The impact on total battery costs is dependent on other factors; but for targeted chemistries, the weight would be 33% lower and the cost would be 25% lower than the best available alternatives. Primary applications of this technology include electric vehicles and grid storage of electricity, Dr. Suppes suggests.

Rechargeable lithium-metal batteries differ from rechargeable lithium-ion batteries, which are commercial and widely available. A key problem facing rechargeable lithium metal batteries is the formation of dendrite metal crystals during charging. These crystals form in the separator between the negative and positive electrodes and eventually form a short circuit between the two electrodes. Not only does this ruin the battery, it can lead to overheating and even fires.

Lithium-ion batteries use large amounts of graphite in the negative electrode which forms a carbon cage around lithium ions (referred to as intercalation) and reduces the activity of lithium to the extent that dendrite crystals do not form. To do this, about 10x more graphite is needed than lithium, which increases cost and weight. An additional disadvantage of using graphite is that it lowers total battery voltage which further increases weight and cost.

Concentration profiles of the soluble lithium salt in a lithium battery by location. The dashed line is the profile of the convection battery; the solid of a conventional diffusion battery. Lithium cations are assumed to be paired with anions throughout solution. During charging (as illustrated) the lower lithium cation concentration in the separator with the convection battery leads to an absence of driving force for dendrite formation in the separator. Source: Dr. Galen Suppes. Click to enlarge.

Dr. Suppes, who is also the Chief Scientific Officer for Homeland Technologies Inc, which has licensed the patent-pending convection battery technology from the University, had hypothesized that the unique fluid flow possible in a convection battery would prevent dendrite formation in the separator. This hypothesis was tested and validated by PhD candidate Don Dornbusch.

Through the pumping of electrolyte, the convection battery is able to avoid the high lithium activity in the separator that forms dendrites. The researchers have studied the details of convection battery operation both through experiments and computer-based modeling.

They first performed control experiments that established conditions where a normal lithium metal battery would consistently form dendrites in the separator with short circuit failure. In the same laboratory battery operated as a convection battery (with flow of electrolyte) the battery operated without failure.

In a paper recently published in the AIChE Journal (American Institute of Chemical Engineers), the researchers validated that the convection battery is able to displace the diffusion driving force with convection of electrolyte resulting in an essentially constant level of lithium ion activity throughout the battery’s electrolyte.

These analyses substantiated the following conclusions on the performance of the convection battery: (a) flow in the convection battery can reduce concentration overpotentials by 99.9%, (b) both the ionic and electron conductivities of solid phases can have a major impact on convection battery performance, and (c) the solid-phase ionic conductivity of a porous separator is an important design parameter and is not considered by porous electrode theory as published to date.

In view of the ability of the convection battery to overcome both bulk diffusion and liquid-phase effective conductivity limitations; the convection battery has an unprecedented potential to redefine the performance of large batteries.

—Gordon and Suppes (2013a)

The University of Missouri team is researching the convection battery as part of a $300,000 National Science Foundation grant that started in July of 2012.

Resources

• Gordon, M. and Suppes, G. (2013a) Convection battery—modeling, insight, and review. AIChE J. doi: 10.1002/aic.14080

• Gordon, M. and Suppes, G. (2013b) Li-ion battery performance in a convection cell configuration. AIChE J., 59: 1774–1779. doi: 10.1002/aic.13950