Preparation And Properties Of Cathode Materials For Lithium Sulfur Batteries

Lithium ion batteries(LIBs) as a universal commercial batteries play an important role in social life, because its high specific capacity, high voltage, long cycle life, green property and none pollution, a wide range of applications in electric vehicles and portable electronic devices, more and more in-depth research is carring on. However, the conventional LIBs cathode materials(such as LiMn2O4, LiCoO2, LiNiO2, LiFePO4, etc.) can not meet the needs of society due to its low theoretical capacity. Elemental sulfur which has a high theoretical specific capacity, abundant resources, low cost, non-toxic, etc., as a new cathode material, is considered to be one of the most developmented potential cathode materials. Currently there are many problem in cathode material of lithium-sulfur batteries(LSBs), for example: conductivity of elemental sulfur, generated polysulfide which is easy to dissolved electrolyte, volume expansion during the electrode reaction, “shuttle effect”, low active material utilization and cycle life, which seriously hinder its application. In order to solve these problems existed in the sulfur electrode, in this paper, starting from the improved conductivity, we design and synthesis new LSBs composites, and carried out the research on modified sulfur cathode and performance improvement.1. This article describes S/PPy/MWCNTs composites with the starting materilas of the elemental sulfur, polypyrrole(PPy) and multi-walled carbon nanotubes(MWCNTs). Firstly, MWCNTs were acidified, then PPy/MWCNTs composites were synthesized by in-situ chemical polymerization, then S/PPy/MWCNTs composites were obtained by melt diffusion method. To study the effect of MWCNTs content on PPy/MWCNTs composites, we added 15%, 25%, 35%(mass ratio) MWCNTs of PPy-MWCNTs to improve the electrochemical properties of the composites. Compared with other materials, S/PPy/MWCNTs-25% composites exhibit the best electrochemical stability and discharge capacity. At a current density of 200 mA g-1, the discharge capacity of only pure phase sulfur electrode is 647 mAh g-1, after 60 cycles, the discharge capacity only has 101 mAh g-1, however, S/PPy/ MWCNTs-25% composites display the initial discharge capacity of 1282 mAh g-1, the discharge capacity remain at 510 mAh g-1 after 60 cycles.2. We synthesized tubular polypyrrole by the method of template, using elemental sulfur as raw materials and reasonably choosing of the preparation craft and processes, we prepare the S-PPy composites which is dispersed uniformly by simple and fast ball milling method.we study the morphology and structure of the S-PPy composites by XRD, SEM, etc. various means, and teste cycling performance and rate capability. At a current density of 200 mAh g-1, S-PPy initial discharge specific capacity reaches 1178 mAh g-1, discharge specific capacity of S-PPy composites remains at 675 mAh g-1 after 150 cycles. However, the initial discharge specific capacity of pure sulfur electrode is 939 mAh g-1, and the discharge specific capacity is only 150 mAh g-1 after 150 cycles. Even under the current density of 1 C(1 C = 1675 mAh g-1), the initial discharge specific capacity of S-PPy composites is as high as 1085 mAh g-1, and discharge specific capacity keeps at 617 mAh g-1 after100 cycles.