The Modification Of Sulfur Host And Separator For Enhanced Performance In Lithium Sulfur Battery

Lithium-sulfur battery?Li-S?is considered as one of the most promising candidates for the next-generation energy storage system due to its inherent merits such as high theoretical specific capacity of 1675 mAh g^-1 and energy density of 2600 Wh kg^-1,abundant source,low cost and environmental friendliness.However,the poor conductivity of sulfur leads to low active material utilization;the huge volume change during cycling would easily destroy the electrode structure.The dissolution of intermediate polysulfides leads to the so-called " shuttle effect" resulting in severe active material loss,low coulombic efficiency,and poor cycling life.To address these problems,graphitic carbon nitride?g-C₃N₄?was explored for inhibiting polysulfides shuttling in Li-S batteries inspired by the chemisorption of N-doped materials to polysulfide.g-C₃N₄ was combined with carbon materials to improve the sulfur electrochemistry properties.The influence of carbon structure on cell performance was also studied.Firstly,the highly conductive Super P was mixed with g-C₃N₄ as the sulfur co-host for enhancing cycling efficiency.Then g-C₃N₄ was mixed with macroporous carbon?MPC?which held much higher porosity and specific surface area,to serve as a stronger polysulfides absorber.Moreover,a g-C₃N₄@MPC hybrid was designed by generating g-C₃N₄ inside the MPC pores to provide more efficient sulfur confinement attributed to the synergistic combination of the components.The weight ratio of MPC to melamine?the precursor of g-C₃N₄?was optimized as 1:1.The obtained hybrid delivered a high nitrogen content of 8.08 wt.%and the according sulfur electrodes exhibited excellent cycling and rate performances.Though N-doped porous carbon could effectively absorb polysulfides and improve cell performance,the synthesis processes were complex and high cost which limited their practical applications.Therefore,the biomass watermelon rind?WR?riching in cellulose and citrulline was used as both carbon and nitrogen source.A sustainable approach of one-pot synthesis was also developed for the low-cost and high commercial availability of nitrogen-doped porous carbons?N-PCs?.The effects of pyrolysis temperature on pore properties and electrochemical properties of resultant Li-S battery were also investigated,resulting in an optimized condition of 900 ?.The resultant porous carbon?WR900?was endowed with a nitrogen content of 2.20 wt.%,high specific surface area of 1639.1 m2 g^-1,abundant microporous,and high conductivity.The WR900 based cells showed excellent electrochemical performance with high capacity of 847 mAh g^-1 after 200 cycles at 0.2 C rate,a minimized capacity decay rate of 0.14%per cycle,and outstanding rate capability of 640 mAh g^-1 and 496 mAh g'1 at 5 C and 10 C rate,respectively.By using WR900 as sulfur host could significantly improve the cell performance,but the slight capacity degradation still revealed the polysulfide shuttling in certain extent in the obtained Li-S batteries.Considering this,a novel separator?PBI-PEO@PP?functionalized by of polybenzimidazole?PBI?and polyoxyethylene?PEO?on conventional propene membrane?PP?was investigated to further prevent polysulfides escaping from PCs.The PEO in the modified membrane with strong electrolyte adsorption offered favorable Li+ transfer,while PBI was capable of building up a blocking layer for inhibiting polysulfide penetration.The balanced weight ratio of PEO to PBI was optimized as 1:1 for the best cell performance.Combining the WR900 based electrode with PBI-PEO@PP separator,the obtained Li-S cells achieved excellent electrochemical performance,i.e.,high initial discharge specific capacity of 1404 mAh g^-1,and high capacity retention of 920 mAh g^-1 after 200 cycles at 0.2 C.This study shows that the joint improvements of sulfur host materials and separator offer an effective way to enhance Li-S battery performances.The use of biomass in fabricating sulfur host materials delivers great practical potential in promoting the commercialization of Li-S battery.