|Cycling stability of different C/S electrodes. Schuster et al. Click to enlarge.|
A research team led by Thomas Bein of LMU Munich (Ludwig-Maximilians-Universität München) and Linda Nazar of the University of Waterloo (Canada) has developed new cathode materials for lithium-sulfur batteries. The materials are spherical ordered mesoporous carbon nanoparticles featuring very high inner porosity (pore volume of 2.32 cm3 g−1 and surface area of 2445 m2 g−1), synthesized in a two-step casting process. (Earlier post.)
Applied as cathode materials in Li-S batteries, they showed high reversible capacity up to 1,200 mAh g−1 and excellent cycling efficiency. A paper on the work is published in the journal Angewandte Chemie.
Li-S batteries are of interest due a high theoretical specific energy density that exceeds that of conventional lithium-ion batteries by about a factor of five, good low-temperature performance, and its use of inexpensive and nontoxic raw materials. The sulfur plays a special role in this system; under optimal circumstances, it can absorb two lithium ions per sulfur atom. It is therefore an excellent energy storage material due to its low weight.
As opposed to the intercalation of Li+/e- into a host structure with minor perturbation to the framework, the reduction of sulfur involves a reversible chemical reaction with lithium to form another material. We call this an “integration” reaction. Extensive studies have shown that this occurs via a redox cascade of intermediate “molecular” polysulfides (in order of decreasing sulfur oxidation state: Li2S8 ↔ Li2S6 ↔ Li2S5 ↔ Li2S4), followed by the formation of insoluble Li2S2 and finally Li2S.
The overall redox couple, described by the reaction S8 + 16Li ↔ 8Li2S, lies at an average of about 2.1 V with respect to Li+/Li. The potential is about 2/3 of that exhibited by conventional positive electrodes, but this is offset by the very high theoretical capacity afforded by the nontopotactic integration process, of 1675 mA h/g. Thus, compared to conventional batteries, Li-S batteries have the opportunity to provide a gravimetric energy density that is a factor of at least 3-5 times higher. Theoretical values can approach 2500 W h/kg or 2800 W h/L on a weight or volume basis, respectively, assuming complete reaction to Li2S.—Ellis et al. 2010
However, among the challenges faced by the chemistry, sulfur is a poor conductor, meaning that electrons can only be transported with great difficulty during charging and discharging. To improve this battery’s design the scientists strove to generate sulfur phases with the greatest possible interface area for electron transfer by coupling them with a nanostructured conductive material.
The team first developed a network of porous carbon nanoparticles. The nanoparticles have 3- to 6-nanometer-wide pores, allowing the sulfur to be evenly distributed. In this way, almost all of the sulfur atoms are available to accept lithium ions. At the same time they are also located close to the conductive carbon.
They prepared materials with different percentages of sulfur in the mix; the C/S cathode materials were slurry-cast onto a carbon-coated aluminium current collector. Typically, 82 wt% of C/S composite, 10 wt% Super S carbon and 8 wt% PVDF were mixed with cyclopentanone.
The electrochemical performance of the prepared cathodes was evaluated in 2325 coin cells cycled galvanostatically at room temperature between 1.5 V and 3.0 V, with lithium metal foil as the anode. An electrolyte was chosen to optimize the high rate behavior.
The sulfur is very accessible electrically in these novel and highly porous carbon nanoparticles and is stabilized so that we can achieve a high initial capacity of 1200 mAh/g and good cycle stability. Our results underscore the significance of nano-morphology for the performance of new energy storage concepts.—Thomas Bein
The carbon structure also reduces the polysulfide problem faced by Li-S cells. Polysulfides form as intermediate products of the electrochemical processes and can have a negative impact on the charging and discharging of the battery. The carbon network binds the polysulfides, however, until their conversion to the desired dilithium sulfide is achieved. The scientists were also able to coat the carbon material with a thin layer of silicon oxide which protects against polysulfides without reducing conductivity.
Schuster, J., He, G., Mandlmeier, B., Yim, T., Lee, K. T., Bein, T. and Nazar, L. F. (2012), Spherical Ordered Mesoporous Carbon Nanoparticles with High Porosity for Lithium–Sulfur Batteries. Angew. Chem. Int. Ed., 51: 3591–3595. doi: 10.1002/anie.201107817
Brian L. Ellis, Kyu Tae Lee, and Linda F. Nazar (2010) Positive Electrode Materials for Li-Ion and Li-Batteries. Chem. Mater. 22, 691–714 doi: 10.1021/cm902696j