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Researchers in China propose new strategy for Li-S batteries: smaller sulfur molecules

1 November 2012

Xin
Electrochemical properties of sulfur-carbon composite. (a) GDC voltage profiles of S/(CNT@MPC) at 0.1 C. (b) Cycling performance of S/(CNT@MPC) and S/CB at 0.1 C (blue circles show the Coulombic efficiency of S/(CNT@MPC)). (c) GDC voltage profiles of S/(CNT@MPC) at different discharge/charge rates. (d) Rate capabilities of S/(CNT@MPC) and S/CB. Credit: ACS, Xin et al. Click to enlarge.

Researchers in China are proposing a new strategy for addressing some of the issues limiting the commercialization of high energy density lithium-sulfur batteries (earlier post): using small sulfur allotropes (different forms of the same element)—i.e., S2−4—confined in a microporous carbon (MPC) matrix with pore size of ∼0.5 nm.

In a paper published in the Journal of the American Chemical Society, they reported that a sulfur−carbon composite containing S2−4 molecules (S/(CNT@MPC)) shows “admirable” electrochemical properties in terms of specific capacity, cycling stability, and high rate capability and suggests a practical Li−S battery with high energy density for applications in portable electronics, electric vehicles, and large-scale energy storage systems.

Among the best candidates for next-generation high-energy-storage systems, metal−sulfur batteries, such as Li−S, Na−S, and Mg−S, hold high theoretical energy densities, making them especially attractive. Of these, the Li−S battery has the highest theoretical energy density of 2567 Wh kg−1, calculated on the basis of the Li anode (∼3,860 mAh/g) and the S cathode (∼1675 mAh/g), making it a promising choice for the next generation of high-energy rechargeable batteries.

The application of Li−S batteries suffers from two major issues. One involves with the use of lithium metal as anode, which may raise safety concerns for practical applications due to the growth of lithium dendrite...The other issue lies in the sulfur cathode. It is known that a Li−S battery with a cyclooctasulfur (cyclo-S8) cathode usually discharges stepwise, with two plateaus in its voltage profile. At the first plateau (∼2.3 V vs Li+/Li), sulfur is reduced from S8 to S42−, during which various electrolyte-soluble polysulfides (Li2Sn, n = 4−8) form. The second plateau (∼1.95 V vs Li+/Li) corresponds to the transformation from Li2S4 to insoluble Li2S2 and finally Li2S.

The performance of the Li−S battery is therefore limited by the insulating problem with sulfur and the dissolution and shuttling problem with polysulfides in liquid electrolyte. Many efforts have been made to improve the performance of sulfur, including the use of various conducting substrates and new electrolytes, e.g., solid-state and ionic liquid types. Though the former leads to improved electrical conductivity, it does not solve the intrinsic problem with dissolution of polysulfides due to the contained cyclo-S8 molecules. The latter, intended to relieve the dissolution problem of polysulfides, suffers from the low ionic conductivity of electrolyte at room temperature.

—Xin et al.

In their study, the team from the Key Laboratory of Molecular Nanostructure and Nanotechnology and Beijing National Laboratory for Molecular Sciences, Beijing National Laboratory for Condensed Matter Physics, and Bosch Research & Technology Center, China, found that the confined small S2−4 molecules avoid the unfavorable transition between S8 and S42− during discharging/ charging and typically give a single long output plateau at ∼1.9 V.

To create the material, they started with multi-walled carbon nanotubes (CNTs, average diameter = 50 nm) and then coated them with a MPC layer via a solution-based method (CNT@MPC). The resulting cumulative pore volume for the micropores <0.6 nm was calculated to be 0.46 cm3/g—corresponding to a theoretical loading of 49 wt% sulfur in the micropores, based on the density of sulfur (2.07 g/cm3).

Heating a mixture of sulfur and the CNT@MPC results in the diffusion of sulfur as chain-like molecules into the MPC layer to form the sulfur−carbon composite (S/(CNT@ MPC). The S/(CNT@MPC) contained ∼40 wt% S.

