Green Car Congress  
Go to GCC Discussions forum About GCC Contact  RSS Subscribe Twitter headlines

« Nissan introduces updated 2013 LEAF; US assembly in Smyrna | Main | Proposed 5th edition of Worldwide Fuel Charter introduces new Category 5 for fuel efficiency and emission control »

Print this post

Sulfur–TiO2 yolk-shell cathode for Li-sulfur battery shows best long-cycle performance so far

9 January 2013

Seh1
Electrochemical performance of sulfur–TiO2 yolk–shell nanostructures. (a) Charge/discharge capacity and Coulombic efficiency over 1,000 cycles at 0.5 C. (b) Capacity retention of sulfur–TiO2 yolk–shell nanostructures cycled at 0.5 C, in comparison with bare sulfur and sulfur–TiO2 core–shell nanoparticles. Seh et al. Click to enlarge.

Researchers at Stanford University and SLAC led by Stanford associate professor Yi Cui have used a sulfur–TiO2 yolk–shell design for a cathode material for a lithium-sulfur battery that achieved an initial specific capacity of 1,030 mAh g−1 at 0.5 C and Coulombic efficiency of 98.4% over 1,000 cycles. Most importantly, the team reported in a paper in the journal Nature Communications, the capacity decay after 1,000 cycles is as small as 0.033% per cycle, which represents the best performance for long-cycle lithium–sulfur batteries so far.

The advantage of the yolk–shell structure is the presence of the internal void space that can accommodate the large volumetric expansion of sulfur during lithiation, thus preserving the structural integrity of the shell and minimizing polysulfide dissolution and enabling the high capacity retention. The authors say that, to the best of their knowledge, this is the first time that a lithium–sulfur battery with this level of performance has been described.

Sulphur is a promising cathode material with a theoretical specific capacity of 1,673 mAh g-1, which is ~5 times that of existing materials based on transition metal oxides and phosphates. However, many challenges remain in developing a practical lithium–sulphur battery for commercialization. It is known that sulphur particles suffer from the problems of (a) poor electronic conductivity, (b) dissolution of intermediate polysulphides and (c) large volumetric expansion (~80%) upon lithiation, which results in rapid capacity decay and low Coulombic efficiency.

Over the years, extensive efforts have been devoted to addressing the first two problems, by encapsulating sulphur particles with conducting materials, including porous carbon, graphene oxide and conductive polymers, in an attempt to improve their electronic conductivity and limit polysulphide dissolution. However, insufficient emphasis has been placed on dealing with the third challenge—the large volumetric expansion of sulphur during lithiation coupled with polysulphide dissolution. This poses a critical problem because volume expansion of the sulphur core will cause the protective coating layer to crack and fracture, rendering the conventional core–shell morphology ineffective in trapping polysulphides.

...Herein, we demonstrate for the first time, the design of a sulphur–TiO2 yolk–shell nanoarchitecture for stable and prolonged cycling over 1,000 charge/discharge cycles in lithium–sulphur batteries.

—Seh et al.

Seh2
Schematic of the lithiation process in various sulfur-based nanostructure morphologies. (a) Bare sulfur particles undergo large volumetric expansion and polysulfide dissolution upon lithiation, resulting in rapid capacity decay and low Coulombic efficiency. (b) Although the core–shell morphology provides a protective coating, cracking of the shell will occur upon volume expansion of sulfur during lithiation, leading to polysulfide dissolution as well. (c) The yolk–shell morphology provides internal void space to accommodate the volume expansion of sulfur during lithiation, resulting in a structurally intact shell for effective trapping of polysulfides. Seh et al. Click to enlarge.

To prepare the material, the team reacted sodium thiosulfate with hydrochloric acid to create monodisperse sulfur nanoparticles (NPs); these NPs were then coated with TiO2, resulting in the formation of sulfur–TiO2 core–shell nanoparticles. This was followed by partial dissolution of sulfur in toluene to create an empty space between the sulfur core and the TiO2 shell, resulting in the yolk–shell morphology.

For electrochemical testing, they fabricated 2032-type coin cells with the material; using lithium foil as the counter electrode, the cells were cycled from 1.7–2.6 V versus Li+/Li.

