Recent studies have shown that synthetic and bio-derived polymers which contain carboxy groups, such as polyacrylic acid (PAA) and carboxymethyl cellulose (CMC), demonstrate promising characteristics as binders for Si-based anodes...

In this communication we report that alginate, a high modulus natural polysaccharide extracted from brown algae, yields remarkably stable battery anode. Unlike many polysaccharides commonly found in terrestrial plants, alginates, a major constituent of brown algae and many aquatic microorganisms, contain carboxylic groups in each of the polymer’s monomeric unit.

The higher content of carboxylic group in the binder should lead to a larger number of possible binder-Si bonds, and thus better Si electrode stability.

—Kovalenko et al.

The project was supported by the two universities as well as by a Honda Initiation Grant and a grant from NASA.

The alginate is extracted from the seaweed through a simple soda-based (Na₂CO₃) process that generates a uniform material. The anodes then can be produced through an environmentally friendly process that uses a water-based slurry to suspend the silicon or graphite nanoparticles. The new alginate electrodes are compatible with existing production techniques and can be integrated into existing battery designs.

Use of the alginate may help address one of the most difficult problems limiting the use of high-energy silicon anodes. When batteries begin operating, decomposition of the lithium-ion electrolyte forms a solid electrolyte interface on the surface of the anode. The interface must be stable and allow lithium ions to pass through it, yet restrict the flow of fresh electrolyte.

With graphite particles, whose volume does not change, the interface remains stable. However, because the volume of silicon nanoparticles changes during operation of the battery, cracks can form and allow additional electrolyte decomposition until the pores that allow ion flow become clogged, causing battery failure. Alginate not only binds silicon nanoparticles to each other and to the metal foil of the anode, but they also coat the silicon nanoparticles themselves and provide a strong support for the interface, preventing degradation.

The electrodes showed a moderate rate capability, the team found, inferior to that achieved in Si-C composite anodes with hierarchical porosity or in nanowires. However, the researchers said their advantage is higher volumetric capacity, higher CE and compatibility with existing manufacturing techniques. The team suggested that further electrode optimization and introduction of additional pores may significantly increase the rate performance.

For the future, the researchers hope to explore other alginates, boost performance of their electrodes and better understand how the material works.

Resources

• Igor Kovalenko, Bogdan Zdyrko, Alexandre Magasinski, Benjamin Hertzberg, Zoran Milicev, Ruslan Burtovyy, Igor Luzinov, and Gleb Yushin (2011) A Major Constituent of Brown Algae for Use in High-Capacity Li-Ion Batteries. Science doi: 10.1126/science.1209150