Preparation Of Fluorine-doped Tin Oxide And Its Application In Lithium-sulfur Batteries

Elemental sulfur owns highly theoretical specific capacity ?1672 mA h g-1?, highly theoretical specific energy ?2600 W h kg-1?, extensive source and low cost, so lithium-sulfur batteries as a clean, pollution-free and efficient energy storage and conversion apparatus are received attention widely. However, lithium-sulfur batteries are still facing with many severe challenges, such as electrically and ironically insulated nature of sulfur, polysulfide dissolved in organic electrolyte, the volume expansion of sulfur and so on. These problems lead to low utilization of active material, low rate capability and poor cycle stability, thus seriously hindering the pace of the practical application for lithium-sulfur batteries.The matrix of lithium-sulfur battery cathode composites mainly includes carbon materials, conductive polymer and metal oxide to improve conductivity and adsorption properties of sulfur, as well as modified carbon material and conductive polymer, but the study of modified metal oxide matrix is rarely reported. In order to improve the conductivity of the cathode, this paper improve the conductivity of metal oxide using doped method, which finally realizes to improve the electrochemical properties of lithium-sulfur battery. The paper has prepared hollow tin dioxide with controllable fluorine doping ?SnO₂/F-n? used templated and hydrothermal method, SnO₂/F-n were physically characterized by SEM, TEM and XRD. Then, the SnO₂/F-n/S-76 composites were prepared using SnO₂/F-n and 76 wt% of elemental sulfur with the method of immersion and diffusion, the physical characterization of SnO₂/F-n/S-76 were tested by TGA, FESEM and STEM, the electrical characterization of SnO₂/F-n/S-76 were tested by constant current charge-discharge method, CV and EIS. The paper also studied the influence of different current densities, different binder and collector modification for electrochemical properties of composite materials. In this paper, the main research contents and conclusions are as follows:?1? If fluorine-doped amount is less than 3.70 At%, the hollow SnO₂ shell is intact. The amount of fluorine-doped increases to 4.38 At%, the hollow nanostructures SnO₂/F-3 are collapsed. The thickness for hollow tin oxide with different amounts of doping fluorine ?SnO₂/F-n? is approximately 10 nm. SnO₂/F-n has different specific surface area and resistivitv. As the amount of fluorine-doped is 3.70 At% in SnO₂ ?SnO₂/F-2?, the minimum resistivity is 62.3 ? cm. SnO₂/F-n/S-76 composites which were prepared with SnO₂/F-n and 76 wt% sulfur were tested electrical properties, it is find that the electrical properties of SnO₂/F-2/S-76 composites are best. The highest reversible specific capacity of SnO₂/T-2/S-76 is 899.8 mAh g-1 and 797.5 mAh g-1 after 50 cycles at 0.5 C and 1 C rate. The redox peaks intensity of SnO₂/F-2/S-76 is highest, the minimum combination resistance ?Ro? and charge transfer resistance ?Rct? of SnO₂/F-2/S-76 is 4.87 ? and 7.43 ?, indicating that SnO₂/F-2 is favorable for anchoring polysulfide and better able to suppress polysulfide dissolved organic electrolyte. Therefore, the optimum amount of fluorine doped in hollow tin oxide is approximately 3.70 At%.?2? Based on the optimum amount of doping fluorine, SnO₂/F-2/S-X ?X is 67,76 and 81? composites with different carrying capacity of sulfur were prepared and tested, it is obviously find that SnO₂/F-2/S-76 composites has the most excellent cycle performance. SnO₂/F-2/S-81, SnO₂/F-2/S-76 and SnO₂/F-2/S-67 composites own capacities of 556.2, 899.8 and 665.3 mA hg-1 and own capacity retention rate of 37.5%,66.6% and 66.2% after 50 cycles at 0.5 C rate. Besides, SnO₂/F-2/S-81, SnO₂/F-2/S-76 and SnO₂/F-2/S-67 composites have capacities of 466.3,797.5 and 558.4 mA h g"1 and have capacity retention rate of 34.0%,62.8% and 61.1% after 50 cycles at 1 C rate. With increasing sulfur loading, the Ro and Rct of SnO₂/F-2/S-X increases. Based on the analysis for the cycle performance and EIS curves of SnO₂/F-2/S-X, composites with about 76 wt% sulfur loading ?SnO₂/F-2/S-76? possess better electrical properties.?3? SnO₂/F-2/S-76 composites can still maintain good cycle characteristics at higher rate, the specific capacity and capacity retention rate is 616.7 mA h g-1 and 51.5% after 50 cycles at 2 C rate, respectively. Ethyl cellulose ?EC?, polyvinylidene fluoride ?PVDF? and polyvinyl pyrrolidone K30 ?PVPK30? as binder for SnO₂/F-2/S-76, the cycle performance of them at 1 C rate is contrasted, the initial discharge capacities of SnO₂/F-2/S-76 composites is 1167.1,1269.9 and 1163.5 mAh g-1, the capacity retention rate is 57.7%,62.8% and 70.8%, respectively. And the Ro and Rct are minimum with PVPK30 binder, the Ro and Ret are maximum with EC binder. So the sequence of bonding properties for the above three binders is PVPK30> PVDF> EC. SnO₂/F-2/S-76 composites as cathode, the initial discharge capacity and reversible capacity of battery is 1402.6 mA h g-1 and 826.0 mA h g-1 using Ni collector modified with Pt after 50 cycle at 1 C rate. Compared with the unmodified Ni collector, cycle performance is enhanced, and Ro and Rct resistance is reduced, demonstrating that Pt plays a catalytic role in the electrochemical reaction.