Supercritical Fluid Deposition Of Cathode Materials For Lithium-sulfur Battery

Lithium-sulfur?Li-S?battery has attracted extensive attention nowadays on account of its surpassing theoretical specific capacity?1675 mAh/g?and energy density?2600 wh/kg?,as well as low cost of raw materials and pollution-free feature.However,its practical application is hindered by the poor electronic conductivity of sulfur and product of discharging?Li2S or Li2S2?as well as the well-known shuttle effect.In this context,confine of sulfur in three-dimensional porous structure of conductive materials,has been proposed as a promising solution to alleviate the shuttle effect and improve the electrical conductivity of the sulfur cathode.For this purpose,a variety of methods,including incipient wetness and the widely adopted melting infiltration,have been developed.Nevertheless,due to the small diameter,complicated shape,high aspect ratio or poor wettability of the highly desirable micropores,efficient impregnation of sulfur into conductive host materials remains challenging.In order to load sulfur onto host materials efficiently,a novel preparation method,supercritical CO₂?SCCO₂?assisted deposition has been developed in this work.SCCO₂ possesses low viscosity,low surface tension and high diffusivity like gas as well as appreciable density and solvation power like liquid,enabling uniform dispersion of sulfur into porous carbon and intimate contact of sulfur with carbon,which provides a new solution for improving the utilization of sulfur.In this thesis SCCO₂ assisted precipitation technology is used to load sulfur onto commercial activated carbon as cathode materials of Li-S batteries.The main results are summarized as follows:?1?For comparison,melting infiltration,incipient wetness and their combination as described by literature,were used to prepare carbon-sulfur composites,noted as C-S-M,C-S-W,C-S-W-M,respectively,while the commercial activated carbon ACS-2000 with specific surface area of 1700 m2/g was selected as the conductive host material.Those composites were characterized by transmission electron microscopy?TEM?,nitrogen adsorption/desorption test,X-ray diffraction?XRD?,Raman spectra and thermogravimetric analysis?TGA?,and fabricated as coin-cells to observe the electrochemical performance using a battery testing system.The results show that sulfur is unevenly dispersed in C-S-M,and the size of the agglomerated sulfur particles is about 20?30 nm.Most of sulfur in C-S,W and C-S-W-M is highly dispersed,with only a small amount of 5 to 10 nm sized particles visible.The cycling performance of three composites was tested at 0.5 C?1 C=1675 mA/g?.The discharge capacity is initially 650,610 and 670 mAh/g,and after 150 cycles,355,422 and 391 mAh/g for C-S-M,C-S-W and C-S-W-M,respectively.?2?Activated carbon ACS-2000 was also used to prepare carbon-sulfur cathode materials by SCCO₂ assisted precipitation.By varying the temperature,time and amount of CS2,the preparation of the carbon-sulfur composite materials was optimized and the one with the best electrochemical properties is named C-S-SCF,which was characterized by the same methods with the composites of?1?and tested in lab coin-cells to explore the possible performance improvement as cathode materials for Li-S batteries.The results show that sulfur was highly dispersed into the small pores of ACS-2000 and despite the sparse scattering as particles on the surface of ACS-2000,the size of sulfur particles in the C-S-SCF composite material is about 5 nm.Meanwhile,the composite showed excellent electrochemical reaction activity and cycling performance.The cycling performance of C-S-SCF cathode was tested at 0.5 C,the same conditions applied to the three composites in part?1?.The initial discharge capacity of the C-S-SCF is 850 mAh/g,after 150 cycles,C-S-SCF cathode displays a capacity retention of?71%,and the capacity is 600 mAh/g. Comparing the results of?1?and?2?,the cycling performance of the C-S-SCF prepared by SCCO₂-assisted precipitation was greatly improved versus the carbon-sulfur composites by conventional loading methods.?3?Another commercial activated carbon QR1500 was also used to prepare the carbon-sulfur composites as cathode materials of Li-S batteries by SCCO₂ assisted deposition and melt infiltration and the composites was noted as C-S-SCFQ and C-S-Melting,respectively.The discharge capacity is initially 810 and 139mAh/g,and after 150 cycles,450 and 103 mAh/g for C-S-SCFQ and C-S-Melting,respectively.The results demonstrate that the preparation method of SCCO₂-assisted precipitation is of versatility.