Antimony nanocrystals show promise for sodium-ion batteries
The research team led by Maksym Kovalenko have taken a step closer to identifying alternative battery materials using uniform antimony nanocrystals, which have properties that make them prime candidates for an anode material for both lithium-ion and sodium-ion batteries.
Antimony has been regarded as a promising anode material for high-performance lithium-ion batteries because it exhibits a high charging capacity. The metalloid’s charging capacity is a factor of two higher than that of graphite which is commonly used in lithium-ion batteries. 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 have managed to chemically synthesize uniform ‘monodisperse’ antimony nanocrystals that were between ten and twenty nanometers 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.
Kovalenko and his team used electrochemical tests to show that electrodes made of these antimony nanocrystals perform equally well in sodium and in lithium ion batteries. The result makes antimony 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 are not 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.
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 rate-capability. These features even allow using polydisperse antimony particles to obtain the same performance as with monodisperse particles, as long as their sizes remain within this size-range of 20 to 100 nanometers.
“This greatly simplifies the task of finding an economically viable synthesis method,” explained Kovalenko. “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.
“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,” admitted Kovalenko. The ETH-Zurich professor estimates it could be another decade or so before a sodium-ion battery with antimony electrodes could hit the market.
The results of the scientists’ study have been published in Nano Letters.
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