By contrast, the flow battery requires two separate electrolytes because active species are dissolved in the electrolyte(s)—flowing through the separator would mix reactive species outside the desired half-cell scheme.

Suppes and his colleagues initially validated their concept of the convection cell using a zinc-alkaline chemistry. In the paper reporting that work (Suppes et al. 2011), they concluded that:

Flow batteries have a few major advantages over traditional diffusion-based battery designs. For instance, higher power output is possible with the flow (reported to 500 mA/ cm²) of electrolyte when one of the reagents is stored in the electrolyte. For these high-power output batteries, the flow of electrolyte also facilitates heat removal. Elimination or substantial reduction of membrane separators is another advantage of flow batteries.

The flow battery proposed in this work is different than the traditional flow batteries because all reagents and products reside as solids, which are part of the electrodes. Ionic intermediates flow through the electrolyte but do not accumulate. An advantage of having solid reagents is that the activity of the reagents remains nearly constant during use, whereas both the concentration and voltages from soluble reagents will decrease with use. Solid-substrate batteries also tend to have higher energy densities because of a more constant voltage output and the higher molar densities of pure solid reagents vs. solvated reagents.

...This flow battery has the potential to gain higher energy densities than other types of flow batteries and provides an excellent platform for rechargeable lithium batteries that allow use of metallic lithium in the anode.

—Suppes et al. 2011

For small batteries, the team notes, conventional diffusion-based architectures are superior because the cost of the circulating pump alone would be more than the cost of the traditional diffusion-based battery. However, for larger batteries, the savings in separator and cathode intercalation material costs can more than offset the cost of a pump. Typical applications of the new convection battery would be plug-in hybrid electric vehicles and electric grid energy storage, they suggested.

Other advantages of the convection battery could include:

• Better use of thicker electrodes due to a more-uniform distribution of potential in the electrodes.

• Reduced separator and current collector materials which results in lower cost and higher energy density batteries.

• Mitigation of dendrite formation through multiple mechanisms including shear forces from flow which can inhibit single-crystal growth.

• Elimination of drying of electrodes through replenishing of electrolyte.

• Easy temperature management by flow of electrolyte through heat exchangers.

• Ability to routinely drain electrolyte from cells during non-use to increase cell life.

• Ability to increase electrode separation distance due to decreased concentration gradients across the separator.

• Ability to implement measures that will better-allow use of metallic lithium.

In the new paper submitted to AIChE Journal, they validated their concept using lithium iron phosphate chemistry. The selected LiFePO₄ for study because it is a lithium-ion battery technology widely regarded as having a good combination of high energy density, good cyclability, safety benefits, and reasonable cost.

It is important to recognize that these are only initial, validating studies, the team cautions. Future studies will be needed to increase capacity utilization, identify the best match of materials with the needs of the convection battery, better distinguish between advantages of flow-by versus flow-through separators, and generally optimize performance.

Research on the convection battery, initiated in 2008, has received funding from both the National Science Foundation and the Energy Innovations Small Grant Program of the California Energy Commission. Research results have been published in the American Institute of Chemical Engineers (AICHE) Journal and the Journal of Applied Electrochemistry. They were also presented at the AICHE annual meeting in October 2011. Since then, definitive data has been collected that validates increased power output and recharging capabilities.

In the validating studies, the convection battery provided about six times as much power output as an identical, traditional battery without a pump, and a series of use and charge cycles yielded outstanding performance.

One of the most promising aspects of this technology, the team notes, is that it can be used to improve the best available battery chemistries, as well as future breakthroughs in materials and chemistries. This compatibility is with traditional battery chemistries such as the lead-acid battery and the batteries currently used in electric vehicles—the convection battery is not necessarily compatible with “flow battery” or “air battery” chemistries.

Multiple patents have been filed on the convection battery, and it is anticipated that the intellectual property will be preserved to allow major commercial opportunities in the US, estimated to be available by the end 2014.

Professor Suppes was the recipient of the 2006 Presidential Green Chemistry Challenge Award. He is the co-author of the book entitled “Sustainable Nuclear Power”.

Resources

• Suppes, Galen J.; Bryan D. Sawyer; Michael J. Gordon (2011) High-Energy Density Flow Battery Validation. AIChE Journal 57(7): p. 1961-1967 doi: 10.1002/aic.12390

• Bryan D Sawyer; Michael J Gordon; Michael G Heidlage; Galen J Suppes (2011) Impact of electrode separator on performance of a zinc/alkaline/manganese dioxide packed-bed electrode flow battery. Journal of Applied Electrochemistry, 41(5), 543-550 doi: 10.1007/s10800-011-0264-5