Study On Halloysite Nanotubes-modified Cathode Materials For Lithium Sulfur Batteries

Lithium-sulfur (Li-S) batteries possess a high theoretical capacity of 1675 mA·h/g and a high energy density of 2600 W h/kg, which are much higher than those of traditional cathode materials based on transition metal oxides.Furthermore, sulfur is abundant in nature, inexpensive and environmentally friendly. Therefore, lithium-sulfur batteries have been recognized as one of the most promising next generation rechargeable battery. However, there are still some problems preventing the practical application of Li-S batteries, such as (1)the low electrical and ionic conductivity of elemental S and its solid discharge products (Li2S or Li2S2); (2)the large volumetric expansion of approximately 76% based on full transformation of sulfur to Li2S; (3) the shuttle effect of polysulphides (PS) in the organic electrolyte, which can lead to the low utilization of active materials in the cathode, poor cycle ability and rate performance. To address these issues, in this paper the polar HNTs/S and RGO@HNTs/S composites were designed and synthesized in order to improve electrochemical properties of Li-S batteries and the feasibility of poar halloysite nanotubes as the matrix of sulfur was discussed. The main research contents are listed as below:(1) We investigated the effects of acid etching time and temperature on the physico-chemical and pore characteristics of halloysite nanotubes (HNTs).XRD results indicate that the acid treatment will destroy the crystal Structure of HNTs and form amorphous silica. The BET surface area and the pore volume whose sizes were less than 20 nm was increased with the increase of acid treatment temperature and lots of micropores was formed in the wall of HNTs.However, the BET surface area and total pore volume of HNTs changes slightly with acid etching time increases from 2 to 10h. XPS measurements showed that with the increase of acid etching time and temperature, the Al-OH and Al-0 functional groups contents on the HNTs surface were decreased due to the dissolving of Al3+.(2) Using HNTs-80 as the matrix of sulfur, the effects of different composite methods (melting method, chemical deposition method) on the electrochemical properties of HNTs/S composites were studied. The electrochemical analyses show that the initial discharge capacities of HNTs/S prepared by chemical deposition method was 912mAh/g at 0.1C and the capacity remained at 674mAh/g after 60 cycles with a retention of 61.8%.HNTs/S composites prepared by chemical deposition method increase the utilization of active material comparison with HNTs/S by melting method, lead to significantly improve the discharge specific capacity of Li-S batteries. The results show that the HNTs/S electrode prepared by chemical deposition method has better electrochemical performance.(3) We used the HNTs with different etching temperature as a functional material in the Li/S batteries. Meanwhile, the effect of HNTs with different etching temperature on the electrochemical properties of the composites was studied. On the one hand, acid etching HNTs with large specific area and abundant oxygen-containing functional groups on its surface can improve the wettability between the electrodes and electrolyte and reduce contact resistance.On the other hand, the wall of acid treated HNTs form porous structure, which will improve the adsorption capacity of polysulphides and facilitate the transport of electrolyte/Li+ during the cycle, reduce the ionic and electronic conduction distance. Electrochemical tests showed that HNTs-80/S composites have better electrochemical performance.(4) A novel RGO@HNTs-80/S composite has been synthesized by a simple aqueous one-Pot synthesis process. The capacity of RGO@HNTs-80/S composite electrode remained at 655mAh/g and the retentive efficiency of capacity could keep 57.8% after 100 cycles at current density of 0.1C. When the rate is up to 1C, a discharge capacity of 556 mA h/g is still attainable.Compared with RGO@S composites, the RGO@HNTs/S composites exhibit better capacity retention, and the RGO@HNTs/S composites show better rate capability than the HNTs/S composites.