Tackling the Volume Increase Problem of Lithium-Sulfur Batteries During Cycling

Abstract

Lithium-sulfur cells are of big interest due to their high potential as a successor of Li-ion batteries. They have a significantly higher teoretical capacity than todays Lithium-ion batteries and have been of high interest for several years. However, they still suffer from several problems, especially from poor cyclability. One of the reasons for the low cyclability is the large volume increase of the cathode during cycling. The volume expansion and contraction requires the cathode to withstand the induced forces. In this thesis the synthesis and use of helical carbon nanofibers (HCNFs) as a potential flexible cathode is explored. Due to the HCNFs very special shape, that reminds of a telephone cord, the cathode is believed to be able to "breath". HCNFs are synthesized by chemical vapor deposition and activated with potassium hydroxide to increase the porosity. The electrodes are prepared by mixing elemental sulfur and HCNFs by a simple melt-diffusion approach. The activation significantly increases the porosity of the HCNFs. The specific capacity is highly increased in the activated HCNFs compared to the prestine HCNFs at low discharge rates. The capacity is almost twice as high in the activated HCNFs compared to activated multi-walled carbon nanotubes. The activated HCNFs retain a specific discharge capacity above 560 mAh/g over 50 cycles with a coulombic efficiency of 97.0 %, which is similar to published results with straight nanofibers.