September 08, 2011

Battery Storage Could Get a eight times higher capacity by using Seaweed and silicon anodes

Lithium-ion batteries could hold up to 10 times as much energy per cell if silicon anodes were used instead of graphite ones. But manufacturers don't use silicon because such anodes degrade quickly as the battery is charged and discharged.

Researchers at the Georgia Institute of Technology and Clemson University think they might have found the ingredient that will make silicon anodes work—a common binding agent and food additive derived from algae and used in many household products. They say this material could not only make lithium-ion batteries more efficient, but also cleaner and cheaper to manufacture.

Researchers have demonstrated that the alginate can produce battery anodes with reversible capacity eight times greater than that of today's best graphite electrodes. The anode also demonstrates a coulombic efficiency approaching 100 percent and has been operated through more than 1,000 charge-discharge cycles without failure.

They hope to explore other alginates, boost performance of their electrodes, understand how the material works. Alginates are natural polysaccharides that help give brown algae the ability to produce strong stalks as much as 60 meters in length. The seaweed grows in vast forests in the ocean and also can be farmed in wastewater ponds.

Journal Science - A Major Constituent of Brown Algae for Use in High-Capacity Li-Ion Batteries

Identifying similarities in the material requirements for applications of interest and those of living organisms provides opportunities to use renewable natural resources to develop better materials and design better devices. Here, we harness this strategy to build high-capacity Si nanopowder–based Li-ion batteries with improved performance characteristics. Si offers more than an order of magnitude higher capacity than graphite, but exhibits dramatic volume changes during electrochemical alloying and de-alloying with Li, which typically leads to rapid anode degradation. We show that mixing Si nanopowder with alginate, a natural polysaccharide extracted from brown algae, yields a stable battery anode possessing reversible capacity 8 times higher than that of the state-of-the-art graphitic anodes.

Lithium-ion batteries store energy by accumulating ions at the anode; during use, these ions migrate, via an electrolyte, to the cathode. The anodes are typically made by mixing an electroactive graphite powder with a polymer binder—typically polyvinylidene fluoride (PVDF)—dissolved in a solvent called NMP. The resulting slurry is spread on the metal foil used to collect electrical current, and dried.

If silicon particles are used as the basis of the electroactive powder, the battery's anode can hold more ions. But silicon particles swell as the battery is charged, increasing in volume up to four times their original size. This swelling causes cracks in the PVDF binder, damaging the anode. In research published today by Science, the Georgia Tech and Clemson scientists show that when alginate is used instead of PVDF, the anode can swell and the binder won't crack. This allows researchers to create a stable silicon anode that has, so far, been demonstrated to have eight times the capacity of the best graphite-based anodes.

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