Research On Modified Multiwall Carbon Nanotube-Coated Separator For Lithium-Sulfur Batteies

Due to the advantages of high theoretical capacity, environmental friendly, low cost and so on, lithium-sulfur （Li-S） battery is becoming competitive for the next generation rechargeable batteries. However, there are some drawbacks hindering the commercialization of Li-S battery, such as the insulating nature of sulfur and its discharge products, the "shuttle effect" that polysulfides produced in discharge process dissolve in organic electrolyte and migrate to the anode, the large volume expansion of active materials due to the decrease in density of discharge products. All of these lead to the low sulfur utilization and bad cycle performance. Recently the method of applying a conductive coating on separator has been proved to be an effective way to improve the performance of Li-S battery, while the existing research reports are all based on the low sulfur mass loading (≤2 mg cm^-2) of cathode and there is no clear conclusion for the effect of the coating applied into the high sulfur mass loading (≥4 mg cm^-2) of cathode. And the high sulfur loading is essential for making the high specific energy density Li-S battery. To solve these above problems, two kinds of modified MWCNT have been prepared, one is the oxidized multiwall carbon nanotube （o-MWCNT）, and the other is the multiwall carbon nanotube/sulfonated polyaniline （MWCNT/SPANI） composite. These two modified MWCNT are made into the corresponding coated separators, then apply the coated separators in the Li-S batteries with sulfur mass loading of 5 mg cm^-2. Applying the tests of constant current charge-discharge, different current density charge-discharge and the battery impedance, explore their effects on the cycle and rate performances of batteries. Here comes the main conclusions:1. Research on the o-MWCNT-coated separator. MWCNTs are oxidized with the mixture of concentrated sulfuric and concentrated nitric, controlling the time of acid treatment leads to o-MWCNTs with different oxidation o degrees. Then these o-MWCNTs are mixed with some little water-soluble binder LA132 in dispersant and then coated onto the separator, getting the corresponding coted separators. The Li-S batteries are assembled with the high sulfur mass loading of cathodes and these o-MWCNT-coated separators, preparing for the electrochemical performance test. The results indicate:with acid treatment of 2h, the batteries with the corresponding o-MWCNT-coated separator show the best cycle performance, which exhibit an initial capacity of 1105 mA h g^-1, after 50 cycles, the discharge capacity remains in 925 mA h g^-1, the capacity retention reaches to 83.7%, there are remarkable improvements compared to those of batteries with the pristine separator and untreated MWCNT-coated separator in cycle performance. In addition, excessive oxidization for MWCNT will damage its structure and decease its electric conductivity. Thus, with acid treatment of 4h, the batteries with the corresponding o-MWCNT-coated separator show worse performance compared with that of MWCNT with the acid treatment of 2h.2. Research on the MWNCT/SPANI-coated separator. The polyaniline is coated on the surface of MWCNT through in-situ polymerization, getting the MWCNT/PANI composite. And then take this composite into sulfonated processing with chlorosulfonic acid, getting the MWCNT/SPANI composite. The different covering amounts of SPANI are prepared by controlling the adding of aniline. The MWCNT/SPANI composites are mixed with binder LA 132 and then coated on separator. The MWCNT/SPANI-coated separator with the high sulfur loading of cathodes are assembled into the Li-S batteries and implement the electrochemical performance tests. The results show that when the adding of aniline is 25% of the quantity of MWCNT, the batteries with the corresponding coated separator exhibit an initial capacity of 1125 mA h g^-1, after 50 cycles, the discharge capacity remains in 930 mA h g^-1. There are some improvements compared to the pristine separator and untreated MWCNT-coated separator in cycle performance. The adding of aniline is 10% of the quantity of MWCNT, the batteries with the coated separator is similar to that of 25% of the quantity of MWCNT in cycle performance, which is due to the SPANI give decreases in the electric conductivity of composite and weakened the effect of coating on improving the performance of batteries.