Hierarchical Porous Carbon Based Cathode Materias For High Performance Lithium-Sulfur Battery

With electric vehicles and grid-level energy storage showing up,the limited theoretical energy density of conventional Li-ion batteries makes them difficult to meet the requirments.Lithium-Sulfur?Li-S?battery is considered as the most promising electrochemical energy storage technology due to their remarkably high theoretical specific capacity?1675 mA·h/g?abundant natural content,low cost and environmental benighty.However,there are also several challenges that ought to overcome.First,the low conductivity of sulfur and Li₂S₂ cause low utilization of active materials.Second,serious volumetric expansion of sulfur on lithiation?up to80%?leads to irreversible structural damage.Last,high soluble intermediate polysulfides(S^2-_4-6)in organic electrolyte deposit on the surface of lithium,which seriously reduces the utilization rate of active material and gives rise to severe capacity fading.In order to overcome these problems,we have investiaged microprous carbon,mesoporous carbon and find if we combine these two kinds of carbon together to fabricate hierarchically meso-microporous carbon can improve the low conductivity of sulfur and Li₂S,and also relieve the volume expansion and shuttle effect.The main contents and results are summarized as follows:?1?Nitrogen-doped hierarchically meso-microporous carbon?N-MMC?nanocage and used it as sulfur host.We also prepare nitrogen doped mesoporous carbon?N-MesoC?,nitrogen doped microporous carbon?N-MicroC?and pure sulfur for comparasion.The results show N-MMC possesses large specific surface area,high pore volume and inner-connected nanometer pores.It provides enough space for high sulfur loading and volume expansion facilitates electron and lithium ions transportation and confines the loss of soluble polysulfides,thus leading to high capacity and improvements in cycling stability.N-MesoC exhibits continuously decreased capacity indicative of a high dissolution of polysulfides.The cycle stability of N-MicroC/S was almost as good as N-MMC/S,but it maintained lower initial specific capacity because of relatively low sulfur loading.?2?The traditional method using hard and soft template to synthesize hierarchically porous carbon is complicated,and hard template remaining is difficult to remove completely.This will cut down the proportion of active material,decrease the current density and capacity of the battery,and shorten the cycle lifespan.The biomass material original from carbon source is used to prepare nitrogen-doped porous carbon.After using KOH to activate carbon,the pores blocked by impurities such as tars were eliminated and the pore volume,the specific surface area also improved.The results of a series of electrochemical tests show that the initial specific capacity and the long cycle stability of WSAC-2/S improved immensely compared with the pure sulfur.?3?The contents of mesopore and micropore have great influence on the specific capacity and cycling stability of the battery.However,there are few papers focusing on micropore content in hierarchically porous carbon.The WSAC-8 and WSAC-5 are synthesized by changing the ratio of KOH and carbon,and the heating rate.The proportion of micropore volume in material were 68.3%and 77.6%respectively,and the pore diameter was about 0.46 nm,and the diameter of mesoporous pore was also closed.The results of a series of comparative studies on electrochemical properties show that the WSAC-8/S has the best initial capscity and cyclic stability,indicating that the higher the micropore content does not mean better the performance of the battery,but the appropriate content between the mesopore and the micropore contributes to better performance of the cathode material.