|TEM image (false colored) of monodisperse antimony nanocrystals. (Photo: Maksym Kovalenko Group / ETH Zürich) Click to enlarge.|
Researchers from ETH Zürich and Empa have succeeded for the first time in producing uniform (monodisperse) antimony (Sb) nanocrystals (NCs). The nanocrystals possess high and similar Li-ion and Na-ion 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 all C-rates (0.5−20C), capacities of 20 nm Sb particles are systematically better than for both 10 nm and bulk Sb. At 20C-rates, retention of charge storage capacities by 10 and 20 nm Sb nanocrystals can reach 78−85% of the low-rate value, indicating that rate capability of Sb nanostructures can be comparable to the best Li-ion intercalation anodes and is so far unprecedented for Na-ion storage. A paper on their work appears in the ACS journal Nano Letters.
Antimony has long been regarded as a promising anode material for high-performance lithium-ion batteries as this metalloid exhibits a high charging capacity—a factor of two higher than that of commonly used graphite. Initial studies revealed that antimony could be suitable for both 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 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.
Electrochemical tests showed 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-ion 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 nanometers aren’t sufficiently stable due to the massive volume expansion and contraction during the operation of a battery. The researchers achieved the best results with 20 nanometer large particles.
The researchers also 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 very 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. Development of such cost-effective synthesis is the next step for us, together with our industrial partner.—Maksym Kovalenko
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.
However, production of a sufficient number of high-quality uniform antimony nanocrystals is still too expensive to make short-term commercialization possible. Kovalenko estimates that tt will be another decade or so before a sodium-ion battery with antimony electrodes could hit the market.
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.—Maksym V. Kovalenko
He M, K Kravchyk, Walter M, Kovalenko MV (2014) “Monodisperse Antimony Nanocrystals for High-Rate Li-ion and Na-ion Battery Anodes: Nano versus Bulk,” Nano Lett. doi: 10.1021/nl404165c