Lithium-sulfur (Li-S) batteries have attracted great attentions over the past few years due to their appealing performance that can potentially deliver a theoretical energy density of 2500 Wh/kg. Sulfur characterizes not only a high theoretical capacity of 1672 mAh/g with average working voltage at around 2.1 V (V vs. Li/Li⁺), but also the natural abundance, low toxicity, and reasonable cost, which makes it an ideal cathode material for high-energy density lithium batteries.

However, main challenges on how to achieve high sulfur utilization with high Coulombic efficiency and long cycle life are remained to get superior Li-S batteries performance. These challenges are mostly associated with the shuttle effect of dissoluble polysulfide species that cause active materials loss and side reactions on lithium metal anode. It will directly results in capacity fading, coulombic efficiency decrease, lithium metal corrosion, and dendrite formation. To suppress the shuttle effect, it is necessary to completely preserve sulfur and polysulfides locally in cathode area.

—Kim et al.

In the study, the team presented a sulfur-carbon composite coated with a conducting polymer as polysulfide block to increase electrochemical performance. The sulfur-carbon composite consists of a free-standing single-walled carbon nanotube (SWNT) film loaded with thermo-infused sulfur.

• The free-standing SWNT film is an ideal carbon matrix for sulfur materials to function as a current collector and electrolyte/polysulfides reservoir due to the the high electronic conductivity, structural high porosity, and mechanical stability.

• Polyaniline (PANI) is adopted to the S/SWNT composite to block the out-diffusion polysulfides while function as a gel electrolyte. Due to the electronic conductivity of PANI, it can also help to improve the composite conductivity, resulting in high sulfur utilization.

As part of the study, the team compared the electrochemical performance of SWNT film coated with sulfur (S/SWNT) and PANI-coated S/SWNT (PANI-S/SWNT) film.

Electrochemical performances of S/SWNT and PANI-S/SWNT. (a) The initial discharge-charge curves, (b) Cycling performance, (c) Rate retention, (d) Nyquist plots of S/SWNT and PANI-S/SWNT electrodes. Kim et al. Click to enlarge.

… the biggest difference between S/SWNT and PANI-S/SWNT is the reversibility of the electrochemical reaction during the cycles. The intensity and the position of anodic peak in S/SWNT electrode is negatively shifted after second cycle, implying that the sulfur reaction kinetics was not stable and electrode was still electrochemically activating during the repetitive cycles. In contrast, anodic peak in PANI-S/SWNT sample was repeatedly overlapped after second cycle, indicating that conducting PANI can stabilize the sulfur reaction kinetics and facilitate the electrochemical activation.

—Kim et al.

In addition, the rate capability of PANI-S/SWNT showed a higher capacity retention of 56% than that of S/SWNT (37%) at 2 C.

Resources

• Joo Hyun Kim, Kun Fu, Junghyun Choi Kichun Kil, Jeonghyun Kim, Xiaogang Han, Liangbing Hu & Ungyu Paik (2015) “Encapsulation of S/SWNT with PANI Web for Enhanced Rate and Cycle Performance in Lithium Sulfur Batteries” Scientific Reports 5, Article number: 8946 doi: 10.1038/srep08946