Soluble polysulphide sorption using carbon nanotube forest for enhancing cycle performance in a lithium-sulphur battery

The rapid capacity decay of lithium–sulphur batteries has been a significant obstacle for practical application, which is generally considered to arise from dissolution of lithium polysulphide in the electrolyte and diffusion away from the cathode. As the lithium content in the polysuphide increases with further discharge, capacity decay occurs also from the passivating effects by the formation of insoluble sulphides, further amplified by volume increase. More recently, weakening of sulphur adhesion to carbon with progress in discharge is also an important factor in the sulphur cathode degradation. In order to overcome capacity decay caused by all the above mechanisms, we have prepared a composite cathode made of sulphur and high density carbon nanotube (HD-CNT) forest scaffold that is able to interfacially adsorb and volumetrically confine the polysulphide species and accommodate the expansion of sulphur discharge products effectively. This cathode demonstrates very high electrochemical stability and high discharge capacity up to 200 full discharge/charge cycles even with the use of the basic organic ether electrolyte where polysulphide shows high solubility, thus providing evidence for confinement and interfacial contact. Retention and surface adsorption favoured by minimizing the wall-to-wall distance between the aligned CNTs arise from a decrease in the reaction energy of the adsorption. Computational simulation of the interface between polysulphide species and carbon nanotube surface provides first-principle confirmation of improved binding between C and S in the polysulphides as wall-to-wall distance is decreased. The HD-CNT scaffold is self-binding and highly-conducting thus the conventional additives of binder and carbon black are also fully eliminated. A high discharge capacity of 812 mA h g⁻¹ of sulphur (corresponding to 503 mA h g⁻¹ of the whole cathode material mass) is stably retained after 200 cycles at 400 mA g⁻¹ with a small average capacity decay of only 0.054% per cycle on average These encouraging results provide novel approaches to designing and fabricating long cycle life cathode in a lithium–sulphur battery.Financial support from EC project Technotubes is greatly appreciated. Kai Xi wishes to thank the Cambridge Overseas Trust.This is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.nanoen.2014.12.02