Uniform antimony nanocrystals — capable of storing a great quantity of both lithium and sodium ions — have been created for the first time by researchers at ETH Zurich and Empa.
Such uniform nanocrystals could be of great use in the creation of potential super-high-density batteries thanks to the previously mentioned storage abilities, and a number of other notable qualities.
It’s long been known that antimony was a promising anode material for high-performance lithium-ion batteries — primarily due to its high charging capacity, which is a factor of two higher than the graphite that is currently used, but also due to the fact that it’s suitable for both lithium- and sodium-ion batteries.
There’s an obstacle in the way of antimony’s use for this purpose, though. The high storage capability is only exhibited by a “special” form — the so-called “monodisperse” form. This form consists of antimony nanocrystals between ten and twenty nanometers in size.
That’s where the new work comes in.
The press release from ETH Zurich explains:
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.
Electrochemical tests showed lead researcher Maksym 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.
One of the other important results of this new work was the identification of “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 rate-capability.”
Interestingly, in this size range, even polydisperse antimony particles perform about as well as monodisperse particles — but only so long as their sizes remain within this range.
“This greatly simplifies the task of finding an economically viable synthesis method,” Kovalenko states. “Development of such cost-effective synthesis is the next step for us, together with our industrial partner.”
Something important to note, though — this new material won’t be used commercially for some time. The cost of synthesis is still too high.
“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,” explains Kovalenko.
The researchers guess that it’ll be at least a decade or so before a sodium-ion battery with antimony electrodes could hit the market, as the topic is still in its infancy with regard to research.
“However, other research groups will soon join the efforts,” he notes.
The new findings are detailed in a paper just published in the journal Nano Letters.