They estimated the volume of empty space in the yolk–shell nanostructures to be 37%; this space can accommodate ~60% volume expansion of the sulfur present within the shell, allowing for 1,250 mAh g-1—i.e., 75% of the maximum theoretical capacity of sulfur, to be utilized (assuming volume expansion is linearly dependent on the degree of lithiation). Experimentally, they were able to achieve a maximum discharge capacity of 1,215 mAh g-1.

It basically worked the first time we tried it. The sulfur cathode stored up to five times more energy per sulfur weight than today’s commercial materials.

After 1,000 charge/discharge cycles, our yolk-shell sulfur cathode had retained about 70 percent of its energy-storage capacity. This is the highest performing sulfur cathode in the world, as far as we know. Even without optimizing the design, this cathode cycle life is already on par with commercial performance. This is a very important achievement for the future of rechargeable batteries.

—Yi Cui

Funding for the project came from the DOE Office of Basic Energy Sciences through SLAC’s Laboratory Directed Research and Development Program, which directs a percentage of the lab’s funding to high-risk, high-payoff research that, if successful, can lead to future program opportunities.

Over the past seven years, Cui’s group has demonstrated a succession of increasingly capable anodes that use silicon rather than carbon because it can store up to 10 times more charge per weight. Their most recent anode also has a yolk-shell design that retains its energy-storage capacity over 1,000 charge/discharge cycles.

The group’s next step is to combine the yolk-shell sulfur cathode with a yolk-shell silicon anode to see if together they produce a high-energy, long-lasting battery.

Resources

  • Zhi Wei Seh, Weiyang Li, Judy J. Cha, Guangyuan Zheng, Yuan Yang, Matthew T. McDowell, Po-Chun Hsu & Yi Cui (2013) Sulphur–TiO2 yolk–shell nanoarchitecture with internal void space for long-cycle lithium–sulfur batteries. Nature Communications 4, Article number: 1331 doi: 10.1038/ncomms2327

January 9, 2013 in Batteries | Permalink | Comments (3) | TrackBack (0)

TrackBack

TrackBack URL for this entry:
http://www.typepad.com/services/trackback/6a00d8341c4fbe53ef017ee724c299970d

Listed below are links to weblogs that reference Sulfur–TiO2 yolk-shell cathode for Li-sulfur battery shows best long-cycle performance so far:

Comments

Nice work, smart approach, very good results but applications seems far away...

More research groups (worldwide) should contribute actively to develop this (and similar) approaches to accelerate the development and mass production of 1000 Wh/Kg batteries capable of sustaining 2000+ cycles, for future extended range BEVs.

Nice approach, but TiO2 may not be the optimum shell layer. There is definitely room for improvement.

They should be able to tell us whether they did in fact prevent sulfur transfer to the anode. aI say that because their degradation rate is still too high. However, typically when new materials are used in a electrochemical cell, the electrolyte and surfaces have to be optimized. We know the main problem with sulfur is polysulfide migration to the anode, so if they could provide evidence that they do indeed prevent polysulfide migration it would suggest that this cell is simply some materials optimization work away from the solution. On the otherhand, if they still have some polysulfide contamination, they would need to either change the shell material or improve their synthesis problem.

It is very common for modern researchers to confuse the issues intentionally. Often they justify this as protection of proprietary information, however, it is often really just obfuscation of the eventual shortcomings and failure of the proposed system. We don't do a good job of knowing who is capable and who just acts capable when deciding on funding, indeed, just as most people vote for the talking heads of superficiality, it's easier to select a researcher that talks a good talk. You know, the fakers that say what they know the evaluaters want to hear.

Verify your Comment

Previewing your Comment

This is only a preview. Your comment has not yet been posted.

Working...
Your comment could not be posted. Error type:
Your comment has been posted. Post another comment

The letters and numbers you entered did not match the image. Please try again.

As a final step before posting your comment, enter the letters and numbers you see in the image below. This prevents automated programs from posting comments.

Having trouble reading this image? View an alternate.

Working...

Post a comment

Green Car Congress © 2014 BioAge Group, LLC. All Rights Reserved. | Home | BioAge Group