Synthesis And Performance Of Carbon-based Composite Materials In Lithium-sulfur Batteries

The depletion of the traditional fossil fuels and environmental pollution have become the key constraints to sustainable development.Therefore,it is urgent to develop environmental benign and green energy storage system.Lithium-sulfur batteries（LSBs）based on multi-electron redox conversion reaction mechanism have been considered as the most promising next generation energy storage system due to their high theoretical capacity(1675 m Ah g^-1)and mass energy density(2600 Wh kg^-1).Nevertheless,several challenges impede the commercialization of LSBs,including the poor electrical conductivity of sulfur,volume expansion,shuttle effect,sluggish redox kinetics and flammable and leaky electrolyte.In order to solve these problems,this thesis focuses on the carbon-based composites materials and solid-state electrolyte.The main research contents are as follows:（1）Using a facile approach to directly synthesize P-doped ultra-high specific surface area carbon materials from abundant biomass molecules—phytic acid.Se_0.07S_0.93/P-C was prepared by melt-diffusion method and then investigated its application as cathode material of LSBs The results show that,P-C has strong chemisorption and physical confinement on lithium polysulfide（Li PSs）,which can effectively inhibit the"shuttle effect".The doping of Se improves the conductivity of the active substance,accelerates the transport of electron and enhances the kinetics of redox reaction.With these advantages,Se_0.07S_0.93/P-C exhibits a high initial discharge capacity of 1392 m Ah g^-1 at 0.3A g^-1.After 150 cycles,coulombic efficiency also close to 100%.Under the high current density of 2 A g^-1,the initial discharge capacity is1190 m Ah g^-1 and the capacity remains at 475 m Ah g^-1 after 780 cycles.The average capacity decay rate is 0.077%per cycle.（2）Fe,Ni bimetallic materials supported on nitrogen doped graphene（Fe,Ni/N-C）were prepared by high temperature pyrolysis precursor method and used as sulfur host materials for LSBs.Fe,Ni/N-C material with large specific surface area and abundant pore structure not only chemically adsorb Li PSs,but also physical confinement on Li PSs,which can effectively inhibit the"shuttle effect".The loading of polar transition metals Fe,Ni provided the active sites for the catalytic conversion of Li PSs,promoted the redox reaction and accelerated the redox reaction kinetics of sulfur species.With these advantages,S/Fe,Ni/N-C exhibits a high initial discharge capacity of 1100 m Ah g^-1 at 0.2 C.Fe,Ni/N-C has a initial discharge capacity of 983.5 m Ah g^-1at 0.5 C,and the capacity can be maintained at 662 m Ah g^-1 even after 100cycle.After 700 cycles at 1 C,the capacity remained at 421.4 m Ah g^-1,with an average capacity decay rate of 0.07%per cycle.（3）PEO-based solid-state electrolyte was prepared by casting method.The properties of polyethylene oxide（PEO）/Li TFSI solid-state electrolytes with different ratios[EO]/[Li⁺]were explored.Herein,PEO/Li TFSI（[EO]/[Li⁺]=16）with higher ionic conductivity,stable interface with lithium metal,and wide voltage window was selected,and applied to solid-state LSBs.The influence of S/C,S/Fe,Ni/N-C and Se_0.07S_0.93/P-C cathode materials on the performance of PEO-based SSLSBs was further investigated.The PEO-based SSLSBs with Se_0.07S_0.93/P-C as cathode materials exhibits high discharge specific capacity of 1197 m Ah g^-1at 60℃and 0.1 C,the discharge capacity is maintained at 826.8 m Ah g^-1 after 50 cycles,and the coulomb efficiency is kept above 97.5%.The performance of PEO-based SSLBSs is improved.