Research paper of the month
July 2015 - Soluble Polysulphide Sorption using Carbon Nanotube Forest for Enhancing Cycle Performance in a Lithium Sulphur Battery
A lithium-sulphur battery embodies energy which is several times superior to the conventional Li-ion battery. Rapidy decay of this energy has been a significant obstacle hampering practical applications, the loss of capacity mainly arises from dissolution of lithium polysulphide in the electrolyte followed by diffusion away from the cathode. In order to overcome this problem, we have prepared a composite cathode made of sulphur and a 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 in tests carried out up to 200 full discharge/charge cycles. We have purposely used the least favourable electrolyte with a high polysulphide solubility to demonstrate the concept forcefully thus providing evidence for confinement. Computational simulation of the interface between polysulphide species and carbon nanotube surface provides first-principles confirmation of improved binding between C and S in the polysulphides as wall-to-wall distance is decreased with increasing density of the CNT forest. The HD-CNT scaffold is self-binding and highly-conducting thus the conventional additives of binder and carbon black are also fully eliminated reducing the weight of the battery. A high discharge capacity of 812 mAh g-1 of sulphur (corresponding to 503 mAh g-1 of the whole cathode material mass) is stably retained after 200 cycles at the current density of 400 mA g-1 with an average capacity decay of only 0.054% per cycle.
Figure: Cycle performance of the lithium sulfur battery and a schematic diagram of the lithiation process in Sulphur/CNT (Carbon nanotube) array composite (HD- high density, MD-meium density, LD-low density)
K. Xi, B. Chen, H. Li, R. Xie, C. Gao, C. Zhang, R.V. Kumar, and J. Robertson, "Soluble polysulphide sorption using carbon nanotube forest for enhancing cycle performance in a lithium–sulphur battery", Nano Energy, 12 (2015) 538–546.