A Study On Improvement Of Performance Stability Of Li-S With High S-loading By Using Porous Li-anode And Multifunctional Intermidiate Layer

Lithium-sulfur(Li-S)battery owns high theoretical energy density(2500 Wh/kg)with low cost,using sulfur as the positive active substance and metallic lithium as the negative active substance.Recently,the great progress in the specific capacity and rate performance demonstrates its promising potential for next-generation energy storage technology.However,most of developing Li-S batteries use sulfur electrode with low S-loading(≤2 mg/cm2),leading to the lower energy density battery than that of traditional Li-ion batteries.Therefore,it is necessary to fabricate high S-loaded electrode with high performance stability.Li2S segregation in the high S-loaded electrode causes the decrease in electrochemical reaction area,leading to the poor electric and ionic conductivities.As a result,the high S-loaded electrode usually shows sluggish electrode dynamics,low utilization of active substances,and rapid capacity decay.On the other hand,the increase in Li-ion transfer flux in the high S-loaded electrode leads to the acceleration of local lithium deposition,promoting growth of Li-dendrite accelerate the occurrence of local micro-shortcut in Li-S battery,resulting in the fast deterioration of the S-electrode and rapid failure of the Li-S battery.In order to solve the problems mentioned above,this work fabricates a MnO2-modified MWCNTs as the electrocatalyst which fills into an active carbon fiber felt(MWCNTs@MnO2/CF)to improve the electric and ionic conductivities of the high S-loaded electrode by using the MWCNTs@MnO2/CF as the intermediate layer between separator and the S-electrode.The fabricated MWCNTs@MnO2/CF intermediate layer enhances polysulfide(LiPSs)absorption and catalyzes electrochemical reaction of LiPSs significantly.This work also fabricate a porous Li-anode(Li@CuO/Cu-foam)for decrease in occurrence of Li-dendrite.The resultant battery using the MWCNTs@MnO2/CF and the Li@CuO/Cu-foam demonstrates excellent cycle life.The major achievements are summarized as follows:(1)Through investigation on the components in the cycled battery,it is found that the occurrence of Li-dendrite and the volume expansion derived from the development of Li-dendrite lead to the occurrence of local micro-shortcut which result in the rapid battery failure.A simulated cell with S anode and Li2S cathode is assembled for understand the cycling behavior of each active substance during battery cycling.It is found that the formation of the irreversible Li2S during cycling blocks mass transfer path,leading to the capacity decay of the S-electrode.As an example,irreversible formation of Li2S is promoted,leading to the serious Li2S segregation which induces Li-dendrite growth,when a Li-S battery with high S-loading is charged and discharged with a small current density.Therefore,the batteries cycled at smaller current densities show faster failure.(2)A multifunctional intermediate layer is fabricated through filling an MnO2-modified MWCNTs into an active carbon fiber felt(MWCNTs@MnO2/CF).The MWCNTs@MnO2/CF presents a 3D-conductive network rich in micro pores with a high specific surface area of 978.9 m2/g,which improves the wettability of the S-electrode toward electrolyte,decreases interfacial resistance,and promotes Li-ion transportation.MnO2 is able to catalyze conversion between Li2S2 and Li2S to decrease the LiPSs in electrolyte during electrodeposition.The flexible MWCNTs@MnO2/CF can adjust the volume expansion arising from the development of Li-dendrite to ease occurrence of local micro shortcut,enhance LiPSs adsorption to suppress polysulfide shuttle,and catalyze the conversion reaction of the absorbed LiPSs to depress segregation of Li2S.The porous MWCNTs@MnO2/CF enhances Li-ion transfer to decrease the induction of Li2S segregation toward Li-dendrite growth,thereby improving the cycle life of the Li-S battery with the high S-loading.Using the S-electrode with S-loading of 10 mg/cm2,the assembled Li-S battery with the MWCNTs@MnO2/CF presents an initial specific capacity of 1290 mAh/g at 0.1C,and keeps a capacity of 495.4 mAh/g after 501 cycles.(3)A porous Li-anode is fabricated through electrodeposition of Li to a CuO-modified Cu-foam(Li@CuO/Cu-foam).The fabricated Li@CuO/Cu-foam provides homogeneous electric-field in Cu-foam,and Li-affinitive interface for metallic Li nucleation,thereby reducing the overpotential of metallic Li nucleation to suppress the development of Li-dendrite.Furthermore,the large pore volume of the Li@CuO/Cu-foam provides sufficient spaces for metallic Li growth to reduce the induction of Li-dendrite toward the Li2S segregation.Using the S-electrode with S-loading of 10 mg/cm2,the Li-S battery with the Li@CuO/Cu-foam and the MWCNTs@MnO2/CF presents a specific capacity as high as 1221.8 mAh/g,and remains a specific capacity 425 mAh/g after 641 cycles at 0.1 C rate.