## Pellion Technologies leveraging high throughput computational materials design to make progress on Mg-ion rechargeable batteries

##### 13 June 2011

While the current generation of lithium-ion batteries has enabled the very visible return of a still-nascent plug-in electric vehicle market, industry stakeholders on all sides are clear about the need for more advanced energy storage technologies that transcend the limitations of current Li-ion couples in terms of cost and/or capacity in order to support a mass market for these plug-ins. (Earlier post.)

Last week’s 4th Symposium on Energy Storage: Beyond Lithium-ion, hosted by the Pacific Northwest National Laboratory (PNNL), brought together researchers tackling the “Beyond Li-ion” problem by working on a number of different platforms (e.g., Li-O2, Li-S, Mg-ion, Zn-air, Na-metal halide) and components (electrodes, electrolytes) and exploring different synthesis techniques. One basic challenge in developing a “Beyond Li-ion” solution is identifying the appropriate materials.

At the symposium, Dr. Kristin A. Persson, a scientist at Lawrence Berkeley National Laboratory, described the benefit of using high throughput computation to tackle this problem in battery research. It is now possible to leverage computational power to calculate a wide variety of properties that are relevant for advanced energy storage systems, she said, enabling researchers to identify performance bottlenecks in known materials as well as screen hypothetical materials to accelerate new materials discovery.

The use of such an approach combined with database capabilities enables the creation of a “materials genome”, Persson said.

Persson is also a co-founder of Pellion Technologies—an MIT spin-off co-founded by Dr. Gerbrand Ceder, funded by Vinod Khosla, and recipient of an ARPA-E grant (earlier post) that is developing a magnesium-ion (Mg-ion) rechargeable battery. Pellion is leveraging such high throughput computational materials design, coupled with accelerated materials synthesis and electrolyte optimization to identify new high-energy-density magnesium cathode materials and compatible electrolyte chemistries.

Pellion, said ARPA-E’s Dr. Dawson Cagle in an earlier talk at the symposium, is targeting the development of a Mg-ion battery with 2–4x the energy density of Li-ion batteries, that would be compatible with the existing Li-ion manufacturing infrastructure. “Pellion is one of our higher-risk projects,” Cagle said.

Persson noted as part of her talk that by leveraging such high throughput techniques, Pellion has already screened some 9,600 cathode candidates and moved 15 of those into the lab, where they are cycling them under different conditions. This represents a significant speed-up in development time.

Magnesium-based couples have been discussed for some time as having the potential to achieve a high specific energy and eventually replace Li-based couples.

Magnesium is divalent, thereby displacing double the charge per ion, Persson noted (i.e., Mg2+ rather than Li+). As an element, magnesium is much more abundant than lithium, and more stable. Magnesium-ion batteries theoretically could offer good electrochemical performance, while being safer and less expensive than Li-ion batteries.

However, Mg-ion batteries have suffered from a number of limitations. Mg2+ ion insertion into ion-transfer hosts proceeds slowly compared with Li+, so it is necessary to realize fast Mg2+ transport in addition to other requirements, noted a 2003 study by researchers at Nankai University in China. Researchers from Bar-Ilan university (one of Pellion’s partners in the ARPA-E award, focusing on the electrolyte) in a paper published in Chemical Materials in 2009 outlined more specific limitations:

• Anode/electrolyte incompatibility. Magnesium, like lithium, develops surface films upon exposure to oxygen, humidity, and most polar organic solvents as a result of reduction reactions. Unlike lithium’s, however, Mg interphase formed is a real passivation barrier since it is both electron and ion insulating. In other words, in most polar organic electrolytes such as those commonly used in lithium batteries, magnesium anodes do not function as reversible electrodes.

However, earlier studies of Mg cathodes were performed in such electrolytes, Levi et al. point out, with some of the works not using Mg anodes, but different sources of Mg2+ ions)

Hence, once any of these cathode materials proves viable for practical use, a full compatibility study of these materials with proper electrolyte should be necessary. Such a study will be particularly crucial for hydrated cathodes because of incompatibility between water and magnesium or between water and some of the feasible electrolytic solutions.

• Narrow electrochemical windows of the electrolyte solutions. With a narrow electrochemical stability window, organohaloaluminate/ether electrolytes limits greatly the choice of cathodes for Mg-ion batteries, and hence its potential specific energy.

• Slow solid-state diffusion of Mg2+ cations. The majority of the intercalation compounds, proved to be good Li cathodes, show poor, if any, electrochemical activity with Mg, usually because of the exceedingly slow solid-state diffusion of the divalent cations.

The idea was that we need a high energy density cathode that works with the electrolyte and a new anode. We are using exactly the same screening criteria [as described in her talk for Lithium system] only pertaining to magnesium. What we did find out though and as is well known in the literature is that we are diffusion limited.

But, here’s where computations come in. We can find a few outliers there, where it actually moves quite well in the compound. We don’t have to go through all the literature, we can pick those out by our high throughput screening of everything that is out there. While in general, it’s true magnesium moves slower, there are outliers. We have synthesized some cathodes already.

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Post Lead, NIMH, Li-On batteries (e-storage units) are bound to come about and may be more numerous than we think by 2020+. It's all a question of how much worldwide efforts and resources are available and effectively used for e-storage development.

The world will find ways to progressively improve performance by up to 4x or to about 1000 Wh/Kg and reduce mass production cost from $400/Kwh to$100/Kwh in the next 20 years or so. That's what is required for future affordable highway capable e-vehicles to replace the current ICE fleets.

Meanwhile, we will have improved HEVs,/PHEVs and progressively improved city BEVs. City e-buses will be out in large numbers very soon.

I am still holding out for rechargeable air batteries, the capacity is what we are looking for and they can use supercaps or regular lithium ion for a current buffer.

What? You guys have given up on EEStor??

The evidence for a Poe accumulates.

I look back at the NiMH batteries of the EV1 and RAV4EV and wonder. Now that Bosch owns the Cobasys patents, we could see NiMH pouch cells and prismatic cases. A 200 Wh battery could appear again. I know, size, cost, weight...but when you look at plug hybrids, it is tempting.

NiMH is worse than most lithium chemistry in all respects. Why exactly would we see it in anything?

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