Studies On The Highly Prohibited Polysulfides Membrane In Lithium-sulfur Batteries

With the rapid development of society and the continuous growth of population,people's demand for sustainable energy is gradually on the increase.Lithium ion batteries are widely used in household appliances,smart phones,computer and electric vehicles.However,due to the low capacity and energy density of traditional lithium-ion batteries,the needs of people cannot be satisfied for high-energy equipment.Therefore,lithium-sulfur batteries have attracted people's attention.The specific capacity and energy density of lithium-sulfur batteries is 1675 mAh?g^-1 and2600 Wh?kg^-1.Currently,the commercialization of lithium-sulfur batteries has been seriously hindered by the following aspects:poor conductivity of sulfur as cathode material,volume change caused by the formation of lithium sulfide(density 1.66 g?cm^-3)from sulfur(density 2.07 g?cm^-3)and dissolution of polysulfides in electrolyte.In this thesis,thiourea-treated stereotaxically-constructed multiporous graphene?SCMG?has been used as a kind of sulfur carrier,and titanium dioxide nanotubes?TNT?are grown in the inner pore of porous graphene?SCMG?to modify commercial membrane,showing excellent electrochemical properties.In addition,hierarchical nanoporous carbon materials?HNCM?are prepared with a simple method to modify commercial membrane?SCMG₁ as sulfur carrier?.The main research contents are as follows:?1?Study of SCMG-TNT composite modified commercial membrane in lithium-sulfur batteriesThe preparation of SCMG is a cationic exchange resin generated at high temperature under the catalysis of Ni⁺.Then,SCMG-TNT composites are synthesized by one-step hydrothermal method under alkaline conditions.The composites have multistage porous structure.SCMG have low surface active site and the adsorption capacity of polysulfides is poor.Therefore,thiourea is used to modify the surface of SCMG?SCMG₁?to improve the adsorption of polysulfides.SCMG₁ can be adopted as sulfur carrier?69 wt%and 78 wt%?to improve the conductivity and surface adsorption capacity of sulfur.At the same time,SCMG-TNT is modified commercial membrane?SCMG-TNT/PP?.This battery structure delivers a high second cycle capacity of 1594 mAh?g^-1 at a 0.2 C current rate and demonstrates a stable cycling performance of 710 mAh?g^-1 after 100 cycles at 0.5 C.?2?Study of HNCM modified commercial membrane in lithium sulfur batteriesUsing pretreated cation exchange resin as raw material,HNCM is formed at high temperature under the treatment of KOH,PTFE and Al₂O₃,and it is adopted to modify the commercial membrane?SCMG₁ as sulfur carrier?.The micropore and mesopores in HNCM can block the shuttling of polysulfides,while the macropore can promote ion migration and enhance the wettability of the membrane.The Li–S cells deliver a specific discharge capacity of 978 mAh?g^-1 at 1 C on the first cycle and demonstrate a stable cycling performance of 532 mAh?g^-1 after 300 cycles.