Research On Controllable Construction Of Microspherized Sulfur Cathodes For Lithium Storage Applications

Lithium-sulfur batteries（LSBs）exhibit high theoretical weight/volume energy density(E_G/E_V:2600 Wh kg^-1/2800 Wh L^-1)on account of two-electron reactions of sulfur（S₈）cathodes（S₈+16 Li⁺+16 e^-→8 Li₂S）;such specific energies are much higher than that of lithium-ion batteries(Max.:600 Wh kg^-1).Due to the abundant reserves of S₈and its environmental-friendly nature,LSBs have become the most promising secondary battery system,with high energy density and free of any heavy metal pollutions.It may play a crucial role in pushing forward the battery-related industries,such as portable mobile power supplies and large-scale energy-storage systems.However,several inherent problems have emerged during the practical application of LSBs like:i)low Coulomb efficiency and short cyclic life,which are induced by the insulation property and volume changes of sulfur and its derivatives in discharging stages,and the adverse polysulfide shuttle effects;ii)Imbalance between E_Gand E_Vof LSBs in practical applications.To achieve high-energy and practical LSBs,we propose to improve the tap density and S₈loading in cathodes by means of pathways below:（1）Constructing S₈microspheres via high-temperature methods to achieve high tap density and high-sulfur-content cathodes,and balance E_G/E_Vin LSB pouch cells:A unique high-temperature synthesis approach is developed to improve the heat resistance of S₈（from 200 to 400 ^oC）and prevent S₈loss by gradually encapsulating polypyrrole and dense silica protective layers onto S₈/NiFe₂O₄QDs（quantum dots）microspheres.Under a high-temperature condition,the polymeric shells would be dehydrogenated and further carbonized.Unlike traditional S₈/carbon composite cathodes made via injecting elemental S₈into carbons,our produced S₈/NiFe₂O₄QDs@N-rich carbon microspheres share the closely packed feature similar to commercial ternary oxides,with a high tap density of 2.12 g cm^-3and a large sulfur content more than 70 wt%.Such closely-packed nature greatly facilitates the reduction of charge-transfer resistance.Besides,abundant N atoms involved in shelly regions can serve as polar chemical sites,benefiting the adsorption/physical confinement to polysulfides and mitigating the volume expansion of sulfur-based actives in discharging stages.Moreover,NiFe₂O₄QDs uniformly dispersed in S₈microspheres can catalyze the phase conversions of polysulfides and thus expedite the redox reactions.Even in lean electrolyte/high S₈loading(4.2μL mg^-1/4.95 mg cm^-2)pouch cells,the E_Vand E_Gof S₈/NiFe₂O₄QDs@N-rich carbon cathodes can be well balanced to the upper levels of 567.95 Wh L^-1and 388.37 Wh kg^-1,respectively.（2）Constructing S₈@N-rich carbon@Se_0.06SPAN micro-fibers by electrospinning techniques to achieve advanced carbonate-electrolyte-based LSBs:S₈@N-rich carbon microspheres are trapped in Se_0.06SPAN shell layers via electrospinning methods,and the total content of S₈actives reaches up to a high level of 69.3 wt%.After initial discharge,CEI films would be formed on the outer surfaces of Se_0.06SPAN shells and remain stable in subsequent cycles,preventing the penetration of carbonate electrolyte.The reversible solid-phase conversions（S₈/Li₂S）would completely eliminate the capacity losses caused by polysulfides shuttling.The micro-fiber itself provides the one-dimensional electron-transfer channels;tight connections built between the shell and inner S₈microspheres facilitate Li⁺transportation,thus elevating the S₈utilization rate.Also note the Se doping increases the ion/electron conductivity of cathode systems(13.4×10^-8cm²S^-1/5.12×10^-9S cm^-1).As demonstrated,the configured LSBs exhibit a large capacity of 402 m Ah g^-1at 6 C;when tested in lean carbonate electrolyte(E/S ratio≤4.5μL mg^-1)and high sulfur loading(>4 mg cm^-2)conditions,cells show a remarkable reversible capacity of 632.5 m Ah g^-1after 1000 cycles（current rate:0.2 C）,and the Coulomb efficiency keeps stable around 100%.