A new approach to the design of a liquid lithium battery, using a passive, gravity-fed arrangement similar to an old-fashioned hourglass, could offer great advantages due to the system's low cost and the simplicity of its design and operation.
Liquid flow lithium batteries — in which the positive and negative electrodes are each in liquid form and separated by a membrane — are not a new concept, and some members of this research team unveiled an earlier concept three years ago. The basic technology can use a variety of chemical formulations, including the same chemical compounds found in today's lithium-ion batteries. In this case, key components are not solid slabs that remain in place for the life of the battery, but rather tiny particles that can be carried along in a liquid slurry.
But all previous versions of liquid lithium batteries have relied on complex systems of tanks, valves, and pumps, adding to the cost and providing multiple opportunities for possible leaks and failures.
The new version, which substitutes a simple gravity feed for the pump system, eliminates that complexity. The rate of energy production can be adjusted simply by changing the angle of the device, thus speeding up or slowing down the rate of flow.
Chiang describes the new approach as something like a "concept car" — a design that is not expected to go into production as it is but that demonstrates some new ideas that can ultimately lead to a real product.
The original concept for flow batteries dates back to the 1970s, but the early versions used materials that had very low energy-density — that is, they had a low capacity for storing energy in proportion to their weight. A major new step in the development of flow batteries came with the introduction of high-energy-density versions a few years ago, including one developed by members of this MIT team, that used the same chemical compounds as conventional lithium-ion batteries.
The new version replaces all that plumbing with a simple, gravity-fed system. In principle, it functions like an old hourglass or egg timer, with particles flowing through a narrow opening from one tank to another. The flow can then be reversed by turning the device over. In this case, the overall shape looks more like a rectangular window frame, with a narrow slot at the place where two sashes would meet in the middle.
While a conventional, the basic concept of the flow battery makes it possible to choose independently the two main characteristics of a desired battery system: its power density (how much energy it can deliver at a given moment) and its energy density (how much total energy can be stored in the system). For the new liquid battery, the power density is determined by the size of the "stack," the contacts where the battery particles flow through, while the energy density is determined by the size of its storage tanks. "In a conventional battery, the power and energy are highly interdependent," Chiang says.
The trickiest part of the design process, he says, was controlling the characteristics of the liquid slurry to control the flow rates. The thick liquids behave a bit like ketchup in a bottle — it's hard to get it flowing in the first place, but then once it starts, the flow can be too sudden. Getting the flow just right required a long process of fine-tuning both the liquid mixture and the design of the mechanical structures.
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