Pore Structure Regulation Of Carbon Materials And Its Mechanism For Enhancing The Performance Of Room-temperature Sodium-sulfur Batteries

Although lithium-ion batteries play an important role in electric vehicles and mobile electronic devices as a new generation of energy storage devices,their application in large-scale power equipment has been unprecedentedly hindered due to limited theoretical capacity and the high price of metal lithium.Room temperature sodium-sulfur batteries（7）RT Na-S（8）have attracted widespread attention because of high theoretical capacity（7）1675 m Ah g^-1（8）、non-toxicity、low cost and rich sulfur content in the earth’s crust.However,RT Na-S batteries face many problems,such as low reversible capacity and rapid capacity decay.The low reversible capacity is because of the insulating properties of sulfur and the slow reaction kinetics of sulfur and sodium;Polysulfides dissolve into the electrolyte during the circulation process,that is the shuttle effect occurs,which is the key reason for the rapid capacity decay.Therefore,the reasonable selection of sulfur host plays a vital role in the development of RT Na-S batteries.From the perspective of chemical catalysis,the introduction of catalysts can accelerate the conversion rate of soluble polysulfides to insoluble S or Na₂S,reducing the content of polysulfides in the electrolyte,thereby increasing the utilization and cycle life of S.From the perspective of physical constraints applied to sulfur cathodes,carbon materials have higher conductivity,larger specific surface area,more versatile porous structure and lower cost,which is a better choice than other synthetic materials.Therefore,in this paper,we focus on regulating the pore structure of carbon materials and its enhancement mechanism for room temperature sodium-sulfur batteries.The main research contents and results are as follows:（1）Application of biomass-derived carbon materials with rich mesoporous network structure in room temperature sodium-sulfur batteriesThe waste biomass macadamia nut shells was used as the carbon source and potassium hydroxide as the activator to adjust the pore distribution of the material to create a rich mesoporous network structure to facilitate the mass transfer process,thereby limiting the dissolution of polysulfides.That rich mesopore-networked structure with rational pore structure design enables the reaction kinetics of Na₂S_n（7）n=1～8（8）to be tuned.Achieving excellent rate capability of 912 m Ah g^-1 at 0.1 A g^-1 and 360 m Ah g^-1at 5 A g^-1,which was more superior than most of the reported sodium-sulfur cathode.This study presents a method for predicting cycle life,and opening a promising avenue to achieve ideal electrode kinetics.（2）Application of biomass-derived carbon materials with different pore sizes in room temperature sodium-sulfur batteriesThe waste biomass pecan husks was used as the carbon source,the same method as in the previous chapter was used to control the pore size of biomass-derived carbon materials.Pore sizes of different sizes can match specific polysulfide intermediates in terms of molecular size,enabling unique reaction pathways that limit the shuttling effect.In particular,the ultra-small pore structure（0.37 nm）was tuned to lock the polysulfides in the pores until reduction to the final solid product Na₂S.Na₂S is uniformly deposited on the porous cathode surface to form a uniform Na₂S"film",which enables rapid diffusion of Na ions while suppressing the escape of polysulfides into the electrolyte.The discharge specific capacity is 933 m Ah g^-1 at a current density of 0.1 A g^-1;the decay rate per cycle is only 0.036%at a current density of 1 A g^-1 for 1500 cycles.（3）Preparation of metal organic framework derived porous nitrogen-doped carbon materials and their application in room-temperature sodium-sulfur batteriesThe synthesis method in this study was to introduce cobalt nitrate hexahydrate for co-precipitation together with the synthesis of metal-zinc organic framework compounds,followed by carbonization at high temperature,thereby obtaining nitrogen-doped carbon materials derived from metal-organic frameworks.Sites were occupied during framework formation,and the occupied sites are removed after annealing and cleaning to increase active sites during the reaction.At the same time,the material have a larger specific surface area and pore volume,which can better control the dissolution of polysulfides;and it also provides accommodation space for the volume expansion of polysulfides during battery charge and discharge,so as to improve the long-cycle stability of the battery.The capacity is 303 m Ah g^-1 after 850 cycles at a current density of 1 A g^-1,and the capacity decay rate per cycle is 0.047%.In summary,the systematic manipulation of pore structure and its enhancement of room temperature Na-S battery performance have been investigated,providing a new feasible way to realize high-performance RT Na-S batteries.