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PNNL team develops composite sulfur/Ni-MOF composite cathode for Li-S batteries showing excellent capacity retention

15 April 2014

Researchers at Pacific Northwest National Laboratory (PNNL) have used a novel Ni-based metal organic framework (Ni-MOF) significantly to improve the performance of Li-sulfur batteries by immobilizing polysulfides within the cathode structure through physical and chemical interactions at molecular level.

In a study reported in the ACS journal Nano Letters, the use of a sulfure/Ni-MOF composite cathode resulted in capacity retention of up to 89% after 100 cycles at 0.1 C. The research team attributed the excellent performance to the synergistic effects of the interwoven mesopores (2.8 nm) and micropores (1.4 nm) of Ni-MOF, which provide an ideal matrix to confine polysulfides, as well as the strong interactions between Lewis acidic Ni(II) center and the polysulfide base, which significantly slow down the migration of soluble polysulfides out of the pores.

Master.img-000
Left: Crystal structure of Ni-MOF containing two different types of pores represented by dark yellow sphere and blue sphere: mesopore (yellow sphere indicates pore volume; gray, C; red:,O; green, Ni; blue, N); micropore (blue sphere indicates the pore volume).
Right: Cycling performance of Ni-MOF/S composite at 0.1 and 0.2 C rates at a voltage range of 1.5−3.0 V; inset of panel is a schematic diagram illustrating the interaction between polysulfides (e.g., Li2S8/ Li2S6/Li2S4, and so forth) and paddle-wheel unit in Ni-MOF. C, O, N, S, Li and Ni atoms are represented by gray, red, blue, yellow, pink, and green spheres, respectively.
Credit: ACS, Zheng et al. Click to enlarge.

Lithium–sulfur (Li–S) batteries promise high specific capacity (1,675 mAh g–1 based on sulfur). However, they suffer from rapid capacity degradation, mainly caused by polysulfide dissolution, hampering many practical applications. Developing a solution for that problem is thus a key focus.

Li-S anode work
Lead PNNL Jie Xiao and some of her PNNL colleagues earlier reported designing a lithium–sulfur battery using electrically connected graphite and lithium metal as a hybrid anode to block polysulfides.
Lithiated graphite placed in front of the lithium metal functions as an artificial, self-regulated solid electrolyte interface (SEI) layer that actively controls the electrochemical reactions and minimizes the deleterious side reactions, leading to significant performance improvements.
Lithium–sulfur cells incorporating such hybrid anodes deliver capacities of >800 mAh g−1 for 400 cycles (4x the cycle life compared to a conventional anode) at a high rate of 1,737 mA g−1, with only 11% capacity fade and a Coulombic efficiency of more than 99%. (Earlier post.)

In recent years, many efforts have been pursued to overcome the hurdles in Li−S battery technology. Various approaches have been proposed, spanning from immobilization of sulfur in different kinds of hosting materials, sulfur cathode surface modification, electrolyte modification, and anode protection by employing LiNO3 as the electrolyte additive. While the development of new electrolyte/additive and lithium anode protection remain challenges for a long history in lithium batteries, more progress was achieved in the sulfur cathode modulation.

… Recent work from this group further correlates carbon properties with the real current density and revisits their functions at different current densities. Polymers, porous aromatic framework (PAF), intercalation compounds, and silica have also been extensively investigated for Li−S battery system. Another type of high surface area hosts, metal organic framework (MOF), however, has received much less attention probably due to its poorly conducting nature compared to the carbon scaffold.

… In the present work, we report a novel nickel-based MOF (Ni-MOF) for sulfur impregnation. … Here, we demonstrate that polysulfides can be effectively harnessed by this novel Ni-MOF, displaying remarkably improved cycling performances.

—Zheng et al.

Metal organic frameworks (MOFs) are crystal-like compounds made of metal clusters connected to organic molecules, or linkers. Together, the clusters and linkers assemble into porous 3-D structures. MOFs can contain a number of different elements. PNNL researchers chose the transition metal nickel as the central element for this particular MOF because of its strong ability to interact with sulfur.

