Preparation And Electrochemical Performance Of Cathode Materials For Lithium-Sulfur Batteries

Sulfur possesses high theoretical discharge specific capacity and low cost, which makes lithium-sulfur batteries promising in the future. As coulombic efficiency of lithium-sulfur batteries changes in cyclic charge and discharge, effect of cycle numbers, current densities and lithium nitrate on coulombic efficiency has been analyzed in the thesis. Sulfur-carbon nanofibers mixture has been fabricated by direct milling. Morphology, structure and electrochemical performance of sulfur-carbon nanofibers mixture have been tested. Results show that sulfur is evenly mixed with carbon nanofibers. In the sulfur-carbon nanofibers mixture, sulfur disperses on the surface of carbon nanofibers. Discharge specific capacity of sulfur-carbon nanofibers mixture electrode decreases when the cycle number increases. When there is no lithium nitrate in electrolyte, initial coulombic efficiency of sulfur-carbon nanofibers mixture electrode is 73.4% at current density of 100 mA g-1. The coulombic efficiency rises to 96% when the cycle number increases to 30, which is linked with gradual passivation for the surface of lithium electrode. As the cycle number further increases, the coulombic efficiency keeps relatively stable. Higher current density does not obviously change coulombic efficiency. When there is lithium nitrate in electrolyte, initial coulombic efficiency of sulfur-carbon nanofibers mixture electrode is 95.4% at current density of 100 mA g-1. The coulombic efficiency stabilizes at 98.4% when the cycle number increases. The relatively high initial coulombic efficiency is ascribed to accelerated passivation for the surface of lithium electrode through lithium nitrate. Higher current density does not apparently affect coulombic efficiency. In comparison with coulombic efficiency of sulfur-carbon nanofibers mixture electrode, coulombic efficiency of sulfur-acetylene black mixture electrode exhibits similar change.Sulfur-carbon matrix composite has been synthesized to improve cyclic discharge stability of sulfur. Carbon precursor was fabricated by carbonizing sucrose with concentrated sulfuric acid. Sulfur was obtained through chemical reaction between sulfuric acid and sodium thiosulfate. Carbon precursor was deoxidized to carbon matrix through hydrate hydrazine. Then sulfur-carbon matrix composite was synthesized. Morphology, structure and electrochemical performance of sulfur-carbon matrix composite have been tested. Results show that sucrose is not completely carbonized by concentrated sulfuric acid. There is carbonyl group in the carbon precursor. The carbonyl group in carbon precursor is removed by hydrate hydrazine. Carbon precursor and carbon matrix are composed of lamellas. In sulfur-carbon matrix composite, sulfur is evenly dispersed. There is carbon coating on the surface of sulfur-carbon matrix composite. Compared with sulfur-acetylene black mixture electrode, sulfur-carbon matrix composite electrode exhibits higher discharge specific capacity and cyclic discharge stability. When the current density of charge and discharge is 100 mA g-1, discharge specific capacity of sulfur-acetylene black mixture electrode is 370 mAh g-1 after 100 cycles and discharge specific capacity of sulfur-carbon matrix composite electrode is 846 mAh g-1 after 100 cycles. Carbon coating on the surface of sulfur-carbon matrix composite hinders dissolution of lithium polysulfides in electrolyte, which reduces the dissolution amount of active material.Carbon coating-sulfur-acetylene black composite has been synthesized to reduce the dissolution amount of active material. Sulfur-acetylene black mixture was obtained by direct milling. Aluminum powder was used to deoxidize carbon precursor. The deoxidized carbon wrapped sulfur-acetylene black particles. Then carbon coating-sulfur-acetylene black composite was synthesized. Morphology, structure and electrochemical performance of carbon coating-sulfur-acetylene black composite have been tested. Results indicate that sulfur is evenly mixed with acetylene black particles. In sulfur-acetylene black mixture, sulfur envelops acetylene black particles. There are no Raman bands of sulfur in the Raman spectrum of sulfur-acetylene black mixture. In acid solution, aluminum powder removes the carbonyl group in carbon precursor. Carbon coating wraps sulfur-acetylene black mixture particles in carbon coating-sulfur-acetylene black composite. When current densities of charge and discharge are 100 mA g"1 and 200 mA g"1, discharge specific capacities of carbon coating-sulfur-acetylene black composite electrode are 886 mAh g-1 and 808 mAh g-1 respectively after 100 cycles. In comparison with sulfur-acetylene black mixture electrode, carbon coating-sulfur-acetylene black composite electrode has higher cyclic discharge stability. The higher cyclic discharge stability is attributed to hindrance for dissolution of active material in electrolyte through the carbon coating on the surface of sulfur-acetylene black mixture particles.Sulfur-carbon-carbon nanofibers composite has been synthesized to increase the utilization rate of sulfur. After sulfur-carbon precursor composite sol was obtained, carbon nanofibers were dispersed in the sulfur-carbon precursor composite sol to obtain sulfur-carbon precursor-carbon nanofibers composite. The carbon precursor was deoxidized subsequently. Then the sulfur-carbon-carbon nanofibers composite was synthesized. Morphology, structure and electrochemical performance of sulfur-carbon-carbon nanofibers composite have been tested. Results indicate that sulfur is evenly dispersed in the sulfur-carbon-carbon nanofibers composite. Sulfur-carbon composite envelops carbon nanofibers. In the X-ray diffraction pattern of sulfur-carbon-carbon nanofibers composite, there are diffraction peaks of sulfur, carbon and carbon nanofibers. In sulfur-carbon composite, sulfur chemically binds with carbon. The chemical bond forms in the process of deoxidizing the carbon precursor. There are no Raman bands of sulfur in the Raman spectrum of sulfur-carbon nanofibers mixture. When the current density of charge and discharge is 100 mA g-1, discharge specific capacity of sulfur-carbon nanofibers mixture electrode is 440 mAh g-1 after 100 cycles and discharge specific capacity of sulfur-carbon-carbon nanofibers composite electrode is 813 mAh g-1 after 100 cycles. Discharge specific capacity of sulfur-carbon-carbon nanofibers composite electrode is higher than that of sulfur-carbon nanofibers mixture electrode, which is attributed to adsorbability of carbon for active material in the sulfur-carbon-carbon nanofibers composite.