Lithium-Sulfur Batteries with an Ultrahigh-Sulfur-Loading Carbon-Cotton Cathode

Abstract

Going beyond lithium-ion chemistry, the lithium-sulfur (Li-S) chemistry could offer 2 – 3 times higher energy density than the current Li-ion battery technology.1,2 However, as reported in recent publications, the progress is far from adequate with respect to the high-loading capability of the active material in building high-energy-density Li-S batteries.1-3 Although the sulfur cathode has a high theoretical capacity (1,675 mA h g-1), a high-loading cathode fabricated by conventional electrode coating method has, unfortunately, proved it to be difficult to attain the necessary electrochemical conversion/utilization of sulfur. Given that a traditional cathode configuration is not ideal for a Li-S battery, advanced electrode structures have been extensively studied for increasing sulfur loading and achieving a high areal capacity.2,3 However, new challenges arise with respect to how to simultaneously improve the sulfur loading and maintain the dynamic and static cell stability.2,4 We present here a carbon-cotton cathode that possesses hierarchical macro-/micro-porous architecture for establishing the high-loading capacity of Li-S cells and simultaneously enhancing their dynamic and static stability.4 Fig. 1 shows the microstructure of the carbon cotton and carbon-cotton cathode. The carbon cotton is fabricated by carbonizing the cotton at 900 °C for 6 h with a heating rate of 2 °C min-1 under argon atmosphere. The carbon-cotton electrode (Fig. 1a) has a cross-linked spiral carbon-fiber network that is armored with microporous reaction sites, forming a hierarchical macro-/micro-porous architecture. The macroporous channels allow the carbon-cotton cathode to encapsulate a high amount of catholyte (Fig. 1b). The high-sulfur-loading carbon-cotton cathode is fabricated by impregnating the carbon cotton with a high amount of Li2S6 catholyte (80 µL). The cells have a fixed electrolyte-to-sulfur ratio of 6.8. The high tortuosity of the carbon-cotton cathode benefits the catholyte infiltration and the ensuing active-material retention. As a result, the carbon-cotton cathodes with the highest values of both sulfur loading (61.4 mg cm-2) and sulfur content (80 wt.%) demonstrate enhanced electrochemical utilization with the highest areal capacity (56 mA h cm-2), volumetric capacity (1,121 mA h cm-3), and gravimetric capacity (724 mA h g-1) simultaneously (Fig. 2a). On the other hand, the abundant microporous reaction sites (micropore surface area: 557 m2 g-1) spread throughout the carbon cotton enlarge the accessible reaction area among electrons, catholyte, and active material, facilitating the redox chemistry and guaranteeing the smooth operation of the high-loading/content Li-S system. Therefore, in Fig. 2, the high-loading carbon-cotton cathode exhibits (i) enhanced cycle stability with a good dynamic capacity retention of 70% after 100 cycles and (ii) improved cell-storage stability with a high static capacity retention of above 93% and a low time-dependent self-discharge rate of 0.12% per day after storing for a long period of 60 days. In conclusion, the electrochemical enhancements and engineering designs of the carbon-cotton cathodes make them advanced cathode architectures for the development of high-loading/content sulfur cathodes in high-energy-density Li–S batteries. REFERENCES 1. S. Urbonaite, T. Poux and P. Novák, Adv. Energy Mater., 5, 1500118 (2015). 2. A. Manthiram, S.-H. Chung and C. Zu, Adv. Mater., 27, 1980 (2015). 3. M. Hagen, D. Hanselmann, K. Ahlbrecht, R. Maça, D. Gerber, J. Tübke, J. Adv. Energy Mater., 5, 1401986 (2015). 4. S.-H. Chung, C.-H. Chang and A. Manthiram, ACS Nano, 10, 10462 (2016). Figure 1