Preparation Of Modified Separator Based On CoS₂ And Its Application In Li-S Batteries

As one of the most promising energy storage devices,lithium-sulfur（Li-S）batteries have received extensive attention because of its extremely high theoretical energy density(2570 Wh kg^-1),along with the low cost and environmental benignity properties of sulfur（S）.Despite these attractive merits,its practical application is still constrained by several inherent and critical drawbacks.Firstly,the insulating nature of S and its discharge products（Li₂S and Li₂S₂）result in sluggish electrochemical kinetics.Secondly,during the cycling process,highly soluble polysulfides（Li PSs）dissolve in the electrolyte and migrate to the anode side under the effect of the concentration field.Subsequently,undesired parasitic reactions occur between Li PSs and Li metal,leading to the loss of active species.These obstructions will lead to low utilization of sulfur,inferior cycling stability and poor rate performance.Therefore,it is crucial that Li PSs are anchored and their conversion reactions are catalyzed in order to improve the performance of Li-S batteries.In this thesis,ZIF-67-derived carbon polyhedra decorated with ultrafine cobalt disulfide nanoparticles（CoS₂）are interconnected by carbon nanomaterials.The uniformly distributed CoS₂nanoparticles exhibits a strong polar Co-S bond with Li PSs,thereby can anchor Li PSs and act as an electrocatalyst in propelling the Li PSs conversion process.Therefore,CoS₂is regarded as one of the ideal materials for separator modification.Furthermore,both the ZIF-67-derived carbon polyhedra and carbon nanomaterials contribute to fast electron transport due to their excellent electrical conductivity,leading to excellent rate and cycling performance.In this dissertation,CoS₂-NC@RGO and CoS₂-NC@CNTs composite structures are designed based on CoS₂nanoparticles for the separator modification.The specific research contents are as follows:A special three-dimensional structure was formed by in situ growing a large number of carbon polyhedra embedded with CoS₂nanoparticles on two-dimensional reduced graphene oxide nanosheets（RGO）.The anchor-catalysis modified separator（CoS₂-NC@RGO-pp）was prepared by coating it on conventional separator（labelled as PP）using a simple vacuum filtration method.The presence of CoS₂nanoparticles can immobilize Li PSs and catalyze their conversion of solid-to-liquid-to-solid reactions,which enhances the redox kinetics.At the same time,the high conductivity of RGO is crucial to the performance of the battery.Thanks to the anchor-catalytic function of CoS₂-NC@RGO,the specific capacity of the battery reaches 848.6 m Ah g^-1at the current density of 2 C.Under the high current density of 0.5 C,the discharge specific capacity of the first cycle reaches 1165.8 m Ah g^-1.In order to solve the agglomeration problem that occurs during the in situ growth of carbon polyhedra,a novel three-dimensional conductive network structure material was prepared using one-dimensional carbon nanotubes（CNTs）instead of RGO.Subsequently,the anchor-catalysis modified separator（CoS₂-NC@CNTs-pp）was prepared by coating it on commercial PP separator using a vacuum filtration method.This three-dimensional network structure formed by CNTs can alleviate the agglomeration of CoS₂nanoparticles and maximize the exposure of catalytic active sites for strong chemisorption and catalytic conversion of Li PSs.Furthermore,the introduction of CNTs provides strong mechanical strength preserving the structural integrity in the charge/discharge process.Simultaneously,it promises fast electron transportation,which contributes to the high rate performance of the battery.Besides,the three-dimensional network structure with abundant micro/mesopores enables better penetration of the electrolyte and physically blocks Li PSs.The batteries were assembled using CoS₂-NC@CNTs-pp and show a high initial discharge capacity of 1408.5 m Ah g^-1at 0.1 C and 1186.7 m Ah g^-1at 1 C,and they remain outstanding cycling stability for over 500 cycles with a low capacity decay rate of 0.095%per cycle.Even at a high rate of 5 C,it still can deliver a high discharge capacity of 832.6 m Ah g^-1.