Performances Of Noble Mental-Loaded Carbon As Sulfur Host For Lithium-Sulfur Batteries

Lithium-sulfur?Li-S?batteries,with high-capacity(1675 mAh g^-1)and low-cost sulfur as the cathode as well as metallic lithium as the anode,have received great attentions owing to the high theoretical energy density of 2600 Wh kg^-1 that is one order of magnitude higher than that of the commercial lithium-ion batteries.However,several major issues to be addressed hinder the pratical application of Li-S batteries.These include the insulating nature of sulfur and its reduced product?Li₂S?,shuttle effect during discharge-charge process,and large volumetric expansion,leading to low sulfur utilization and poor cycle ability.The main method to improve the sulfur cathode performance is to composite sulfur with various carbon materials with good conductivity.However,due to the poor affinity between non-polar carbon surface and polar polysulfides,the carbon host materials is hard to strongly adsorb the polysulfides,and thus shuttling suppression is limited.In order to solve these problems,noble metals are deposited on the surface of various carbon materials.By polar adsorption and catalytic properties of noble metal nanoparticles?NPs?,the electrochemical performance of lithium-sulfur batteries is improved.The main results are shown as follows:Firstly,Pt and Au NPs are deposited on ordered mesoporous carbons?OMCs?by hydrogen reduction,and then the OMCs are composited with sulfur to produce the electrode materials OMC-Pt/S?H?and OMC-Au/S?H?.It is found that the particle size of noble metal NPs is about 10-15 nm.OMC-Pt/S?H?and OMC-Au/S?H?deliver the initial discharge capacity of 746.8 and 730.2 mAh g^-1,respectively.After 100 cycles at0.2 C rate,the capacity retention rate is 61.1%for OMC-Pt/S?H?and 60.0%for OMC-Au/S?H?,respectively.Conversely,the initial discharge capacity of OMC/S without metal NPs is 708.2 mAh g^-1,with a capacity retention rate of 50.8%after 100 cycles.The results display that the incorporation of metal NPS results in enhancing cycle stability of sulfur cathode.In addition,CV measurments demonstrate that,after metal deposition,the oxidation peak and reduction peak of the materials shift negatively and positively,respectively.This indicates that the deposition of Pt and Au NPs can enhance the redox kinetics.Secondly,the particle sizes of Pt and Au NPs obtained by aforementioned hydrogen reduction are large,most likely due to applied high-temperature condition.In order to downsize Pt and Au NPs and improve their size monodispersity,NaBH₄ is used as reductant to deposit NPs on OMCs under low-temperature condition.The OMC-NPs are then composited with sulfur to produce the electrode materials OMC-Pt/S and OMC-Au/S.It is found that the diameter of Au NPs is less than 10 nm and the Pt NPs is about 2 nm.OMC-Pt/S and OMC-Au/S deliver the initial discharge capacity of 764.6and 756.2 mAh g^-1,respectively.After 100 cycles at 0.2 C rate,the capacity retention rate is 68.4%for OMC-Pt/S and 64.3%for OMC-Au/S,respectively.Additionally,compared with hydrogen reduction,CV measurments demonstrate that the oxidation peak and reduction peak of the materials shift negatively and positively,respectively,and the stability and reversibility are better.The results suggest that the smaller size of NPs,the better performance of the cathode.Thirdly,reduction Graphene Oxide?rGO?,features with high specific surface area,good conductivity,and polar functional group-rich surface.Pt NPs are deposited on rGO by using NaBH₄ as the reductant,and then the rGO is composited with sulfur to produce the electrode materials rGO-Pt/S.rGO-Pt/S delivers the initial capacity of1504.7 mAh g^-1,and the utilization of active sulfur is 90%.In comparison,the initial capacity of rGO/S without metal NPs is 1395.0 mAh g^-1.In addition,at the current density of 0.5,1.0 and 2.0 C,after 300 cycles,the discharge capacity of rGO-Pt/S is513.2,533.0,and 453.0 mAh g^-1,respectively.The results suggest that the synergy of noble metal NPs and rGO can effectively improve the overall electrochemical performance of sulfur cathode.