Study On Preparation And Applications Of Porous Carbon Materials For Lithium-Sulfur Batteries

With the rapid development of modern industrial society,the environment has suffered tremendous damage and the society has also fallen into an energy crisis.The over-exploitation of primary energy makes the task of developing new clean energy and new-generation energy storage systems imminent.Lithium-Sulfur batteries（LSBs）have been considered to one of the most potential energy storage devices in modern life,thanks to its excellent energy density(2600 Wh kg^-1),high theoretical weight capacity(1675 m Ah g^-1),low cost and nontoxicity.However,there are series problems limit their development in practical applications,such as the insulating properties,the changing of volume of elemental sulfur,the shuttle effect of intermediate lithium polysulfides（Li Ps）during charge and discharge and so on.So,it is important to prepare host materials with novel structures,for lithium-sulfur batteries to become a viable energy storage device.Host materials can be divided into two categories,one is metal compounds,such as metal carbides,nitrides,phosphides,oxides,hydroxides,sulfides,selenides and so on.This type of methods used chemical bonds to inhibit the shuttle of Li Ps,but the shortcomings are also obvious（Low sulfur loading,poor conductivity,and high price）.So inorganic compounds are not a good host material for LSBs.The second is carbon material,such as porous materials,carbon nanotube,carbon nanofiber,biomass and so on.Carbon materials mainly limit the"shuttle effect"of polysulfides through the pore structure.Such materials have good electrical conductivity,low price,and the prepared cathode materials are more suitable for high-load electrodes.It has the potential to be commercialized.Among many types of carbon materials,porous carbon frameworks can limit the shuttle effect of lithium polysulfides effectively during charging and discharging.However,its inhibition of the"shuttle effect"of polysulfides mainly depends on the pore structure,and limiting ability is not strong.It can enhance the adsorption of polysulfides and the cycling stability of the material by incorporating heteroatoms into the porous carbon framework,and it can also improve the electrical conductivity of the material and reduce the ion transport path.Common heteroatom-doped elements include nitrogen,phosphorus,sulfur,boron,oxygen and so on.Compared with other doped elements,The atomic radius of nitrogen atom is more similar with carbon,so it is easier to be incorporated into the carbon skeleton.The preparation methods of nitrogen-doped porous carbon materials are divided into two types:（1）React with raw materials containing nitrogen to obtain nitrogen-doped porous carbon;（2）Sinter the prepared porous carbon in an ammonia gas atmosphere,and ammonia gas Pyrolysis generates free radicals,corrode porous carbon,increases porosity,and can replace oxygen-containing functional groups in the original material,make it surface functionalization.In addition,the method of nitrogen-doped porous carbon is simple,easy to operate and low in cost.It is a good opportunity for industrialization.Besides,the other way to enhance the ability of porous carbon materials to suppress the"shuttle effect"and enhance the physical confinement of lithium polysulfides of lithium-sulfur batteries is to adjust their pore size structure for achieving efficient cycling stability.Among them,due to their tunable pore structure,metal-organic framework derived porous carbon are favored as carbon-based host materials for lithium-sulfur batteries.They are designed to build strong confinement of lithium polysulfides,cycle stability and reduce resistance.After carbonizationd and acid washing,the metal ions can be removed,the chemical bonds would be broken,and more active sites can be exposed to functionalize the surface.It can promote the binding,charge transfer of polysulfides,and accelerate the electrochemical reaction kinetics.Therefore,this paper focuses on three topics.The specific work of this paper is as follows:1.Preparation of nitrogen-doped porous carbon materials by solid-state method and their electrochemical behavior as cathodes for lithium-sulfur batteries:Nitrogen-doped porous carbon was prepared by high-temperature solid-phase method with L-serine and melamine.Nitrogen-doped three-dimensional porous carbon materials（NPC-6,NPC-8 and NPC-10）with different specific surface area and pore size were prepared by adjusting different concentrations of melamine.During the high-temperature carbonization process,melamine is converted into a C₃N₄ skeleton and adsorbs small molecules to form a porous carbon skeleton on the surface.Among of them,NGC-8 has the highest specific surface area(442.33 m²g^-1)and capacity(1387.4m Ah g^-1).It also has a specific capacity of 940.6 m Ah g^-1 at 1 C current density.After1000 cycles,the performance is stable,there is still a specific capacity of 437.3 m Ah g^-1,the single-cycle capacity decay rate is only 0.05%.2.Preparation of La@metal-organic framework-derived mesoporous carbon structures and study on the inhibition of lithium polysulfide shuttle effect:We employ the spherical nickel-cobalt double Metal-Organic Frameworks as the matrix to tailor the mesopores by tuning the amount of doping La³⁺for a sub-3 nm mesopores-rich carbon material as sulfur host with high specific surface area(1644.49m²g^-1)and abundant mesopores（61.95%,most of them were 2.136 nm mescopores）.This structure can achieve superior specific capacity(1374.2 m Ah g^-1 at 0.25 C)and cycle stability（after 1000 cycles with fading rate of 0.044%per cycle）.More importantly,the cathode with sulfur loading of 5 mg cm^-2 still maintains～100%capacity after 150 cycles at 1 C.3.Preparation of La@porous carbon structures and study on the inhibition of lithium polysulfide shuttle effect:Because of the advantage of tunable pore size of metal-organic frameworks,we find metal-organic frameworks with La³⁺as the skeleton vertex as the precursor of porous carbon.And by changing the quality of La³⁺in the reaction as a comparative analysis.Porous carbon materials with 2-3 nm with lanthanum nanoparticles as the main body were successfully prepared.The material has a high specific surface area(2000.49m²g^-1)and a lot of small mesopores.At a current density of 0.5 C,it exhibits capacity is1360.4 m Ah g^-1.The highest capacity at a current density of 1 C is 1125 m Ah g^-1,still maintains a capacity of 616.7 m Ah g^-1 after 900 cycles,with a single cycle decay rate is0.05%.And it also has an excellent rate performance,678.3 m Ah g^-1 at a current density of 6 C.Compare with work 2,the more 2-3 nm mesoporous structures can reduce charge transport and ion transport paths,speed up their reaction kinetics,and provide excellent electrochemical performance.The results show that with the increase of the number of small mesopores,the limitation of polysulfides and the utilization of sulfur elements are significantly improved.In summary,this paper discusses nitrogen-doped porous carbon and small mesoporous porous carbon as host material for sulfur,and investigates its electrochemical performance and reaction kinetics.The experimental results show us:（1）Nitrogen-doped can not only provide more active sites to promote electrochemical reaction kinetics,also provide chemisorption with polysulfides,inhibit the"shuttle effect"and enhance cycling stability effectively.（2）2-3 nm mesopores have a significant inhibitory effect on the volume expansion and the"shuttle effect"of lithium-sulfur batteries during charging and discharging.With the increment of 2-3 nm mesopores in the material,the better the electrochemical reaction kinetics and cycling stability exhibited.