Enabling Prominent High-rate And Cycle Performances In One Lithium-Sulfur Battery: Designing Permselective Gateways For Li~+ Transportation In Holey-CNT/S Cathodes

Lithium sulfur (Li-S) battery has a high theoretical capacity (1675 mAh g-1) and a high theoretical density (2600 Wh Kg-1). Additionally, the low cost, natural abundance, and environmental benignity of element sulfur make it attractive for large-scale practical applications. Howerver, the insulating nature of sulfur, the volume expansion, and the high solubility of lithium polysulfides in the liquid electrolyte lead to low sulfur utilization, serious capacity fading and poor cycle stability of Li-S battery, inhibiting its commercial applications.In recent years, considerable efforts have been devoted to tackle these issues for delivering the high capacity of the S cathodes by a large number of researchers and some positive progress has been acquired. For instance, encapsulating sulfur in the micropores of carbon spheres or mesopores of hollow carbon and so on is an excellent strategy to enhance the adsorption of polysulfides and decrease remarkly the shuttle-effect. Due to relatively high specific surface area, outstanding conductive properties, the strong adsorption between pore and polysulfides and palmary capabilities of endure volume variation during the charge and discharge, the corresponding cathode materials demonstrate stable cycle performance under low rate (<0.5 C). However, as pursuing high power output of Li-S batteries, unfortunately, the cathodes succeeding at low- and medium-rates usually encounter dramatic performance reduction at high-rates. Recently, only several novel C/S composites had reported excellent high-rate performances of Li-S batteries. According to literature, it is not difficult to find that it is rather necessary not only to encapsulate to prevent the substantial dissolution of polysulfides but also to provide sufficient ion transportation channels. Obviously, it significantly essential to own excellent electrical conductivity for former and remarkable ion conductivity for the latter. Therefore, it is pretty vital to obtain an ideal cathode material which can display both super electrical and ion conductivity.Hence, in this thsis, original MWCNTs (o-CNT) were activated by potassium hydroxide (KOH) to prepare holey carbon nanotubes (h-CNT) and sulfur is incorporated in holey carbon nanotubes. By incorporating medium ZrO2 to the S-incorporated h-CNTs and the effect causing of improved electrochemical performances are concluded. h-CNT is a perfect candidate for supplying both excellent electrical and ion conductivity, which is to extremely increase the utilization of sulfur. Meanwhile, by incorporating ZrO2 to the S-incorporated h-CNTs, permselective gateways for Li+ transportation could be assembled at the mesopore openings, which can suppress efficiently the dissolution of polysulfides as well as sustaining high energy density. More details are as follows:1) The holey carbon nanotube (h-CNT) is prepared by the KOH-activated method and the holey carbon nanotube (h-CNT/S) is synthetised by the the melting-diffusion method. Compared with the original carbon nanotube (o-CNT), h-CNT displays big specific surface area and abundant mesopores so that it demonstrate outstanding rate performance. The capacity of h-CNT/S are nearly 300-400 mAh·g-1 higher than that of o-CNT/S, with rate ranging from 0.2 C to 3 C. In addition, h-CNT/S depicts an excellent capacity just around 679 mAh·g-1 at 3 C, while the capacity of o-CNT/S is only approximately 268 mAh·g-1. Moreover, h-CNT/S can still remain 424 mAh·g-1 at 6 C. However, the abundant mesopores will bring about rapid dissolution of polysulfides, which may result in poor cycle performance campared with o-CNT/S. The initial discharge capacity of o-CNT/S and h-CNT are 1189 mAh g-1 and 712 mAh·g-1, with retentions of 47.0% and 54.3% respectively. In conclusion, the rich mesopores of h-CNT enhances the rate capability, but accelerates the dissolution of polysulfides, leading to weak cycle performance.2) Via incorporating ZrO2 to the h-CNT/S composite, a cathode material composed by KOH-activated holey carbon nanotubes (h-CNTs), sulfur (S) and zirconia (ZrO2) is designedly developed to deliver the high energy density of lithium-sulfur (Li-S) battery at high rates with excellent cycle performances. The tube and mesoporous nature of h-CNTs not only provides sufficient transportation channels for Li+ intercalation/deintercalation but can endure the volume variation of cathode during cycle. Upon appropriate ZiO2 modification, the rich mesopore openings of h-CNTs are tuned to be permselective only for Li+ ions but impenetrable to lithium polysulfides. Highly efficient Li+ transportation through the pore openings can be achieved and the soluble polysulfides are trapped in tube interior. Such a designed architecture enables significantly improved high-rate and cycle performances of the test cells. With a S loading of 46 wt%, the h-CNT/S/ZrO2 cathode delivers an initial capacity of 1138 mAh g-1 and reserves 879 mAh g-1 at 0.5 C upon 200 cycles. At the ultrahigh rate of 10 C, remarkably, the discharge capacity averages to be 870 mAh g-1 within 200 cycles. Our strategy offers a facile and low-cost cathode candidate for applying Li-S batteries in practical applications which demand both high energy density and power density.