A High-Flux Polysulfide Blocking Membrane Fabricated By Shear Alignment of Discotic Nematic Liquid Crystals of Graphene Oxide on Sulfur Cathode

Abstract

With consistent technological advances, demands for batteries with considerably higher specific energy than current Lithium ion devices are apparent. As an extremely promising alternative to Li-ion, the electrochemistry of Lithium-Sulfur (Li-S) has received strong attention due to its theoretical specific capacity of 1675 mAh g-1 and theoretical specific energy of 2500 Wh kg-1. Turning the principle of the Li-S concept into practice though faces a number of key challenges. Among these challenges, the continuous crossover of highly soluble polysulfides – the electrode active material from the cathode through the separator to the lithium anode and vice versa - is primarily responsible for rapid capacity fading of Li-S cells.[ 1 ] Accordingly, the focus of the research effort in this area has been devoted to the retention of sulfur and its reduction products on the cathode side of the battery. These studies have produced some striking improvements in both the sulfur cathode durability and the role that the separator plays in a Li-S battery.[ 2 , 3 ] The separator is a critical component in liquid electrolyte energy storage devices. Sitting between the positive and negative electrode to prevent physical contact between them, the membrane should show sufficient wettability, porosity, chemical, mechanical, thermal and dimensional stability while demonstrating high permeability and high flux. However, for electrochemical systems where redox-active species are dissolved or dispersed in the electrolyte, such as redox flow batteries, fuel cells, and Li-S batteries, permselectivity is also a crucial property. Little attention has been paid to the usage of permselective membranes in the Li-S cells while tremendous efforts have been devoted to the structure and composition of the cathode. A few recent articles have explored the use of permselective membrane separator in mitigating the issue of migration of polysulfides.[ 3 , 4 ] Despite partial success with retention of polysulfides, the presence of such permselective membranes results in a natural decrease in the ion flux across the membrane. This may potentially hinder the industrial use of permselective membranes in Li-S battery where the rate capability is already an issue due to the slow kinetics of the reactions. Accordingly it is highly desirable to achieve an ion-selective transport and mitigate the consequences of polysulfides whilst enabling unimpeded transport of ions. In here we report, for the first time, the integration of a high-selectivity, high-flux GO membrane directly supported onto the sulfur electrode in a Li-S battery. This simple approach dramatically resolves the issue of dissolution of electrode material in battery electrolyte and delivers properties close to those required for a practical rechargeable battery. Our shear-aligned GO membrane which is directly coated onto the electrode, is uniquely composed of well-ordered stacks of the graphene sheets in the membrane plane and we unambiguously demonstrate that its integration in the Li-S battery hardly interferes with free ionic flow in the device. For example, alongside with excellent capacity retention arising from the inherent surface charge of the GO, the highly ordered structure of the thin (~ 0.75 µm) membrane enables the cells to deliver unprecedented high initial discharge capacities of 1616, 1400 and 1170 mAh g-1 at 0.2 C, 0.5 C and 1 C rates, respectively. The remarkable rate capability results reported in this work provides a reliable foundation to challenge the long-held belief that integrating permselective membranes in electrochemical energy storage devises suffering from electrode instability issues, although critical for capacity retention are not suitable for power characteristics. [1] M. Barghamadi, A. S. Best, A. I. Bhatt, A. F. Hollenkamp, M. Musameh, R. J. Rees, T. Rüther, Energy & Environmental Science 2014. [2] J. Song, M. L. Gordin, T. Xu, S. Chen, Z. Yu, H. Sohn, J. Lu, Y. Ren, Y. Duan, D. Wang, Angewandte Chemie 2015. [3] J.-Q. Huang, T.-Z. Zhuang, Q. Zhang, H.-J. Peng, C.-M. Chen, F. Wei, ACS nano 2015, 9, 3002. [4] C. Li, A. L. Ward, S. E. Doris, T. A. Pascal, D. Prendergast, B. A. Helms, Nano letters 2015, 15, 5724.