A novel approach in designing a liquid battery by means of a passive, gravity-fed arrangement that is comparable to an old-fashioned hourglass could be advantageous in many ways owing to the simple design, operation, and low cost of the system, according to a group of MIT scientists who have created a demo version of this novel battery.
Liquid flow batteries, wherein the negative and positive electrodes are both in liquid form and isolated by a membrane, are not a new idea. An earlier concept was unveiled by some members of this group three years before. A range of chemical formulations, which include the same chemical compounds used in existing lithium-ion batteries, can be utilized in the basic technology. Here, the major components are tiny particles that could be carried along within a liquid slurry as opposed to standard solid slabs that stay in place throughout the service life of a battery. Hence, larger tanks are required to hold the slurry in order to increase the storage capacity.
But all the earlier versions of liquid batteries have depended on complicated systems of tanks, pumps, and valves. Therefore, they are not only high in cost but also have a high risk of potential failures and leaks.
This complexity is eliminated in the new version, which uses a simple gravity feed in place of the pump system. The energy production rate can be modified by simply altering the device angle, thereby increasing or slowing down the flow rate.
This concept is explained in a paper in the Energy and Environmental Science journal. The co-authors of the paper include Kyocera Professor of Ceramics Yet-Ming Chiang, Pappalardo Professor of Mechanical Engineering Alexander Slocum, School of Engineering Professor of Teaching Innovation Gareth McKinley, and POSCO Professor of Materials Science and Engineering W. Craig Carter, along with Xinwei Chen, postdoc, Brandon Hopkins, graduate student, and four other researchers.
Chiang relates the latest approach to a “concept car,” a design that is not necessary to enter production as it is, however, helps develop some new concepts that can eventually result in a real product.
The concept for flow batteries originally emerged in the 1970s, but the previous versions were made of low energy density materials which have a low capacity for storing energy that is in proportion to their weight. A major development in flow batteries came with the advent of high-energy-density variants a few years ago, including a version developed by members of this group of MIT researchers who used the same chemical compounds as traditional lithium-ion batteries. This MIT version was advantageous in many ways over other flow batteries, but had the drawback of complex plumbing systems.
The latest variant substitutes the complex plumbing systems with a plain, gravity-fed system. It works like an egg timer or old hourglass, wherein particles flow between different tanks through a narrow opening. By turning the system over, the flow could then be reversed. Here, the overall shape resembles like a rectangular window frame, having a narrow opening at the point where two sashes would convene at the center.
Only one side of the proof-of-concept version consist of flowing liquid, but the other side is a solid form (a lithium sheet). The group was trying to test its idea in a simpler form before achieving their ultimate goal, a variant consisting of both positive and negative electrodes in liquid form that flow alongside through a slot while being separated by a membrane.
Both solid and liquid batteries have their own advantages, based on their specific applications.
The concept here shows that you don’t need to be confined by these two extremes. This is an example of hybrid devices that fall somewhere in the middle.
The latest design should enable potentially simpler and further compact battery systems, which should be both economical and modular, facilitating the widespread adoption of grid-connected storage systems to satisfy rising demand, says Chiang. Such storage systems would be vital for increasing the usage of intermittent power sources like solar and wind.
While traditional, all-solid batteries need electrical connectors for all the cells that constitute a huge battery system, these contacts are required for only a small area in the middle of the flow battery that resembles the neck of the hourglass, thus making the system’s mechanical assembly greatly simpler, says Chiang. The parts are simple enough such that they can be fabricated using injection molding or 3-D printing, he says.
Additionally, the fundamental concept of a flow battery allows for selecting the two major features of a battery system of interest independently: energy density and power density. Energy density defines the amount of energy that is storable in the system and power density describes the amount of energy that can be delivered at a particular moment. The power density of the new liquid battery is determined by the stack size, i.e., the contacts through which the battery particles traverse. Its energy density is measured by the size of its storage tanks.
In a conventional battery, the power and energy are highly interdependent.
The most difficult aspect of the design was controlling the liquid slurry characteristics in order to control the rate of flow, says Chiang. The behavior of the thick liquids can be related to that of ketchup within a bottle, i.e., making the liquid to flow is initially difficult, but becomes too sudden once it starts flowing. An extensive process of fine-tuning is required for both the design of the mechanical structures and the liquid mixture in order to obtain the appropriate flow.
Adjusting the device angle could help control the flow rate, says Chiang, and the group discovered that at an extremely shallow angle (nearly horizontal), “the device would operate most efficiently, at a very steady but low flow rate.” The fundamental concept ought to work with various chemical compositions for the different battery components, he says, however “we chose to demonstrate it with one particular chemistry, one that we understood from previous work. We’re not proposing this particular chemistry as the end game.”
Venkat Viswanathan, research scientist at Lawrence Berkeley National Laboratory who was not part of this study, says: “The authors have been able to build a bridge between the usually disparate fields of fluid mechanics and electrochemistry,” and by doing so created a promising novel advance towards battery storage. “Pumping represents a large part of the cost for flow batteries,” he says, “and this new pumpless design could truly inspire a class of passively driven flow batteries.”
The research work received support from the Joint Center for Energy Storage Research, funded by the U.S. Department of Energy. Graduate students Ahmed Helal and Frank Fan, and postdocs Kyle Smith and Zheng Li also contributed to the study.