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Batteries from Sand

July 16, 2014

Early scientific research was focused on simple materials, since those were the only ones available. Materials scientists learned a lot about iron before they tackled iron alloys. Now, many centuries hence, all the low-hanging fruit has been harvested, and scientists can only do something really novel if they have a novel material. Fortunately, some of these materials, like graphene, are just different forms of an old standard.

However, most novel materials are novel because they contain exotic, and usually toxic or otherwise dangerous, chemical elements. For example, thin film photovoltaic materials contain cadmium, selenium and tellurium. Advanced semiconductor devices, such as diode lasers, contain arsenic. Lithium batteries and lithium-ion batteries contain reactive compounds of lithium.

NFPA-704 label (blank).No, it's not a poorly hung painting by Piet Mondrian. It's a blank "NFPA 704" label for material hazard identification.

The blue, red and yellow areas would contain number codes, respectively, for health hazard, flammability, and chemical reactivity. The white area is for special hazard information.

(Via Wikimedia Commons.)

Many scientists have started to think "green" by attempting to replace existing materials and the processes for their manufacture with environmentally friendly and renewable alternatives. I wrote about the idea of replacing some structural materials with cellulose fiber composites in a recent article (Strong Cellulose Filaments, July 2, 2014).

In a recent publication, materials scientists and engineers from the University of California - Riverside (UCR) have described their process for the production of nanoscale silicon anodes for lithium ion batteries. Their process incorporates some "green" chemistry by using sand (quartz silica crystals) and table salt (sodium chloride, NaCl) as starting materials. Their anodes are better than the conventional graphite material, producing batteries with a capacity of 1,024 milliamp-hours per gram when discharged at 2 amps per gram after 1000 cycles.[1-2]

Using Mg and NaCl to turn sand into nanosilicon
Using magnesium and sodium chloride to turn sand into nanosilicon. (University of California, Riverside, image.)[2]

Their process, as shown in the above figure, involves the reduction of silica (SiO2) by magnesium to produce magnesium oxide and silicon; viz.,
SiO2 + 2Mg -> Si + 2MgO
One feature of the process is the use of NaCl as a heat absorption medium. The highly exothermic reaction of magnesium with quartz would proceed violently were the NaCl not present.[2] The heat scavenging NaCl also promotes the formation of nano-silicon network with interconnect thickness of 8-10 nm (see figure).[1

One non-green aspect of the process is that an acid mixture of hydrochloric and hydrofluoric acid is required to remove the magnesium oxide.[1] Also, the harvested sand needs to be milled to nanometer scale and then purified.[2] Examples of the starting materials and final nanosilicon product are shown in the photographs, below.[2]

Figure caption
From start to finish: (a) unpurified sand, (b) purified sand, and (c) vials of unpurified sand, purified sand, and nano silicon. (University of California, Riverside, image. modified for clarity.)[2]

This research was prompted by the fact that the optimization limit has been reached for graphite, the present anode material for lithium-ion batteries. Silicon, being conductive like graphite, is an alternative, but it's difficult to produce nanoscale silicon in large quantity. The porosity of the nanoscale silicon produced by the UCR process is what makes this anode material ideal for its purpose.[2]

Batteries made from this nanoscale silicon could have three times the useful lifespan of batteries used for electric vehicles, and they would power mobile electronic devices three times longer. The research team is scaling their research from coin-sized batteries to larger sizes.[2] The University of California, Riverside, has filed patent applications on this invention.[2]

References:

  1. Zachary Favors, Wei Wang, Hamed Hosseini Bay, Zafer Mutlu, Kazi Ahmed, Chueh Liu, Mihrimah Ozkan and Cengiz S. Ozkan, "Scalable Synthesis of Nano-Silicon from Beach Sand for Long Cycle Life Li-ion Batteries," Scientific Reports, vol. 4, Article no. 5623 (July 8, 2014), DOI:10.1038/srep05623. This is an open access article with a PDF file available here.
  2. Sean Nealon, "Using Sand to Improve Battery Performance," University of California at Riverside Press Release, July 8, 2014.

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