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New cathode design and understanding of electrolyte delivers greater efficiency in magnesium-ion batteries

25 August 2017

Researchers have achieved a significant boost in the storage capacity of magnesium-ion batteries through a new design for the cathode and a new understanding of the electrolyte. In an open-access paper in the journal Nature Communications, they report a battery chemistry that utilizes magnesium mono-chloride cations in expanded titanium disulfide.

The battery demonstrates the reversible intercalation of 1 and 1.7 magnesium monochloride cations per titanium at 25 and 60 °C, respectively, corresponding to up to 400 mAh g−1 capacity based on the mass of titanium disulfide. The large capacity accompanies with excellent rate and cycling performances even at room temperature, opening up possibilities for a variety of effective intercalation hosts for multivalent-ion batteries.

Magnesium rechargeable batteries (MRBs) are emerging as an attractive candidate for energy storage in terms of safety, energy density, and scalability because magnesium metal has ideal properties as a battery anode: high capacity, low redox potential, dendrite-free deposition, and earth-abundant resources. Since the first MRB prototyped by Aurbach et al., significant progress has been made in cathodes, electrolytes, and anodes. One critical challenge for MRBs is the development of Mg storage cathodes with higher capacity and operating voltage than Chevrel phase Mo6S8 cathodes, which operate at ca. 1 V vs Mg/Mg2+ with capacity of ca. 100 mAh g–1.

… In this work, we report a MRB based on a MgCl+ intercalation cathode, a Mg anode, and a standard chloride-based electrolyte. Moving from the divalent Mg2+ to the monovalent MgCl+ as the charge carrier makes Mg- ions similar to one-electron-transfer alkaline metal ions where (1) only low-energy desolvation (Ea~0.8 eV) but not high-energy Mg−Cl scission (Ea> 3 eV) is necessary before intercalation and (2) the polarization strength ofthe ion, and hence the ion diffusion energy barrier, is low. The new battery chemistry illustrated using interlayer-expanded titanium disulfide (TiS2) cathode as an example demonstrates 1 and 1.7 MgCl+ intercalation per formula of TiS2 at 25 and 60 °C, respectively, corresponding to high reversible capacities of up to 400 mAh g−1 based on the mass of TiS2.

—Yoo et al.

The work was first conceived by Yan Yao, associate professor of electrical and computer engineering at the University of Houston and postdoctoral fellow Hyun Deog Yoo in 2014; the project spanned several years and involved scientists from three universities and three national laboratories, working both experimentally and theoretically.

Magnesium ion is known to be hard to insert into a host. First of all, it is very difficult to break magnesium-chloride bonds. More than that, magnesium ions produced in that way move extremely slowly in the host. That altogether lowers the battery’s efficiency.

—Hyun Deog Yoo, first author

The new battery stores energy by inserting magnesium monochloride into a host, such as titanium disulfide. By retaining the magnesium-chloride bond, Yao said, the cathode demonstrated much faster diffusion than traditional magnesium versions.

Voltage of the new battery remains low at about one volt. That compares to three to four volts for lithium batteries. The high voltage, coupled with their high energy density, has made lithium ion batteries the standard. But lithium is expensive and can develop dendrite growths, which can cause the batteries to catch fire. An earth-abundant resource, magnesium is cheaper—and does not form dendrites.

Until now, however, it has been held back by the need for a better cathode and more efficient electrolytes.

The key, Yoo said, is to expand the titanium disulfide to allow magnesium chloride to be inserted—a four-step process called intercalation—rather than breaking the magnesium-chloride bonds and inserting the magnesium alone. Retaining the magnesium-chloride bond doubled the charge the cathode could store.

The magnesium monochloride molecules are too large to be inserted into the titanium disulfide using conventional methods. Building upon their earlier work, the researchers created an open nanostructure by expanding the gaps in the titanium disulfide by 300 percent, using organic “pillars.”

The increased opening—from 0.57 nanometers to 1.8 nanometers—allowed for the magnesium chloride to be inserted.

We hope this is a general strategy. Inserting various polyatomic ions in higher voltage hosts, we eventually aim to create higher-energy batteries at a lower price, especially for electric vehicles.

—Hyun Deog Yoo

In addition to Yao and Yoo, authors on the paper include Yanliang Liang, Hui Dong, Yifei Li, Qiang Ru, Yan Jing and Qinyou An, all of UH; Junhao Lin and Sokrates T. Pantelides of Vanderbilt University and the Oak Ridge National Laboratory; Wu Zhou of Oak Ridge National Laboratory; Hua Wang and Xiaofeng Qian of Texas A&M University; Yisheng Liu and Jinghua Guo of Lawrence Berkeley National Laboratory; Lu Ma, Tianpin Wu and Jun Lu of Argonne National Laboratory.

