ZigBee alliance launches new Greater China special interest group
New Linde hydrogen station officially begins operating in Emeryville, CA

LMU Munich-Waterloo team develops mesoporous carbon nanoparticle cathodes for Li-S batteries

Schuster
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.

Resources

  • 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

Comments

HarveyD

This could be another major step towards improved future batteries required for extended range BEVs.

DaveD

Always good to see improvements on the cathode side as they always lag so far behind the anode. These are excellent numbers indeed.

Now, the usual questions:
Cost?
safety concerns?
Cost?
operating temperatures?
power density?
cost?
Availability?
Did I mention cost? :-)

HarveyD

DaveD...cost can only be solved (reduced) with mass production in low labor cost countries. That's where they will be built.

SJC

They have lots of work to do on sulfur batteries, but the problems seem like they can be solved. Sulfur is a poor conductor, so they are using nano carbon as the conductor and this is showing results.

I believe we will see two breakthroughs in a few years, first with vanadium and next with sulfur. The capacity may double and double again. That allows them to sell at lower cost not by any economies of scale, but cell capacity that allows them to produce fewer cells for the same pack capacity.

Brotherkenny4

Assuming a relatively low cost synthesis and easy incorporation into current cathode industrial coating techniques, sulfur based cathodes should be considerably less expensive than the transition metal oxide cathodes currently used. Indeed, besides the higher theoretical energy density one of the allures of sulfur is it's plentiful nature and the low cost. Here's the real issue with sulfur, the polysulfides they refer to are somewhat soluble in the organic based electrolyte systems, and will transport to the anode both acting as an internal loss and shuttle mechanism as well as potentially reacting to add insulating sulfur to the anode surface. They do say that their carbon encapsulation reduces that affect, but 100 cycles is not enough data to show that they have truely solved those issues. Here they have used a lithium foil as anode, but what would be a real illustration of feasibility would be to cycle against a carbon or silicon anode. Although, they appear to have come up with a way of getting good energy density on the cathode, they still need to work out some of the other issues. By the way, the batteries need to get 5000 real life cycles. At a 1C discharge and .5C charge rate that means it takes 3 hours to do one charge/discharge cycle and so at best you can do 8 cycles per day and thus it takes 625 days to do the 5000 cycle test. Car companies will not use these batteries based on a projected cycle life. So you can see some of the delay that can happen between initial positive discovery and actual implementation.

SimonDM

By the way, Brotherkenny, you seem to know more of batteries than me. I've read frequently that the solubility of sulfur in the electrolyte is the main problem, yet at the same time I've read that some Japanese company developed a Lithium conducting membrane dubbed LISICON, sounding similar to Ceramatec's NASICON. Wouldn't this be part to a solution for Li-S chemistry and why don't we hear more of that?

The comments to this entry are closed.