized by the solvothermal method and Chemical Vapour Deposition（CVD）method.This kind of material not only maintains the ultra-high specific surface area of porous carbon materials that can absorb and retain electrolyte by capillary force but also possesses abundant metal active sites that can chemically bond with LPS and accelerate the conversion of LPS,and the deposition of Li₂S.When the material is applied to the coating layer of the separator,it can effectively improve the long-term cycle stability of the Li-S battery even under a high current rate of 4 C.The initial discharge capacity is 796 mAh g^-1 and it still possesses a reversible capacity of 420 mAh g^-1 after 700cycles,enabling a low capacity fading of 0.06%per cycle.More impressively,even in a harsh test condition(sulfur loading=5.65 mg cm^-2,ratio of electrolyte to active materials（E/S）=4μL mg^-1),the battery can still enable a higher specific capacity of980 mAh g^-1(area capacity of～5.54 mAh cm^-2)at 0.1 C.（b）Nitrogen-doped porous carbon materials dispersed with Iron（Fe）,Cobalt（Co）and Nickel（Ni）（Fe-Co-Ni）ternary alloy were synthesized by a simple Sol-Gel low-temperature combustion method and annealing process.Among this material,porous carbon material can be provided with a conductive framework to enhance the conductivity of the whole material,while the metal active sites provided by Fe-Co-Ni ternary alloy particles can effectively interact with polysulfides and promote the electrochemical reaction kinetics.The satisfactory long-term cycle stability of the batteries with the FNC@NC//PP separator can be distinguished at a current rate of 0.2C,which not only can deliver an initial discharge capacity of 1325 mAh g^-1 but also can maintain a high capacity of 950 mAh g^-1 over 200 cycles.Not only that,under harsh test conditions(E/S=6μL mg^-1 and sulfur loading=4.7 mg cm^-2),the area capacity of batteries proves still much higher than 4 mAh cm^-2.