Antimony nanocrystals for batteries

TEM image (false coloured) of monodisperse antimony nanocrystals. 
(Photo: Maksym Kovalenko Group / ETH Zurich)

Researchers from ETH Zurich and Empa have succeeded for the first time to produce uniform antimony nanocrystals. Tested as components of laboratory batteries, these are able to store a large number of both lithium and sodium ions. These nanomaterials operate with high rate and may eventually be used as alternative anode materials in future high-energy-density batteries.




The hunt is on – for new materials to be used in the next generation of batteries that may one day replace current lithium ion batteries. Today, the latter are commonplace and provide a reliable power source for smartphones, laptops and many other portable electrical devices.

On the one hand, however, electric mobility and stationary electricity storage demand a greater number of more powerful batteries; and the high demand for lithium may eventually lead to a shortage of the raw material. This is why conceptually identical technology based on sodium-ions will receive increasing attention in coming years. Contrary to lithium batteries, researched for more than 20 years, much less is known about materials that can efficiently store sodium ions.

Antimony electrodes?

A team of researchers from ETH Zurich and Empa headed by Maksym Kovalenko may have come a step closer to identifying alternative battery materials: they have become the first to synthesise uniform antimony nanocrystals, the special properties of which make them prime candidates for an anode material for both lithium-ion and sodium-ion batteries. The results of the scientists’ study have just been published in Nano Letters.

For a long time, antimony has been regarded as a promising anode material for high-performance lithium-ion batteries as this metalloid exhibits a high charging capacity, by a factor of two higher than that of commonly used graphite. Initial studies revealed that antimony could be suitable for rechargeable lithium and sodium ion batteries because it is able to store both kinds of ions. Sodium is regarded as a possible low-cost alternative to lithium as it is much more naturally abundant and its reserves are more evenly distributed on Earth.

For antimony to achieve its high storage capability, however, it needs to be produced in a special form. The researchers managed to chemically synthesize uniform – so-called “monodisperse” – antimony nanocrystals that were between ten and twenty nanometres in size.

The full lithiation or sodiation of antimony leads to large volumetric changes. By using nanocrystals, these modulations of the volume can be reversible and fast, and do not lead to the immediate fracture of the material. An additional important advantage of nanocrystals (or nanoparticles) is that they can be intermixed with a conductive carbon filler in order to prevent the aggregation of the nanoparticles.
Ideal candidate for anode material

Electrochemical tests showed Kovalenko and his team that electrodes made of these antimony nanocrystals perform equally well in sodium and in lithium ion batteries. This makes antimony particularly promising for sodium batteries because the best lithium-storing anode materials (Graphite and Silicon) do not operate with sodium.

Highly monodisperse nanocrystals, with the size deviation of ten percent or less, allow identifying the optimal size-performance relationship. Nanocrystals of ten nanometers or smaller suffer from oxidation because of the excessive surface area. On the other hand, antimony crystals with a diameter of more than 100 nanometres aren’t sufficiently stable due to aforementioned massive volume expansion and contraction during the operation of a battery. The researchers achieved the best results with 20 nanometer large particles.
Performance not so size dependent

Another important outcome of this study, enabled by these ultra-uniform particles, is that the researchers identified a size-range of around 20 to 100 nanometres within which this material shows excellent, size-independent performance, both in terms of energy density and rate-capability.

These features even allow using polydisperse antimony particles to obtain the same performance as with very monodisperse particles, as long as their sizes remain within this size-range of 20 to 100 nanometres.

“This greatly simplifies the task of finding an economically viable synthesis method”, Kovalenko says. “Development of such cost-effective synthesis is the next step for us, together with our industrial partner.” Experiments of his group on monodisperse nanoparticles of other materials show much steeper size-performance relationships such as quick performance decay with increasing the particle size, placing antimony into a unique position among the materials which alloy with lithium and sodium.
More expensive alternative

Does this mean that an alternative to today’s lithium-ion batteries is within our grasp? Kovalenko shakes his head. Although the method is relatively straightforward, the production of a sufficient number of high-quality uniform antimony nanocrystals is still too expensive.

“All in all, batteries with sodium-ions and antimony nanocrystals as anodes will only constitute a highly promising alternative to today’s lithium-ion batteries if the costs of producing the batteries will be comparable,” says Kovalenko.


It will be another decade or so before a sodium-ion battery with antimony electrodes could hit the market, the ETH-Zurich professor estimates. The research on the topic is still only in its infancy. “However, other research groups will soon join the efforts,” the chemist is convinced.

