Study On The Modification And Electrochemical Performance Of Cathode Material For Li-s Batteries

Energy is the focus of attention in this century. To meet pressing needs, safe, efficient and environmental friendly energy storage systems are in huge demands. Lithium ion batteries(LIBS) play an irreplaceable role in the world of energy storage systems owning to the advantages of their high power density, high voltage, green environment protection and long cycle life. Since 1991, LIBs bave dominated the secondary battery market. However, the further development of LIBs is restricted by the low theoretical capacity of commercial cathode material. Elemental sulfur with the merits of abundant resources, low cost, non-toxic and high theoretical specific capacity has become one of the most potential cathode materials. But, commercial applications of lithium-sulfur batteries(LSBs) have not been successfully achieved. Although significant progress has been made, LSBs also have a number of difficult problems to be overcome. The electronic and ionic insulting nature of sulfur leads to a poor electrochemical accessibility and low utilization of the sulfur cathode. Besides, the polysulfides formed during discharge/charge cycles are highly soluble and can diffuse outside the cathode where soluble polysulfides as well as soluble sulfur cause lithium corrosion, which can cause a rapid irreversible capacity fading at repeated cycles. In the meantime, the last major problem for LSBs is the large volume expansion of sulfur during cycling, which will cause the poor cycling performance. So, the common method is coating sulfur with conductive materiala for LSBs.Carbon nanotubes with execllent mechanical properties and electrical conductivity are a new kind of carbonaceous materials and widely used in LIBs. Because of its high electrical conductivity, non-grahitic carbon nanotubes(NGCNTs) can replace carbon nanotubes as substrate to prepare sulfur-based composite, which also are a type of finite carbon structure consisting of needle-like tubes similar to carbon nanotubes. In this paper, NGCNTs are obtained by pyrolysis of polypyrrole nanotubes as the carbon percursor, using it as substrate to prepare sulfur-based composite and the eletrochemical properties were tested. The main research contents and results in this paper are as follows:(1)Polypyrrole nanotubes were fabricated via a reactive self-degraded template method at room temperature and then carbonized at 900 ℃ to obtain NGCNTs with thinner size of carbon tubes than that of the original polypyrrole nanotubes. And then, mixtures of NGCNTs and sulfur in the weight ratio of 4:6 and 3:7 were co-heated to prepare H-NGCNTs/S(4:6) and H-NGCNTs/S(3:7) composites. As seen from the results of electrochemical test, the electrochemical performance of H-NGCNTs/S(4:6) composite is obviously superior to that of H-NGCNTs/S(3:7) composite. So, we chose the 4:6 as weight ration for NGCNTs/S composite. In the case of the weight ration of 4:6, the NGCNTs were mixed with sulfur under the ball-milling and subsequently co-heated to obtain B+H-NGCNTs/S composite. The sample shows good electrochemical properties at the current density of 335 m A g-1. The initial discharge capacity is 1202.1 m Ah g-1sulfur, and the discharge capacity is retained at 625.9 m Ah g-1sulfur after 50 cycles.(2)NGCNTs were actived by KOH to obtain the K-NGCNTs with abundant pores and defects, using it as substrate. Ball-milling and heat treatment were also used to prepare K-NGCNTs/S composite in the weight ration of 4:6. The electrochemical test results show that the initial discharge capacity is 1121.9 m Ah g-1sulfur, and the discharge capacity remains 729.7 m Ah g-1sulfur after 50 cycles at the current density of 335 m A g-1.