Fraunhofer researchers report significant extension to Li-S cycle life with silicon anodes and CNT-sulfur composite cathodes
Lithium-sulfur batteries are of great interest for electromobility applications, among others, due to their high specific energy and relatively low cost, but are challenged by significant capacity decay over cycling. (Earlier post.) Scientists at the Fraunhofer Institute for Material and Beam Technology IWS in Dresden, Germany say they have developed new cathode and anode designs for lithium-sulfur batteries that can increase the cycle life by a factor of seven.
According to Dr. Holger Althues, head of the Chemical Surface Technology group at IWS, by using the combination of silicon anode and composite carbon nanotube/sulfur cathode materials, the team has extended the cycle life of lithium-sulfur button cells from 200 cycles to 1,400 cycles.
The interplay between anode and cathode is the critical factor determining the performance and lifespan of a battery. In the lithium-sulfur model, the cathode is composed of elemental sulfur. However, sulfur also interacts with the liquid electrolyte, impairing the performance of batteries and, in the worst case, causes them to lose capacity entirely. The IWS researchers are using binder-free thick film sulfur cathodes based on a carbon structure with carbon nanotubes (CNT). The vertically aligned CNT are directly connected to the current collector.
Because the CNT electrode contains no polymer binder, the volume change of the active material (S8 ↔ Li2S) can be accommodated more easily, leading to better cycle stabilities. The sulfur mass fraction in the electrode can be as high as 90%.
In work reported in a paper in the Journal of Power Sources, the Fraunhofer teams found that the sulfur mass can be varied between 3 and 20 mg cm−2 electrode, leading to sulfur loads that are several times as high as in slurry electrodes. Reported achieved capacities for these extremely high sulfur loads are around 900 mAh g−1 sulfur of the total composite electrode mass at a current of 0.64 mA cm−2 electrode.
The anode of the team’s prototype is made from a silicon-carbon compound rather than the more common metallic lithium. This compound is significantly more stable, as it changes less during each charging process than metallic lithium. The more the structure of the anode changes, the more it interacts with the liquid electrolyte. This process causes the liquid to break down into gas and solids and the battery to dry out.
The energy density of a sulfur-silicon-lithium cell will be below the achievable values of a lithium-sulfur cell, the researchers note. Over the long term, the Fraunhofer team expects to reach a practical energy density of up to 600 Wh/kg. By comparison, the maximum energy density of the lithium-ion batteries currently in use is around 250 Wh/kg.
In the medium term, figures around the 500 Wh/kg mark are more realistic. In practical terms, this means you can drive twice as far with the same battery weight.—Holger Althues
The work was part of the German joint project “AlkaSuSi-Alkalimetal sulfur and silicon” funded by German ministry BMBF.
The researchers are currently working on further optimizing the material and using it in larger battery models. They are also turning their attention to suitable manufacturing methods.
M. Hagen, S. Dörfler, P. Fanz, T. Berger, R. Speck, J. Tübke, H. Althues, M.J. Hoffmann, C. Scherr, S. Kaskel (2013) Development and costs calculation of lithium–sulfur cells with high sulfur load and binder free electrodes, Journal of Power Sources, Volume 224, Pages 260-268, doi: 10.1016/j.jpowsour.2012.10.004
M. Hagen, S. Dörfler, E.Q. González, H. Althues, J. Tübke, H. Föll (2012) High energy cells: Lithium-sulfur and lithium-sulfur-silicon. Honolulu PRiME 2012.
Arumugam Manthiram, Yongzhu Fu, and Yu-Sheng Su (2012) Challenges and Prospects of Lithium–Sulfur Batteries. Accounts of Chemical Research doi: 10.1021/ar300179v