Preparation Of Triamine/Graphene-based Materials And Its Application In Lithium-Sulfur Batteries

Graphene oxide(GO),as a highly chemical oxidation product with many oxygen-containing functional groups,which not only helps to improve the solution processability,but also has been selected as the precursor to prepare graphene and its hybrid derivatives,thus regulating the structure and performance.Currently,graphene-based three-dimensional interconnected frameworks have received more attention in wrapping,absorbing and storing nanoparticles for a wide application,such as energy storage devices.Lithium-sulfur batteries as one of the most promising energy storage devices still have many challenges,including the poor electronic conductivity and ionic conductivity of elemental sulfur,the volume expansion of the sulfur during charge and discharge cycles,the shuttle effect caused by lithium sulfide,which greatly limit the development of lithium-sulfur batteries.Using nitrogen-doped GO as the sulfur host can form a strong electrostatic interaction with lithium polysulfide,thereby inhibiting the shuttle effect of lithium polysulfide.Based on this concept,we used triamine as a cross-linking agent to prepare three-dimensional nitrogen-doped graphene composites,which are then used in high-stability lithium-sulfur batteries.The specific content is summarized as follows:(1)Preparation and properties of GO/TAPA composites with different nitrogen contents.By changing the content of TAPA in the mixed system,graphene-based composites with different nitrogen content and three-dimensional network structure were prepared according to the amidation reaction mechanism at room temperature.The phase separation phenomenon is also discussed during the process of loading sulfur,and the combination of the solution method and the heat treatment melting method was finally determined to load the sulfur and partially reduce the graphene oxide.The results show that the content of nitrogen in the composite increases with the doses of TAPA,and the nitrogen content is positively correlated with the sulfur loading capacity.Furthermore,a series of electrochemical tests have shown that the S@rGO-TAPA composite with the highest nitrogen content which has the best electrochemical performance.The S@rGO-TAPA cathode shows a high initial discharge capacity of 965 mAh/g and the capacity can be remained at 66.8%after 100 cycles at 0.2 C.(2)Preparation of S@rNGO composites and their application in lithium-sulfur batteries.The physical properties of different supports and the electrochemical properties of the composite electrode were systematically analyzed by introducing amines with different segment structure and polarity into the graphene oxide sheet through covalent bonding.The results indicate that,unlike the TAPA and TABP,the aliphatic TAEA links to the graphene oxide sheet by a nucleophilic reaction.In contrast,the nitrogen content of GO-TAEA reaches 13.35%,which is the highest in the designed system.However,it is precisely because of the soft segment structure of TAEA that the composite carries the lowest sulfur content and has the lowest decomposition temperature of sulfur.TAPB as the most rigid structure in the designed system can be used as the crosslinking interconnector for carrying the highest sulfur content up to 72.6%.With the help of the benzene ring structure in TAPB,the composite cathode shows a fast electrochemical reaction.However,the weak polarity of TAPB results in a fast capacity decay during long cycles.In addition,TAPA that has the proper rigid-flexible structure and strong polarity shows the most prominent of the cycle stability and rate performance.(3)Preparation of GO-TAPA/PP separator and its application in lithium-sulfur batteries.High-performance separators can inhibit the shuttle effect of lithium-sulfur batteries through physical and chemical interactions.The GO-TAPA/PP separator was prepared by vacuum filtration method using GO-TAPA composite as a coating layer.This functionalized separator helps to impregnate and preserve the electrolyte,thereby improving the electrochemical reaction kinetics and effectively suppressing the self-discharge.In addition,the first discharge specific capacity of the battery assembling with GO-TAPA/PP reaches 1267 mAh/g,and the capacity can be remained at 70.2%after 100 cycles at 0.2 C.Compared with the PP separator,the GO-TAPA/PP separator helps to improve the utilization of the active material and the cycle stability due to the polar surface.In this study,nitrogen-doped graphene composites with an intrinsic three-dimensional network structure were constructed.The composites can not only achieve uniform loading of the sulfur by the solution method,but also partially reduce the graphene oxide and further disperse the sulfur particles by the melting method.Furthermore,the nitrogen doping graphene can be used as a coating layer of the PP separator to improve the lithium sulfur battery.