The framework’s positively charged nickel center tightly binds the polysulfide molecules to the cathodes. The result is a coordinate covalent bond that, when combined with the framework’s porous structure, causes the polysulfides to stay put.

Having shown the effectiveness of their MOF cathode, PNNL researchers now plan to further improve the cathode’s mixture of materials to improve its energy capacity. The team also needs to develop a larger prototype and test it for longer periods of time to evaluate the cathode’s performance for real-world, large-scale applications.

MOFs are probably best known for capturing gases such as carbon dioxide. This study opens up lithium-sulfur batteries as a new and promising field for the nanomaterial.

—Jie Xiao

This research was funded by the Department of Energy’s Office of Energy Efficiency and Renewable Energy. Researchers analyzed chemical interactions on the MOF cathode with instruments at EMSL, DOE’s Environmental Molecular Sciences Laboratory at PNNL.

Resources

  • Jianming Zheng, Jian Tian, Dangxin Wu, Meng Gu, Wu Xu, Chongmin Wang, Fei Gao, Mark H. Engelhard, Ji-Guang Zhang, Jun Liu & Jie Xiao (2014) “Lewis Acid-Base Interactions Between Polysulfides and Metal Organic Framework in Lithium Sulfur Batteries,” Nano Letters doi: 10.1021/nl404721h

April 15, 2014 in Batteries, Li-Sulfur | Permalink | Comments (7) | TrackBack (0)

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Comments

89% after 100 cycles at 0.1C?
Come back later, much, much later.

Davemart,

You have no soul for science, this may be pointing in a significant direction. This taken with all the other improvements might add up to a viable path. If it were easy, they would just put a bunch of Chemistry PhDs in a room and have the solution in a week.

SJC: and yet "soul" or no, Davemart is essentially correct. This is a microscale demonstration that establishes a potential improvement path that may be promising. Yes, yay for the guys in the labcoats: it is definitely uplifting. But the best case of the words above bear out Davemart's point: "microscale", "demonstration", "potential", "may be", etc. No one said it is easy --- simply that it is merely interesting at this point.

Further, not to pick on you, but the idea of "taken with all the other improvements" has no real merit. Other chemistries, configurations, etc. that have promise are not necessarily beneficiaries of this anode strategy at all. It could very well be an orphan. In fact, your scientific soul should tell you that this is the correct assumption for now.

Not being mean, so please don't take it as such. But some cold water on the face is important. I have had oversight of far less fundamental R&D than this, and I can tell you that is an important self-discipline behavior. I also have a kid making her mark in some very fundamental bio-research and this is exactly the sort of grounded thinking she is expected to do whenever the first experiment looks promising. Not a sissy's game, that science.

@SJC:
Is that your soul objection to my point? :-)

Yes, I have no comments for Herman, he believes he is right in everything.

SJC, honestly, I don't mean to be critical: I just thought you jumped Davemart too quickly. And yes: OF COURSE I "believe" I'm right. Don't we all perceive that our own positions are correct?

But, there is a difference between how I see "science" and how you do. I am certain (yes! certain!!!) that science is (or should be) as impoverished of "belief" as possible. I know this because I wasted a considerable amount of shareholder resources on stuff that was supported by my fervent faith and that of my team. Turns out that was a bad idea. And though I believed I was right with all my soul, I wasn't and the results did not sit well with my handlers.

When we look at the most basic R&D and begin projecting where it might go, it's fun and exciting and we all do it, but that's not science.

I like the things you post and frequently learn from them. Again, no bad intentions, so I apologize for leaving that impression.

The world may be split into three general groups:

1. the believers.
2. the non-believers.
3. the half full, half empty or the wait and see group.

Many people in Group 1 often pick the wrong guy but many pick the right one a get to be very rich.

Most people in Group 2 cannot make their mind and never pick the wrong or right guy. They rarely win or lose much.

People in Group 3 are more patient and wait till winners are known to buy in. They rarely lose but are often small winners.

All three are smart but different.

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