In recent years, the lithium-sulfur (Li-S) secondary battery has been energetically researched   worldwide as a good candidate for next-generation secondary batteries. The sulfur active material has a theoretical capacity of 1675 mAh g^-1and is abundant and highly inexpensive. However, several issues are associated with the use of Li-S secondary batteries, such as low utilization due to the insulating property of sulfur and the redox shuttle phenomenon caused by the dissolution of polysulfides generated during the discharge process.

In the present work, we have developed a Li-S secondary battery using the carbon nanotube (CNT) technoloy. Multiwalled-CNTs were directly grown on a nickel current collector by using the thermal chemical vapor deposition (CVD) method. The CNT films were used as the electrode for the Li-S battery. The S-CNT cathode was fabricated by impregnating it with molten sulfur. Then, sulfur powder was uniformly placed on the CNT film and heated at more than the melting point of 113°C. Our CNT has a diameter of 10-15 nm and a length of 200-600 µm. Therefore, the CNT electrode has a wide reaction area owing to its large specific surface area. In addition, the active materials can react rapidly because the electrons quickly access through the CNTs that connect directly with the current collector.

At IMLB 2014, we reported that coin-cell-type Li-S batteries with a sulfur loading of up to 10 mg cm^-2 operate at a current density of 0.5 mA cm^-2 [1]. However, more sulfur loading is required, in order to achieve the electrode capacity of 300 Wh kg^-1 for practical usage. As sulfur loading increased to more than 10 mg cm^-2, the discharge capacity decreased dramatically, probably owing to be attributed to inhomogeneous sulfur distribution in the CNT film. Moreover, the CNT films fell off the nickel current collector because of the shrinkage of the CNT bundles owing to the absorption of sulfur. Therefore, we attempted to synthesize harder CNT films with homogeneously impregnated sulfur. A thick amorphous carbon layer was coated on the CNTs by optimizing the thermal CVD process, thus making the CNT films harder. Moreover, the adhesiveness between the CNTs and the nickel current collector was enhanced, which improved CNT exfoliation. For homogeneous distribution, sulfur impregnation was performed in vacuum. The sulfur was considered to spread over the CNT films more uniformly. We could obtain the stable coin-cell-battery performance with sulfur loading of up to 20 mg cm^-2, and the capacity was estimated to be more than 300 Wh kg^-1 [2]. We manufactured 1 Ah-class pouch cell, with electrode surface area of 50 cm².

Reference

[1] N.Tsukahara, presented at The 17^thInternational Meeting on Lithium Batteries, Como, 2014 (unpublished)

[2] T. Yamada, presented at The 56^th Battery Symposium, Nagoya, 2015 (unpublished)