ETH Zürich researchers have succeeded in producing uniform antimony nanocrystals and testing them as components of laboratory batteries.  The nanocrystals are able to store a large number of both lithium and sodium ions, operate at high rates and may eventually be used as alternative anode materials in future high energy-density batteries.

The materials hunt is on for new molecular structures to be used in the next generation of batteries to up rate the current lithium ion batteries. Today, lithium ion technology is commonplace and provides a reliable power source for smart phones, laptops and many other portable electrical devices.

All this growth poses a problem.  Portable electric and stationary electricity storage demands a greater number of more powerful batteries and the high demand for lithium could well lead to a shortage of the raw lithium material.

Monodisperse Antimony Nanocrystals TEM Image

That hard reality drives conceptually identical technology based on sodium-ions to receive increasing attention in the coming years. 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.  Contrary to lithium batteries now researched for more than 20 years, much less is known about materials that can efficiently store sodium ions.

Maksym Kovalenko heads a team of researchers from ETH Zurich and that may have come a step closer to identifying alternative battery materials for both lithium and sodium based battery chemistries.  The team become the first to synthesize uniform antimony nanocrystals, the special properties of which make them prime candidates for an anode material for both battery types. The results of the scientists’ study have just been published in Nano Letters.

Antimony has been regarded as a promising anode material for high-performance lithium-ion batteries for a long time.  The metal exhibits a high charging capacity, by a factor of two higher than that of today’s 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.

The published results have charge storage capacities of 580–640 mAh g–1 at moderate charging/discharging current densities of 0.5–1C (1C-rate is 660 mA g–1).  At 20C-rates, retention of charge storage capacities by 10 and 20 nm antimony nanocrystals can reach 78–85% of the low-rate value.  The big news is antimony nanostructures can be comparable to the best lithium ion intercalation anodes and they are so far, unprecedented for sodium-ion storage

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

Antimony nanocrystals fully packed with lithium or sodium leads to large volumetric changes. By using nanocrystals, these modulations of the volume can be reversible and act fast, and do not lead to the immediate fracture of the material. An additional important advantage of nanocrystals is that they can be intermixed with a conductive carbon filler in order to prevent the aggregation of the nanoparticles into clumps.

Electrochemical tests showed Kovalenko and his team that electrodes made of these antimony nanocrystals perform equally well in sodium and in lithium ion batteries. The result makes antimony particularly promising for sodium batteries because the best lithium-storing anode materials, graphite and silicon, do not work with sodium.

The monodisperse nanocrystals, with the size deviation of 10% 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 nanometers aren’t sufficiently stable due to the massive volume expansion and contraction during the charge discharge cycle of a battery. The researchers achieved the best results with 20 nanometer sized particles.

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 nanometers within which this material shows excellent, size-independent performance, both in terms of energy density and electron flow rate-capability.

The new parameters even allow using polydisperse (multiple crystals in a group) antimony particles to obtain the same performance as with very monodisperse particles, as long as their sizes remain within the size-range of 20 to 100 nanometers.

Kovalenko said, “This greatly simplifies the task of finding an economically viable synthesis method. 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 can alloy with lithium and sodium.

Kovalenko considers the idea antimony nanocrystals are an alternative to today’s lithium-ion batteries any time soon. 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 Professor Kovalenko.  He estimates it will be another decade or so before a sodium-ion battery with antimony electrodes could hit the market and thinks, “However, other research groups will soon join the efforts.”

This looks a like a newly developing field.  Note that Kovalenko already has an industrial partner.  We might see products come a little faster than the professor estimates.  When battery product designers and process engineers get more insight into the facts and the potential sodium batteries might get rolling into the market sooner than later.


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