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Antimony nanocrystals as substitute for energy storage

Posted: 04 Apr 2014     Print Version  Bookmark and Share

Keywords:Empa  ETH Zurich  energy storage  antimony nanocrystal  nanomaterial 

A team of researchers from Empa and ETH Zurich has revealed that for the first time, they were able to produce uniform antimony nanocrystals. Tested as components of laboratory batteries, they can 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, added the researchers.

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. Although researched for 20 years, materials that can store sodium ions remain scarce.

A team from Empa and ETH Zurich headed by Empa researcher Maksym Kovalenko may have come a step closer to identifying alternative battery materials: they have become the first to synthesize uniform antimony nanocrystals, the special properties of which make them prime candidates for an anode material for both lithium ion and sodium ion batteries.

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.

Antimony nanocrystals

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. Nanocrystals have a decisive advantage over particles of larger sizes: the full lithiation or sodiation of antimony leads to large volumetric changes. 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 that electrodes made of 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 per cent or less, allow identifying the optimal size-performance relationship. Nanocrystals of 10nm or smaller suffer from oxidation because of the excessive surface area. On the other hand, antimony crystals with a diameter of more than 100nm 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 20nm large particles.

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