Study On Inhibition Of Shuttle Effect Of Lithium Sulfur Batteries By Surface Modified Hierarchical Porous Carbon Materials

Lithium-sulfur battery（Li-S battery）has the advantages of high theoretical energy density,low cost and environment-friendly,and is the development direction of the next generation energy storage devices.Although Li-S battery has many advantages,the shuttle effect of polysulfide has severely limited their practical applications.It is well known that the construction of sulfur host and separator modification materials are effective methods to solve the shuttle effect.Hierarchical porous carbon materials with abundant pore structure,flexible and controllable surface are commonly used in lithium-sulfur batteries.However,due to the weak interactions between hierarchical porous carbon and polysulfide,the shuttle effect cannot be completely avoided.Herein,four kind of hierarchical porous carbon materials with different surface modifications are prepared via innovative preparation methods.The modified material has the ability of chemisorption and catalytic conversion of polysulfide,which can be used to inhibit the shuttle effect.The main research contents of this work are as following:（1）Nitrogen doped two-dimensional porous carbon（NPC）was synthesized by using guanine as carbon and nitrogen source.Due to the nitrogen heteroatoms introduce,NPC possesses polar adsorption sites of surface,which can chemically adsorb polysulfide.In addition,abundant surface microporous structure is generated by KOH activation,which effectively improved the specific surface area and pore volume of the NPC.Benefiting from the two-dimensional ultrathin structure and high specific surface area,active sites in NPC can be fully utilized.When the sulfur loading is 70%,NPC-13/S shows an initial capacity of 847 mAh/g at 1 C,and the specific capacity remains 659 mAh/g after 350 cycles,the average capacity decay rate is 0.064%.（2）Nitrogen doped hierarchical porous carbon material（NHPC）is prepared using Zn（NO₃）₂as pore-forming agent,ascorbic acid and melamine as carbon and nitrogen sources,respectively.This method is simple and efficient without removing the template.NHPC has ultrathin stacked sheet morphology and abundant micropore,mesoporous and macroporous structure,especially mesoporous.The pore volume of NHPC can reach up to 1.6 cm³/g,which can not only facilitate ion transport and alleviate volume expansion during sulfur electrode cycling,but also has good adsorption effect on polysulfide.It can effectively inhibit the shuttle effect.Benefiting from the above structure,the NHPC/S electrode exhibits good electrochemical cycling performance.At 1C,the initial capacity is 879 mAh/g,and the reversible capacity is 729mAh/g after 300 cycles with a capacity decay of only 0.057%per cycle.The excellent electrochemical performance of NHPC/S can be attributed to its abundant pore structure and heteroatom doping.（3）Nitrogen-sulfur co-doped hierarchical porous carbon（NS-HPC）is synthesized by in situ template method.NS-HPC is rich in nitrogen and sulfur element,and it has a unique multistage pore structure and three-dimensional（3D）network morphology.When NS-HPC is used as a separator modification material,its unique morphology and structure not only provide sufficient storage space for polysulfide,but also facilitate the rapid penetration of electrolyte and ion transport,and strengthen the physical adsorption of polysulfide.More importantly,N and S heteroatom doping not only enhance the chemical adsorption of polysulfide,but also catalyze the conversion of polysulfide,which effectively inhibits the the shuttle effect and significantly improve the performance of the lithium sulfur battery.The lithium sulfur battery with NS-HPC separator has a stable specific capacity of 1180 mAh/g at 0.2 C.In addition,the capacity still retains 662.6 mAh/g after 300 cycles at 1 C,with average decay rate of 0.08%.Even at a high sulfur loading of 3.0 mg/cm²,the cell using NS-HPC separator still exhibits a high discharge specific capacity and cycling stability.The specific capacity under 1 C is 630 mAh/g,and the capacity retention rate is 87%after 100 cycles.（4）Based on the advantage of zinc nitrate as a pore-forming agent to prepare hierarchical porous carbon,a Co and N co-doped hierarchical porous carbon（CoN-CNT/HPC）is synthesized and used as a separator modification material.The as-prepared CoN-CNT/HPC has a hybrid morphology consisting of carbon nanotube（CNT）and hierarchical porous carbon sheet（HPC）,which is beneficial to increase electrical conductivity and promote charge transport.A series of experimental results show that Co and N co-doping has excellent adsorption and catalytic capacity,which can not only chemisorption of polysulfide,but also can catalyze the conversion of polysulfide and promote the nucleation and decomposition of Li₂S.Therefore,the CoN-CNT/HPC modified separator can effectively improve the electrochemical performance.The cell with separator CoN-CNT/HPC delivers a high initial capacity of848 mAh/g with a low decay rate of 0.04%per cycle after 500 cycles at 2 C.Even at high area sulfur loading of 5.5 mg/cm²,the cell using CoN-CNT/HPC separator still exhibits a high initial capacity of 646 mAh/g at 0.5 C and an outstanding cycling stability（561 mAh/g after 100 cycles）.