Lithium-ion batteries

A current lithium-ion battery comprises two electrodes – a cathode and an anode. The anode is often made of graphite, the cathode of metal oxides such as cobalt oxide. The lithium ions lodge themselves in these materials during the charging or discharging processes. The two electrodes are separated by a e wall permeable only for lithium ions traveling between the two electrodes, but not for electrons. During the discharge of a battery, the lithium ions shift from the anode to the cathode. As the electrons do not fit through, they take a detour via an electronic device, which is powered by the resulting electron flux. Electrons and ions meet again in the cathode. When the battery is charging, the ions and electrons are enforced to flow in the opposite direction. For the battery to work effectively and for a long time, the ions need to be able to move in and out of the electrode materials easily. The shape and size of the electrode materials should not change much through the recurrent absorption and release of the ions.

Scientists Invent Self-healing Battery Electrode

An electron micrograph shows cracks left in a self-healing polymer
coating due to swelling of its silicon electrode during charging. Right:
Five hours later, the smaller cracks have healed.
(C. Wang et al, Nature Chemistry)

Researchers have made the first battery electrode that heals itself, opening a new and potentially commercially viable path for making the next generation of lithium ion batteries for electric cars, cell phones and other devices. 

The secret is a stretchy polymer that coats the electrode, binds it together and spontaneously heals tiny cracks that develop during battery operation, said the team from Stanford University and the Department of Energy’s (DOE) SLAC National Accelerator Laboratory.

They report the advance in the Nov. 19 issue of Nature Chemistry.

“Self-healing is very important for the survival and long lifetimes of animals and plants,” said Chao Wang, a postdoctoral researcher at Stanford and one of two principal authors of the paper. “We want to incorporate this feature into lithium ion batteries so they will have a long lifetime as well.”

Chao developed the self-healing polymer in the lab of Stanford Professor Zhenan Bao, whose group has been working on flexible electronic skin for use in robots, sensors, prosthetic limbs and other applications. For the battery project he added tiny nanoparticles of carbon to the polymer so it would conduct electricity.

”We found that silicon electrodes lasted 10 times longer when coated with the self-healing polymer, which repaired any cracks within just a few hours,” Bao said.

“Their capacity for storing energy is in the practical range now, but we would certainly like to push that,” said Yi Cui, an associate professor at SLAC and Stanford who led the research with Bao. The electrodes worked for about 100 charge-discharge cycles without significantly losing their energy storage capacity. “That’s still quite a way from the goal of about 500 cycles for cell phones and 3,000 cycles for an electric vehicle,” Cui said, “but the promise is there, and from all our data it looks like it’s working.”

Researchers worldwide are racing to find ways to store more energy in the negative electrodes of lithium ion batteries to achieve higher performance while reducing weight. One of the most promising electrode materials is silicon; it has a high capacity for soaking up lithium ions from the battery fluid during charging and then releasing them when the battery is put to work.

But this high capacity comes at a price: Silicon electrodes swell to three times normal size and shrink back down again each time the battery charges and discharges, and the brittle material soon cracks and falls apart, degrading battery performance. This is a problem for all electrodes in high-capacity batteries, said Hui Wu, a former Stanford postdoc who is now a faculty member at Tsinghua University in Beijing, the other principal author of the paper.

To make the self-healing coating, scientists deliberately weakened some of the chemical bonds within polymers – long, chain-like molecules with many identical units. The resulting material breaks easily, but the broken ends are chemically drawn to each other and quickly link up again, mimicking the process that allows biological molecules such as DNA to assemble, rearrange and break down.

To show how flexible their self-healing polymer is, researchers coated a balloon with it and then inflated and deflated the balloon repeatedly, mimicking the swelling and shrinking of a silicon electrode during battery operation. The polymer stretches but does not crack. (Brad Plummer/SLAC)

Researchers in Cui’s lab and elsewhere have tested a number of ways to keep silicon electrodes intact and improve their performance. Some are being explored for commercial uses, but many involve exotic materials and fabrication techniques that are challenging to scale up for production.

The self-healing electrode, which is made from silicon microparticles that are widely used in the semiconductor and solar cell industries, is the first solution that seems to offer a practical road forward, Cui said. The researchers said they think this approach could work for other electrode materials as well, and they will continue to refine the technique to improve the silicon electrode’s performance and longevity.

The research team also included Zheng Chen and Matthew T. McDowell of Stanford. Cui and Bao are members of the Stanford Institute for Materials and Energy Sciences, a joint SLAC/Stanford institute. The research was funded by DOE through SLAC’s Laboratory Directed Research and Development program and by the Precourt Institute for Energy at Stanford University.

SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the U.S. Department of Energy Office of Science. To learn more, please visitwww.slac.stanford.edu.

The Stanford Institute for Materials and Energy Sciences (SIMES) is a joint institute of SLAC National Accelerator Laboratory and Stanford University. SIMES studies the nature, properties and synthesis of complex and novel materials in the effort to create clean, renewable energy technologies. For more information, please visit simes.slac.stanford.edu.

Source: https://www6.slac.stanford.edu/news/2013-11-17-scientists-invent-self-healing-battery.aspx

Better batteries through biology?

MIT researchers find a way to boost lithium-air battery performance, with the help of modified viruses.

Lithium-air batteries have become a hot research area in recent years: They hold the promise of drastically increasing power per battery weight, which could lead, for example, to electric cars with a much greater driving range. But bringing that promise to reality has faced a number of challenges, including the need to develop better, more durable materials for the batteries’ electrodes and improving the number of charging-discharging cycles the batteries can withstand.

Now, MIT researchers have found that adding genetically modified viruses to the production of nanowires — wires that are about the width of a red blood cell, and which can serve as one of a battery’s electrodes — could help solve some of these problems.

The new work is described in a paper published in the journal Nature Communications, co-authored by graduate student Dahyun Oh, professors Angela Belcher and Yang Shao-Horn, and three others. The key to their work was to increase the surface area of the wire, thus increasing the area where electrochemical activity takes place during charging or discharging of the battery.

The researchers produced an array of nanowires, each about 80 nanometers across, using a genetically modified virus called M13, which can capture molecules of metals from water and bind them into structural shapes. In this case, wires of manganese oxide — a “favorite material” for a lithium-air battery’s cathode, Belcher says — were actually made by the viruses. But unlike wires “grown” through conventional chemical methods, these virus-built nanowires have a rough, spiky surface, which dramatically increases their surface area.

Belcher, the W.M. Keck Professor of Energy and an affiliate of MIT’s Koch Institute for Integrative Cancer Research, explains that this process of biosynthesis is “really similar to how an abalone grows its shell” — in that case, by collecting calcium from seawater and depositing it into a solid, linked structure. 

The increase in surface area produced by this method can provide “a big advantage,” Belcher says, in lithium-air batteries’ rate of charging and discharging. But the process also has other potential advantages, she says: Unlike conventional fabrication methods, which involve energy-intensive high temperatures and hazardous chemicals, this process can be carried out at room temperature using a water-based process.

Also, rather than isolated wires, the viruses naturally produce a three-dimensional structure of cross-linked wires, which provides greater stability for an electrode.

A final part of the process is the addition of a small amount of a metal, such as palladium, which greatly increases the electrical conductivity of the nanowires and allows them to catalyze reactions that take place during charging and discharging. Other groups have tried to produce such batteries using pure or highly concentrated metals as the electrodes, but this new process drastically lowers how much of the expensive material is needed.

Altogether, these modifications have the potential to produce a battery that could provide two to three times greater energy density — the amount of energy that can be stored for a given weight — than today’s best lithium-ion batteries, a closely related technology that is today's top contender, the researchers say.

Belcher emphasizes that this is early-stage research, and much more work is needed to produce a lithium-air battery that’s viable for commercial production. This work only looked at the production of one component, the cathode; other essential parts, including the electrolyte — the ion conductor that lithium ions traverse from one of the battery’s electrodes to the other — require further research to find reliable, durable materials. Also, while this material was successfully tested through 50 cycles of charging and discharging, for practical use a battery must be capable of withstanding thousands of these cycles.

While these experiments used viruses for the molecular assembly, Belcher says that once the best materials for such batteries are found and tested, actual manufacturing might be done in a different way. This has happened with past materials developed in her lab, she says: The chemistry was initially developed using biological methods, but then alternative means that were more easily scalable for industrial-scale production were substituted in the actual manufacturing.

Jie Xiao, a research scientist at the Pacific Northwest National Laboratory who was not involved in this work, calls it “a great contribution to guide the research on how to effectively manipulate” catalysis in lithium-air batteries. He says this “novel approach … not only provides new insights for lithium-air batteries,” but also “the template introduced in this work is also readily adaptable for other catalytic systems.”

In addition to Oh, Belcher, and Shao-Horn, the work was carried out by MIT research scientists Jifa Qi and Yong Zhang and postdoc Yi-Chun Lu. The work was supported by the U.S. Army Research Office and the National Science Foundation.

Source: http://web.mit.edu/newsoffice/2013/better-batteries-through-biology-1113.html