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Sulfur Embedded In Porous Carbon Materials For High Performance Lithium-Sulfur Batteries And Their Electrochemical Performance

Posted on:2015-09-20Degree:MasterType:Thesis
Country:ChinaCandidate:G Y XuFull Text:PDF
GTID:2181330422980787Subject:Applied Chemistry
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Lithium-sulfur (Li-S) batteries are secondary batteries, which are based on metal lithium as anegative electrode and elemental sulfur as a positive electrode. Li-S batteries are considered to be oneof the most promising energy storage devices for next generation high energy power system, due totheir high specific capacity (1675mAh·g-1) and energy density (2600Wh·kg-1). In addition, sulfur isabundant, low cost, and environmentally friendly. Therefore, Li-S batteries are “green batteries”.However, they still suffer from several drawbacks which restrict their practical application. Firstly,sulfur and its fully reduced compound, lithium sulfide (Li2S), are high insulating materials which leadto low utilization of active material. Secondly, lithium polysulfides are soluble in organic liquidelectrolytes and these soluble lithium polysulfides can spontaneously diffuse to anode through theliquid electrolyte and react with lithium, thus leading to lithium metal corrosion and self-discharge.What’s more, these soluble lithium polysulfdes can also diffuse back to the sulfur cathode whichresults in a loss of active material. The shuttle mechanism gives rise to severe capacity fade duringextended cycling. This paper focuses on improving the specific capacity and cycling performance ofsulfur cathode, and prepares cathode materials with high performance. It is to provide theoretical andscientific basis for the power system of long endurance micro aerial vehicle. The details are asfollows:(1) Sulfur was encapsulated into hierarchically porous carbon (HPC) derived from the solublestarch with a template of needle-like nano-sized Mg(OH)2. HPC has a relatively high specific surfacearea of902.5m2·g-1and large total pore volume of2.60cm3·g-1, resulting that a weight percent ofsulfur in S/HPC is up to84wt%. When evaluated as cathodes for Li-S batteries, the S/HPC compositehas a high discharge capacity of1249mAh·g-1in the first cycle and a Coulombic efficiency as high as94%with stable cycling over prolonged100charge/discharge cycles at a high current density of1675mA·g-1. The superior electrochemical performance of S/HPC is closely related to its unique structure,exhibiting the graphitic structure with a high developed porosity framework of macropores incombination with mesopores and micropores. Such nanostructure could shorten the transport pathwayfor both ions and electrons during prolonged cycling.(2) The high developed porous nitrogen-doped carbon nanotube (PNCNT) with a high specifcsurface area (1765m2·g-1) and large pore volume (1.28cm3·g-1) was synthesized from a tubularpolypyrrole (T-PPY). The inner diameter and wall thickness of PNCNT is ca55nm and22nm,respectively. It shows extremely promising properties for encapsulating sulfur as a superior cathodematerial for high performance lithium-sulfur (Li-S) batteries. When the S/PNCNT composite is tested as the cathode material for Li-S batteries, the initial discharge capacity is1341mAh·g-1at a currentrate of1C. And even after50cycles at the same rate, the high reversible capacity still remains at933mAh·g-1. The promising electrochemical energy storage performance of PNCNT can be attributed toits excellent conductivity, large surface area, nitrogen doping and unique pore-size distribution.(3) Sulfur was confined into hierarchically porous carbon nanoplates (HPCN) derived fromone-step pyrolysis of metal-organic framework (MOF-5). HPCN with an average thickness of ca50nm exhibits a three-dimensional (3D) hierarchically porous nanostructure, high specific surface area(1645m2·g-1) and large pore volume (1.18cm3·g-1). When evaluated as a cathode for lithium-sulfurbatteries, the HPCN/S composite manifests high specific capacity and excellent cycling performance.At a current rate of0.1C, the initial discharge capacity of HPCN/S is1177mAh·g-1. Even at a currentrate of0.5C, it still deliveres a discharge capacity of730mAh·g-1after50cycles and the Coulombicefficiency is up to97%. The enhanced electrochemical performance of HPCN/S is closely related toits well-defined3D porous plate nanostructure that not only provides stable electronic and ionictransfer channels, but also plays the key role of strong absorbent to retain polysulfdes andaccommodate volume variation during the charge/discharge process.(4) Three-dimensional (3D) hierarchical CNTs/GO/S ternary composites were prepared bysolution-based chemical reaction-deposition method using graphene oxide (GO) and carbonnanotubes (CNTs) as precursors. Scanning electron microscopy (SEM) and transmission electronmicroscope (TEM) results indicate that uniform sulfur coating on CNTs/GO is obtained due to thelarge specific surface area of GO. At the same time, CNTs interspersed between the GO layers canform a3D porous structure. The constant current charge-discharge tests show that CNTs/GO/Scomposites have a high discharge capacity and excellent cycle stability. The CNTs/GO/S compositesdeliver a high initial discharge capacity of904mAh·g-1at1C. And after50cycles at the same rate,the reversible capacity remains at578mAh·g-1.
Keywords/Search Tags:Micro aerial vehicle, Lithium-sulfur batteries, Porous carbon, Composite, Cyclingperformance
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