Design And Studies Of Cathode Materials And Separator Modifiers For High Energy Density Lithium-Sulfur Batteries

Lithium-sulfur batteries(LSBs),known for their high theoretical energy density,suffer from the poor conductivity of sulfur(S),the shuttle effect of poly sulfides(PSs),and the volume expansion during cycling.These issues become more severe when LSBs with high sulfur contents and high sulfur loading.Herein,this thesis focuses on the design of cathode and the modification of separator to solve these problems.The main content is as following:(1)S microsphere was encapsulated within a thickness-tunable multilayered thin V/V2O5 nanoshell(S@V/V2O5)as a high-sulfur-content cathode for LSBs.The polar V2O5 can chemically adsorb PSs,the metallic V can ensure efficient electron transfer,and the multilayered nanoshell can also buffer the volume expansion during cycling.Owing to these merits,the S@V/V2O5 electrode with a S content of 87 wt%can deliver high initial specific capacity of 1403 mAh g-1 and areal capacity of 4.4 mAh cm-2 at 0.2 C,high retention capacity of 819 mAh g-1 at 0.5 C after 300 cycles and rate capacity of 804 mAh g-1 at 4 C.The electrode can also exhibit low self-discharge of 759 mAh g-1 after resting 50 days,and high Coulombic efficiency above 90%with electrolyte without LiNO3.Moreover,even the S content up to 93 wt%,the S@V/V2O5 cathode without current collector can achieve a stable capacity of 1000 mAh g-1 after 100 cycles at 0.2 C.(2)NiCO2O4/CNF composite was introduced as a separator modified layer to chemically trap PSs and accelerate its conversion.Experiment and density functional theory(DFT)demonstrated that the polar NiCo2O4 can strongly trap PSs through S-Ni and S-Co bonds.The CNF network can serve as a second barrier for PSs,and provide structural stability.Systematic studies on the solid-liquid-solid conversion reactions of S qualitatively and quantitatively demonstrated that the well-designed NiCo2O4/CNF interface with strong PSs affinity,affluent polar nucleation sites and fast Li+/e-transport can noteworthily catalyze the liquid phase conversion of PSs,the solid phase deposition of lithium sulfide(Li2S),and protect the lithium anode as well.Thus,the LSBs with NiCo2O4/CNF-PP separator can display a high-rate capability of 870 mAh g-1 at 5 C and a capacity retention of 95%at 0.1 C after 200 cycles with a S loading of 6.3 mg cm-2.Even with a S loading of 7.9 mg cm-2 and a lean electrolyte/sulfur ratio of 7.2 μl mg-1 s,the LSBs can exhibit a high areal capacity of 8.1 mAh cm-2.