US researchers have discovered alginate binders that improve lithium ion anodes.
Within the anode, binders are required to suspend the graphite, or more recently silicon, particles that interact with the electrolyte to hold charge.
Inspiration for alternative binders was sought in the earth's natural electrolyte: the sea.
"We specifically looked at materials that had evolved in natural systems, such as aquatic plants which grow in water with a high concentration of ions," said Professor Igor Luzinov at Clemson University, South Carolina. "Since electrodes in batteries are immersed in a liquid electrolyte, we felt that aquatic plants - in particular, plants growing in such an aggressive environment as salt water - would be excellent candidates for natural binders."
Silicon-based anodes theoretically offer far more capacity than carbon anodes, providing provision can be made for the swelling and contraction that silicon undergoes as ions are transferred.
Anodes require an interface on their surface which is stable, porous to lithium ions, and impervious to fresh electrolyte - and interface that forms through the decomposition of electrolyte as the cell is used.
"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," said Georgia Tech, which is working with Clemson. "Alginate not only binds silicon particles to each other and to the metal foil of the anode, but they also coat the silicon particles themselves and provide a strong support for the interface, preventing degradation."
The team claims to have demonstrated anodes with reversible capacity eight times that of today's best graphite electrodes.
"The anode also demonstrates a coulombic efficiency approaching 100%, and has been operated through more than 1,000 charge-discharge cycles without failure," said Georgia Tech.
The argument for retro-fitting alginates into carbon anode production is different, and to do with toxicity.
"Existing electrodes use a polyvinylidene fluoride binder manufactured using a toxic solvent", said Georgia Tech. "Alginates are low-cost materials that are already used in foods."
The alginates in question are natural polysaccharides with uniformly distributed carboxylic groups, and are extracted from brown seaweed through a simple soda (Na2CO3) process.
Anodes that are compatible with existing battery production techniques can be made with from a water-based slurry of alginate and suspended silicon or graphite particles, said Georgia Tech.