FIGURE 6-2 Grid-based energy storage technologies have different energy densities and installed capacities.

SOURCES: ESA (2010); Bradwell (2011b).

lithium battery is actually unstable. It reacts with the electrode and forms a layer on the surface of the electrode that grows slowly. The layer is necessary for battery performance, but it should be stable. As the layer grows, it increases the battery’s impedance and lowers performance.

Flow batteries are an interesting technology in which energy is stored in an electrolyte that is pumped across a membrane surface. Fuel cells are getting increasing attention, but efficiency is the big challenge there. Lithium battery efficiencies reach 90 to 95 percent, while the round-trip efficiency on a fuel cell is typically closer to 50 percent. What is encouraging, said Bradwell, is that most of the new approaches that researchers are pursuing use Earth-abundant materials such as zinc, magnesium, lithium, manganese, sodium, and lead. Very few of the new chemistries use resource-constrained materials.

In his work, Bradwell took inspiration from the metals industry’s experience that prices drop as production increases and that prices drop more when materials are processed as liquids. “Both iron and aluminum are produced as liquid metals [that are] handled and processed in a continuous manner, which also keeps the costs low,” said Bradwell. “In particular, the aluminum smelter was the inspiration for the liquid metal battery project.”

An aluminum smelter is a huge electrolysis machine that works as follows: aluminum oxide is poured into a layer of molten cryolite, which at 960°C is so corrosive that it dissolves aluminum oxide and dissociates it into aluminum ions and oxygen ions. A 700-milliampere/cm2, 4-volt (V) current is passed from cathode to anode, which oxidizes aluminum ions to liquid aluminum. This pools underneath the electrolyte and reduces oxygen to carbon dioxide, which is vented to the atmosphere. Though a simple reaction in theory, it proved to be difficult to carry out in practice. Before this process was developed, aluminum was more expensive than gold.

In essence, Bradwell explained, an aluminum smelter is half of a battery that cannot be recharged because it generates a gaseous product that cannot be reclaimed. The solution was to replace this gaseous component with another liquid metal. In the liquid metal battery, an electropositive liquid metal is separated from an electronegative metal by a liquid electrolyte. The three liquids self-segregate based on contiguous immiscibility between the metal and electrolyte layers. The battery operates at high temperature, though less than that of an aluminum smelter, and uses low cost materials. The aluminum industry’s experience suggests that the electrodes will have a 5- to 10-year lifespan before they need refurbishing.

The first generation battery he and his collaborators built used liquid magnesium and antimony as the two electrodes. At low current densities, this battery was 74 percent efficient, with a coulombic efficiency—a measure of self-discharge— of 99.7 percent per cycle. The voltage of this initial battery was low, only 0.5 V compared to 3.5 V for a lithium-ion battery. Fundamental research and chemical development efforts have raised the output to 0.9 V, which is still low but sufficiently high to continue work on this system, and coulombic efficiency has increased to greater than 99.9 percent. Projected costs for such a battery would be less than $100/kWh. Bradwell acknowledged that a significant amount of

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