Vertically-Aligned Carbon Nanotube Electrodes for Lithium-Sulfur Batteries

Abstract

Elemental sulfur is a particularly attractive cathode material for lithium batteries. A high theoretical capacity (1675 mAh/gsulfur), along with widespread availability and a low cost, makes Li/S one of the most promising battery technologies. Sulfur reduction is a multistep process involving various intermediate species, namely lithium polysulfides Li2Sn (2 ≤ n ≤ 8). Lithium polysulfides are soluble in commonly used electrolyte solvents like glymes, whereas the two final species, S8 and Li2S, precipitate at the end of the charge or the discharge respectively. This unconventional redox process leads to numerous difficulties. The Li/S cells suffer low practical capacity, short cycle life, low coulombic efficiency and high self-discharge, due mainly to the solubility of intermediate species. In its soluble form, sulfur can access and thus react with the negative electrode. Plus, the solubilizing/precipitating cycles of the active material induce strong mechanical strains to the positive electrode.To overcome these difficulties, researchers have employed many different strategies, the most popular being the confinement of the active material at the positive electrode.[i] However, in most cases, a non-negligible capacity fading is observed upon cycling, indicating that a certain amount of sulfur is still leaking from the positive electrode.In our study we use a relatively new and very different approach consisting of the use of a “catholyte” in association with a 3D current collector as positive electrode.[ii] A catholyte is a liquid electrolyte that contains the active material of the cathode (sulfur).[iii] Since this technique does not require the fabrication of a composite electrode, the solubility of lithium polysulfides is no longer an obstacle. However, the choice of the current collector for the positive electrode is crucial. The 3D current collector has to comprise a large specific surface area and has to be both flexible and robust to accommodate the precipitating/solubilizing cycles of sulfur.The properties of vertically-aligned carbon nanotubes (VACNT) structures correspond with the requirements of these 3D current collectors. In fact, a VACNT-sulfur composite electrode fabricated on a nickel foil was successfully tested by Dörfler et al..[iv]In this work, we chose to grow VACNT structures on a lighter, more commonly used 20 µm-thick aluminum foil. The fabrication of carbon nanotubes on aluminum is not trivial. The catalyst supported chemical vapor deposition (CVD) method used to grow VACNTs requires temperatures ranging from 500 to 1000°C, but the aluminum melts at 660 °C leaving a small window for CNT growth. Despite this obstacle, we managed to grow VACNT of different heights, varying from a few microns up to 150 µm, on aluminum foils (fig. 1). The height of the nanotubes is essential, since it broadly impacts the specific surface area of the current collector.The electrochemical properties of CNT-on-aluminum current collectors were investigated in a coin-cell configuration, using a catholyte based on a tetraglyme:dioxolane (1:1 vol.) solvent mix, containing polysulfide Li2S6, 1 M LiTFSI and 0.1 M LiNO3. The cells were tested between 1.7 and 3.0 V at rates ranging from C/50 to C/5. The practical loading, calculated based on the capacity of the first complete discharge, attained 3 mAh.cm-2.As shown in fig. 2, the influence of the VACNTs height on the electrochemical performance of the cells was studied. Increasing the height of the nanotubes improved the active material utilization. However, once the height of the structure reached 50 µm, no significant improvements were observed in the cell's behavior.To further improve the capacity of the cells, studies were carried out on the catholyte itself. Parameters including the concentration of polysulfides, the nature of the solvents, the volume introduced in the cell were optimized and yielded capacities over 1000 mAh/gsulfur.[i] X.Ji, L.F. Nazar, J. Mater Chem. 2010, 20, 9821.[ii] C. Barchasz, F. Mesguich, J. Dijon, J.-C. Leprêtre, S. Patoux, F. Alloin, J. Power Sources 2012, 211, 19.[iii] R. D. Rauh, K. M. Abraham, G. F. Pearson, J. K. Surprenant, S. B. Brummer, J. Electrochem. Soc. 1979, 126, 523.[iv] S. Dörfler, M. Hagen, H. Althues, J. Tübke, S. Kaskel, M. J. Hoffmann, Chem. Commun. 2012, 48, 4097.