Oxide/Sulfide Solid Electrolytes For Realizing The Solidification Of Lithium Sulfur Batteries

Lithium-sulfur(Li-S)batteries comprising elemental sulfur as the cathode active material and lithium metal as the anode possess super high energy density(2600 Wh/kg)and theoretical specific capacity(1675 m Ah/g).In addition,sulfur is abundant in nature,while delivering low toxic and eco-friendly properties.Therefore,Li-S battery is considered to be one of the most promising high-energy secondary batteries.However,in conventional Li-S batteries based on organic liquid electrolyte,polysulfide intermediates will dissolve in the electrolyte and lead to severe shuttle effect,causing loss of active materials and corrosion of metal lithium,which is the main reason for low coulombic efficiency and capacity degradation.Besides,the sulfur and its discharge product Li₂S have poor electronic/ionic conductivity,meanwhile suffering from serious volume effect,which result in decline of the capacity.The metal lithium anode can react with the liquid electrolyte,which not only causes capacity loss,but also cause uneven deposition of metal lithium and growth of lithium dendrites,posing a risk of short circuit caused by puncture of the separator.Moreover,organic ether liquid electrolyte used in Li-S batteries may cause fire or even explosion if heated.In order to completely suppress the shuttle effect and solve the safety problems caused by organic electrolytes,solidification of Li-S batteries based on inorganic solid electrolytes is one of the most promising solutions.From the perspective of the structural design in solid-state lithium-sulfur batteries,the corresponding oxide solid electrolyte and sulfide solid electrolyte were prepared,and unique structures for hybrid electrolyte and sulfur cathode were designed.The quasi-solid-state and all-solid-state Li-S batteries eliminating the shuttle effect of polysulfides were successfully assembled,providing new ideas and insights for practical application of Li-S batteries.The specific research contents are as follows:(1)High-density(96%)LAGP ceramic electrolytes with high ionic conductivity of 3.38×10^-4 S/cm at room temperature were prepared.Li-S batteries containing LAGP ceramic can terminate the shuttle of polysulfides,maintaining a high Coulomb efficiency of 100%.Aimed at reducing the interfacial resistance between the ceramic electrolyte and the electrodes,a small amount of liquid electrolyte was added to wet the interfaces on both sides.A carbon coating layer was constructed on the surface of the LAGP facing the cathode as the second current collector,which can absorb polysulfides dissolved in the wetting electrolyte and promote their conversion in the subsequent electrochemical reaction,thereby reducing the capacity loss of the battery.(2)In the quasi-solid-state Li-S battery with LAGP ceramic as the electrolyte separator,trace amount of liquid electrolyte with high concentration of lithium salt(SIS)was used to wet the interface between the ceramic and the sulfur cathode.SIS not only reduced the interfacial impedance,but also inhibited the dissolution of polysulfides,keeping the polysulfides produced around the sulfur-carbon particles.The dense LAGP ceramic can thoroughly block the shuttle of the very small amount of polysulfides dissolved in the SIS.That is,the SIS-LAGP hybrid electrolyte system has formed a"dual confinement effect"for polysulfides.The quasi-solid-state Li-S battery based on the SIS-LAGP hybrid electrolyte delivers excellent cycling stability.The specific capacity of the first discharge at 0.2 C is as high as 1251.7 m Ah/g,and the capacity decay rate is only 0.07%in 250 cycles.Finally,the Interface Split Effect is put forward to explain the mechanism based on the EIS analysis.(3)Sulfide electrolyte Li₇P₃S₁₁ was synthesized by liquid phase method,and then combined with ionic liquid(IL),elemental sulfur,and conductive carbon to prepare a toothpaste-like sulfur cathode.The Garnet-type ceramic electrolyte LLZTO is selected as the main electrolyte and support of the solid-state Li-S battery.The toothpaste-like cathode is coated on the surface of the LLZTO to form a good interfacial contact.The metal lithium anode is attached onto the other side of LLZTO ceramic with Au coating layer via a melt process.This solid-state Li-S battery has a small internal impedance,and shows a high discharge capacity and cycling stability.(4)With cotton as the natural carbon source,dispersed carbon fiber was obtained through hydrolysis in concentrated sulfuric acid and carbonization at high temperature successively,and then a layer of Li₇P₃S₁₁ electrolyte was coated on its surface in situ by liquid phase method.Mixed and co-heated with elemental sulfur resulted in the formation of composite sulfur cathode material,in which the sulfur simultaneously has close contact with the ionic conductor and the electronic conductor.To cooperate with the composite cathode,we prepared a Li₆PS₅Cl electrolyte with a room temperature ionic conductivity of 5.24×10^-3 S/cm through solid-phase reaction method,and assembled an all-solid-state Li-S battery based on the composite cathode and Li₆PS₅Cl electrolyte,along with metal lithium as the anode.The battery exhibits a very high initial discharge capacity of 32.8 m Ah(corresponding to 22 m Ah cm^-2)at 60 ^oC.