In view of the micropore size of 0.5 nm, only small S2−4 molecules can be accommodated in the micropores of MPC, while the large S5−8 molecules cannot be stored. In this way, the chain-like sulfur molecules in the carbon micropores could not transform to the large S8 rings but remain as S2−4 molecules.

—Xin et al.

For electrochemical testing, they assembled the S/(CNT@MPC) into Li−S batteries. For comparison, a sulfur−carbon composite containing cyclo-S8 was also tested, synthesized by mixing sulfur powder and carbon black (S/CB) with the same sulfur content, 40 wt%.

They found that, when using a carbonate-based electrolyte of 1 M LiPF6 in ethylene carbonate/dimethyl carbonate (1:1 wt%), the composite exhibits an initial discharge capacity of 1670 mAh/g at 0.1 C—close to the theoretical capacity of sulfur (1675 mAh/g), and much higher than that of S/CB (∼573 mAh/g) under the same experimental conditions.

The composite also demonstrated cycling stability with 1149 mAh/g after 200 cycles, and a favorable high-rate capability of 800 mAh/g at 5 C.

The success of the novel S cathode promises a new Li−S battery with higher energy density (785 Wh/kg based on anode and cathode) than state-of-the-art Li-ion batteries (theoretically 387 Wh/kg in a LiCoO/C battery) for powering our future electronics. In view of the wide applications of sulfur such as fertilizers, medicines, fungicides, and cell nutrients, our discovery of these unusual small sulfur allotropes may trigger wide research interest in these fields as well.

—Xin et al.

Resources

  • Smaller Sulfur Molecules Promise Better Lithium–Sulfur Batteries Sen Xin, Lin Gu, Na-Hong Zhao, Ya-Xia Yin, Long-Jie Zhou, Yu-Guo Guo, and Li-Jun Wan Journal of the American Chemical Society doi: 10.1021/ja308170k

November 1, 2012 in Batteries, Li-Sulfur | Permalink | Comments (9) | TrackBack (0)

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Comments

Another possibility to improve batteries performance. Will somebody manage to combine some of the latest improvements to produce super batteries for future extended range EVs by 2020 or so?

They should talk to Calbattery. One has an anode, the other a cathode.

Who could get those two (and others) to try to combine their research and findings to arrive at a much higher performance unit and avoid an endless patent war?

There seems to be a US pre-election swarm of battery breakthroughs.

Show/sell me the money/battery.

It seems silicon anodes will work with sulfur cathodes, so in that case a combination can be done. In other cases the anode must be carefully matched to the cathode, so no combining.

Mass production of very high performance (1000+ Wh/Kg) quick charge/discharge batteries are is essential for the world to free itself from fossil fuels for most ground vehicles.

As such, the Group of 8 or 20 industrial nations should make it an urgent priority, coordinate and finance the development and shared mass production.

HarveyD,

Those same 8 to 20 industrial nations are heavily invested in oil. It's kind of like the drug war now. So many prison guards depend on drug criminals for their living, we will never want to end the cycle of drugs and violence because it is profitable and a job creator. Nor will we want as societies and governments to end the violence and dependency on oil because it's so profitable and jobs depend on it. No, the only way we get better batteries is through a private company that has enough backing to commercialize, and one that in fact want to do that, despite possible buy out attempts by the oilers.

I suspect the enormous fortunes to be made by the first company(ies) that can bring an affordable high capacity EV battery to market is all that is needed to drive development at full speed.

The free market can smell profits and a closing time window due to the number of labs working on the technology. I doubt that we need to do anything to stimulate battery development.

Nice thing about these batteries being developed outside of car companies is that results can be spread across all car manufacturers quickly. Car manufacturers can switch from battery manufacturer to battery manufacturer as capacity/price changes. Each won't have to invent their version of "the wheel".

Good observations Bob.

We have at least three choices.

1. Continue to subsidize Oil to fight high performance battery development.

2. Increase direct and indirect support for accelerated battery development.

3. Do little or nothing and let others (like China) to it and reap in the benefits and jobs.

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