Resources

  • Hyun Deog Yoo, Yanliang Liang, Hui Dong, Junhao Lin, Hua Wang, Yisheng Liu, Lu Ma, Tianpin Wu, Yifei Li, Qiang Ru, Yan Jing, Qinyou An, Wu Zhou, Jinghua Guo, Jun Lu, Sokrates T. Pantelides, Xiaofeng Qian & Yan Yao (2017) “Fast kinetics of magnesium monochloride cations in interlayer-expanded titanium disulfide for magnesium rechargeable batteries” Nature Communications 8, Article number: 339 doi: 10.1038/s41467-017-00431-9

August 25, 2017 in Batteries | Permalink | Comments (10)

Comments

'Voltage of the new battery remains low at about one volt. That compares to three to four volts for lithium batteries.'

No good at all for BEVs, I would have thought?

OK for stationary storage, perhaps.

Davemart, with a higher energy density couldn't these batteries be smaller and connected in series to get the desired voltage with the same volume as lithium. Magnesium is a lightweight substance which makes it a plus for BEVs. I would like to know how they holds up in cold temperatures.

400 mAh/g at 1 V is 400 Wh/kg, which is quite high.

The voltage is really not a concern, unless you go to very low voltage, where several hundreds of batteries need to be connected serially to achieve proper voltage.

Anyway, the voltage in the battery and motor supply is handled by a DC/DC converter. Too low voltage results in too high current, which necessitates heavy and expensive cables.

Hi Jam-Ink.

Dunno,

I would have thought that such a low voltage would make configuring it for a car too difficult, but I am hoping that folk who know a lot more will chime in to advise.

In automotive applications, cell counts are usually high enough that you could reconfigure the pack for more serial cell layouts.

For instance, the Tesla Model 3 has two battery configurations, based on 31 or 46 cell parallel bricks. In either configuration, the bricks are laid out in two groups of 25, two groups of 23, each brick being serially connected, for a 96S31P or 96S46P pack. (Source for that info is Electrek.)

So, the nominal pack voltage on the Tesla Model 3 in either battery configuration is 345.6 V (assuming 3.6 V nominal cell voltage), and I believe full charge will be in the area of 398.4 V (assuming 4.15 V per cell).

If these cells are 1 volt full charge, you're looking at roughly a 400S configuration. So, for the long range pack, 400S11P gives you 4400 cells (instead of the 4416 cells of the current long range pack), and for the standard pack, I'd lean towards 400S8P for 3200 cells instead of the 2976 it has currently (the other way to go would be 400S7P, and that would reduce capacity significantly... unless that capacity isn't needed).

If they're 1 volt nominal, you're looking at roughly a 346S configuration. If there's room, 346S13P gives you 4498 cells, if not, 346S12P gives you 4152 cells. 346S9P is where I'd go for the standard pack, for 3114 cells.

Do note that this is easier on Tesla packs, where there's a lot of parallel smaller cells. Other automakers use large prismatic cells and may need to use more, smaller cells to get the voltage up where they want it.

Low voltage per cell is a soluble issue but slower charge and discharge may be a greater problem to solve?

Toyota is working on magnesium as well, they may sort it out eventually. First solid electrolyte, then sulfur, we will get there a step at a time.

Magnesium Rechargeable batteries (MRBs) are definitely a possible Next Gen battery solution. Though there are still many issues related to these batteries and more research is still required. After reading the article, two possible solutions could resolve some of these issues (like the low voltage problem).
1. There is reference in the article to Toyota research: in the article's Discussion: "halogen-free electrolytes are under development for even wider voltage windows" (reference 24 - Dr. Rana Mohtadi of TRINA is one of the contributors). It further states:
"Attempts to use the same approach for high-voltage cathode (e.g., layered vanadium oxide) was not quite successful so far due to the limitation of the nucleophilic nature of the APC electrolyte, which reacts chemically with oxides. It is worthwhile to re-examine this method to layered oxide cathodes, when non-nucleophilic electrolytes with higher voltage stability window become widely available."
2. Other researchers have tried to resolve this by looking into Hybrid Magnesium Lithium Ion Batteries (also called Daniell-type batteries). These add a Mg−Li dual-salt electrolyte to the Battery (with a Magnesium metal anode and appropriate cathode, e.g. Vanadium Oxide. One reference that has open access - "VO2 Nanoflakes as the Cathode Material of Hybrid Magnesium-Lithium-Ion Batteries with High Energy Density", ACS Appl. Mater. Interfaces, 2017, 9 (20), pp 17060–17066 DOI: 10.1021/acsami.7b02480 Publication Date (Web): May 3, 2017.

A Swiss group/firm has built a sporty prototype car using 48 Volts nanoflow batteries and (2X or 4X) 48 volts e-motors, with a range of 1000+ Km @ up to 200 kph with 2 x 95L of bi-ion liquid fuel.

High amperage cabling doesn't seem to be a major problem. All essential accessories can operate on 48 Volts instead of 12 Volts.

BEVs could also operate efficiently with 48 Volts batteries with similar e-motors and accessories?

The Quantino video shows a car going slowly that could be powered by conventional batteries, nothing is independently verified. The Formula E would be verified.

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