Preparationand Electrochemical Performance Study Of Three-dimensional Graphene Based Lithium/Sulfur Battery Electrode Materials

Compared with traditional lithium-ion battery, lithium/sulfur (Li/S) batteryas a new type of secondary battery, has high theroretical specific capacity, highspecific energy, and the sulfur is abundant in earth, environment friendly andlow-cost, thus Li/S battery is promising in wearable electronic equipments andnew-type energy vehicles. However, there are still several challenges indeveloping practical Li/S batteries, for instance, the poor electrical conductivityof sulfur, the dissolution of lithium polysulfides (Li2Sx,4≤x≤8) into theelectrolyte and shuttling thus leading to significant capacity decay,expansion/contraction of sulfur cathode during cycling resulting cathodecollapse. Based on the above understanding, the following works were carriedout to solve the current problems of Li/S battery:(1) A high-density nanosized sulfur-three dimensional (3D) graphene(dense S-G) was synthesized in this work. The graphene sheets could supplyelectron as needed during the reaction between S and lithium ion (Li+) with the excellent electronic conductivity. The nano-sized S could contact with Li+verywell to promote the redox reaction with the large specific surface area.Furthermore, The dense S-G structures could better immobilize sulfur andrestrain the polysulfide diffusion and shuttle within the abundant more closedpores, on the premise that maintain the ion conductivity, thus obtaining highdischarge specific capacity and cycling performance. The initial dischargecapacity of dense S-G cathode is1360mAh/g at0.05C, nearly85%of thetheoretical capacity of S, and shows a capacity of770mAh/g after300cycles.(2) Anano-sized sulfur-porous activated carbon/3D graphene gel (S-ACG)was synthesized in this work. The active S matrix, ACG could provide ahierarchical outer meso-pores coating inner micro-pores reservoir structure, andrestrain polysulfide dissolution at two interfaces, on the base of suitable Sloading. Compared with S-AC(activated carbon) and S-G(3D graphene)composites with single-layer pores structure, this double-layer pores structurecould improve the resistance to polysulfide effectively, and get higher dischargespecific capacity and longer cycling performance. The discharge capacitymaintain834.7mAh/g at0.2C after300cycles,722.8mAh/g at0.5C after500cycles and410.6mAh/g at1C after900cycles.