Study On Preparation And Performance Of Sulfur-based Cathode Composites For Lithium Sulfur Batteries

Lithium sulfur batteries have high capacity density(1675 mAh g^-1) and energy density(2600 Wh kg^-1), which make lithium sulfur batteries as an ideal choice of power battery. As a cathode material for lithium sulfur batteries,sulfur has many merits, such as inexpensiveness, natural abundance, and environmental friendliness. However, elemental sulfur and its reduction products?Li₂S₂/Li₂S?are electrical-insulating, which affects the utilization of active material. Lithium polysulfide is easy to dissolve in the electrolyte, which causes the loss of active composites. Volume changes happen during the cycles process, which seriously damages the electrode structure.In this thesis, three sulfur-based cathode composites for lithium sulfur batteries were synthesized. The composites can improve the electrical conductivity of sulfur, inhibit the dissolution of lithium polysulfide and possess excellent performances.?1? By controlling the relative content of sulfur and iron phosphate, a series of sulfur/FePO₄ nanocomposites were synthesized by chemical deposition mothod. The particle size is small and distribution is uniform. The results show that S-76.9 has the highest discharge specific capacity with iron phosphate of 23.1 wt %. The discharge capacity can reach 714.1, 496 and 411 mAh g^-1 at 0.1, 0.5 and 1 C, respectively. Based on the results, Sulfur/FePO₄/graphene oxide?SFGO? nanocomposites were fabricated. The electrochemical measurements show that SFGO nanocomposites deliver a high initial discharge capacity of 1337.8 mAh g^-1 at 1 C. After 100 cycles, SFGO nanocomposites still remain a discharge capacity of 608.9 mAh g^-1, with the coulombic efficiency is 94 %. The introduction of sulfur and iron phosphate effectively improve the ultilization of active material, surpress the dissolution of polysulfide and buffer volume change during cycles.?2? Sulfur/carbon nanotubes?SCNT? and sulfur /iron phosphate/carbon nanotubes?SFCNT? composites were synthesized by rheological phase reaction. The results show that most of sulfur infiltrates into carbon nanotubes by heated at 155? for 12 h under vacuum. Carbon nanotubes efficiently modified by iron phosphate. The unique structure effectively improves the stability and discharge capacity. The capacity retention is 21.1 %, 33.9 %, 43.4 %, 56.9 % and 61.6 % for SCNT, SFCNT- 155, SFCNT-500, SFCNT- 600 and SFCNT- 700 after 200 cycles at 0.5 C, respectively. The materials added iron phosphate possess higher cycle stability than the material without iron phosphate. SFCNT-700 shows better electrochemical performance among them. Carbon nanotubes and iron phosphate show a synergistic effect on improving the ultilization of active material, surpressing the dissolution of polysulfide and mantaining good stability of electrode structure.?3? Sulfur/carbon aergel composites were synthesized by chemical deposition mothod. The carbon aerogel is synthesized by hydrothermal method using graphene oxide as template and glucose as carbon source., nanosized sulfur is generated on carbon aerogel and graphene oxide by in situ chemical deposition method. The results show that the particle size is small and distribution is uniform. The cathode composite with sulfur of 66.7 % exhibits good electrochemical performance and high discharge capacity, the discharge capacity is 1258.5, 790.6, 631.4, 278 and 162.2 mAh g^-1 at 0.1, 0.2, 0.5, 1 and 2 C, respectively. The abundant oxygenated functional groups on the surface of graphene oxide enhance the interaction between sulfur and carbon,inhibit dissolution of polysulfide, A good contact with sulfur and carbon can improve the electrochemical performance for lithium sulfur batteries.