| Lithium-sulfur ?Li-S? batteries have extraordinarily high theoretical energy density ?2600 Wh/kg?, much lower cost than lithium-ion batteries and environmental friendliness, which are the most promising candidate for the next generation rechargeable batteries with high energy density.However, improvement on practical energy density and cycling stability is the central issue for Li-S batteries, the key to which is designing the cathode materials with high sulfur loading and long cycling life. In this work, based on the carbon foam ?MFC? with 3D foam structure, flexible integrated cathodes for Li-S batteries were prepared and the performances were studied. The MFC material with high electrolyte adsorption capability endows with excellent cycling performance.Moreover, through further modification of the MFC, better performance can be achieved, indicating the great potential of the MFC as sulfur host for Li-S batteries cathode. The main results are as follows:?1? Flexible carbon foam ?MFC? with 3D hollow framework was prepared by direct carbonization via a specific heating process from the commercial melamine formaldehyde resin sponge ?MF?. The obtained MFC with extremely low density (8 mg·cm-3) can combine with sulfur to fabricate the flexible integrated electrodes. Used as cathodes of Li-S batteries, the MFC-Sulfur composites show outstanding cycling stability even with a high sulfur loading of 80.18%. But the low graphitization degree of MFC results in the unsatisfactory conductivity. Further work was devoted to combine the MFC with GO ?or rGO? to overcome this problem. The prepared MFC/GO-S and MFC/rGO-S flexible integrated electrodes exhibit both brilliant cycling stability and high capacity. The MFC-rGO-S cathode with sulfur loading of 75 wt% shows an initial capacity of 1183.1 mAh·g-1 at 0.1C and remains 653.5 mAh·g-1 after 100 cycles with negligible capacity fading from 20 to 100 cycles.?2? The effect of sulfur loading, MFC thickness and electrolyte amount on the performance of Li-S batteries was studied: A series of MFC-S cathodes with different sulfur loading in a range of 55.88 % -75.14 % reveal similar cycling performance and capacity; the capacity increases along with the MFC thickness increasing when the areal density of sulfur in the MFC-S cathode is lower than 2.4 mg·cm-2; the capacity of Li-S batteries decreases as the electrolyte amount raises when the used electrolyte amount is over 10uL. These results above can be attributed to the special foam structure of the MFC. The foam structure has a strong adsorption ability to the electrolyte, and confine the electrolyte whithin the cathode region efficiently. Thus, together with the electrolyte, the diffusion of the excessive polysulfides to the electrochemically inactive spaces is avoided, and the cycling performance of the Li-S batteries are improved. Furthermore, the traditional Li-S batteries using casted S-C cathode with MF sponge interlayer were fabricated. These batteries show the similar high capacity and excellent cycling performance to the MFC-S flexible integrated cathode, while the batteries without MF sponge do not show any enhanced performance. This phenomenon confirms the validity of our assumption and the important role of the material with foam structure, which is very appropriate for cathode with high sulfur loading and long cycling life in Li-S batteries.?3? Based on the foam structure of MFC, MFC/CNT,MFC/Graphene and MFC/CAC composites were prepared by combining the MFC with carbon nanotubes ?CNT?, graphene and mesoporous carbon ?CAC? in polyvinylidene fluoride ?PVDF? slurry, and a followed pyrolysis process. In these composite materials, MFC acts as framework to provide the flexible foam structure. CNT, graphene and CAC endow the composites with extra conductivity and adsorption, while the PVDF in the slurry can transform into the microporous carbon after carbonization, which can further restrain the polysulfides. The flexible integrated cathodes based on the three composites show outstanding cycling stability with much enhanced capacities. MFC/CNT-S,MFC/Graphene-S and MFC/CNT-S cathodes with sulfur loading of 70 %perform initial discharge capacities of 1358.6 mAh·g-1, 1157.5 mAh·g-1 and 1025.7 mAh·g-1 at 0.1 C, respectively. After 100 cycles at 0.2C, the capacity retentions of MFC/CNT-S, MFC/Graphene-S and MFC/CNT-S carhodes are 997.5 mAh·g-1, 846.2 mAh·g-1 and 735.9 mAh·g-1,respectively. |
PDF Full Text Request