Vertically-Aligned Carbon Nanotubes on Aluminum Foil As a Light-Weight Positive Electrode for Lithium/Sulfur Semi-Liquid Batteries
Authors: Liatard Sébastien , Walus Sylwia , Fournier Adeline , Dijon Jean , Martin Jean-Frédéric , Alloin Fannie , Barchasz Céline 
Published: February 2020

Elemental sulfur is a particularly attractive cathode material for lithium batteries. A high theoretical capacity (1675 mAh/gsulfur), along with widespread availability and low cost, makes Li/S one of the most promising battery technologies.1 Sulfur reduction by lithium involves intermediate species called lithium polysulfides Li2Sn (2 ≤ n ≤ 8). Lithium polysulfides are soluble in commonly used electrolyte solvents like glymes, whereas the final species, S8, Li2S2 and Li2S, precipitate at the end of charge or discharge respectively. This unconventional redox process leads to numerous difficulties. The Li/S cells suffer low practical capacity and short cycle life. These shortcomings mainly result from the solubilizing/precipitating cycles of the active material. This process induces strong mechanical strains to composite-type positive electrodes. Morphological changes occur on the electrode upon cycling, which might lower the specific surface area cycle after cycle, and prevent full utilization of sulfur active material.
In our study we use a new approach involving a “catholyte” (i.e. cathode material which is dissolved in the electrolyte) in association with a 3D current collector as positive electrode.2 This catholyte is associated with a current collector that comprises a large specific surface area and which is both flexible and robust to accommodate the precipitating/solubilizing cycles of sulfur. To this purpose, vertically-aligned carbon nanotubes (VACNTs) structures were grown on a light and flexible commonly used 20 µm-thick aluminium foil. The VACNTs were fabricated using a catalyst supported chemical vapor deposition (CVD) method. The conditions of CVD process had to be modified to match the low melting point of aluminium (660 °C) and yielded VACNTs as high as 150 µm. The electrochemical properties of VACNTs-on-aluminium current collectors were investigated in a coin-cell configuration. The catholyte was prepared by mixing metallic lithium, sulfur powder and LiTFSI lithium salt into glyme solvents. The influence of VACNTs height on the electrochemical performance of the cells was studied. Optimization of the catholyte yielded capacities over 1000 mAh/gsulfur during prolonged cycling, using relatively high sulfur loading (>2 mgsulfur/cm2). Remarkable capacity retention was also observed, which was mainly explained by the good mechanical stability and flexibility of VACNT substrates. Last but not least, the combination of multiple characterization techniques, such as in situ and operando X-ray diffraction and electrochemical impedance spectroscopy,3also allowed to take a step forward regarding the understanding of Li/S battery technology, its discharge process and failure mechanisms.
Figure 1. SEM picture of 30 µm-high VACNTs grown on a 20 µm-thick aluminium substrate.

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