As with yesterday's article on liquid-metal batteries this article speaks to how that approach was created. Prof. Donald Sadoway from MIT, A decade ago, the committee planning the new MIT Energy Initiative approached Donald Sadoway, MIT’s John F. Elliott Professor of Materials Chemistry, to take on the challenge of grid-scale energy storage. At the time, MIT research focused on the lithium-ion battery — then a relatively new tech­nology. The lithium-ion batteries being developed were small, lightweight, and short-lived — not a problem for mobile devices, which are typically upgraded every few years, but an issue for grid use.

One of the basic question that Sadoway had an idea that turned to a process he knew well: aluminum smelting. Aluminum smelting is a huge-scale, inexpensive process conducted inside electrochemical cells that operate reliably over long periods and produce metal at very low cost while consuming large amounts of electrical energy. Sadoway thought: “Could we run the smelter in reverse so it gives back its electricity?”

Like a conventional battery, this one has top and bottom electrodes with an electrolyte between them. During discharging and recharging, positively charged metallic ions travel from one electrode to the other through the electrolyte, and electrons make the same trip through an external circuit. In most batteries, the electrodes — and sometimes the electrolyte — are solid according to the professor. This battery includes three metals. These metals are the negative electrode — the top layer in the battery — is a low-density liquid metal that readily donates electrons. The positive electrode — the bottom layer — is a high-density liquid metal that’s happy to accept those electrons. And the electrolyte — the middle layer — is a molten salt that transfers charged particles but won’t mix with the materials above or below. Because of the differences in density and the immiscibility of the three materials, they naturally settle into three distinct layers and remain separate as the battery operates.

Benefits of going liquid

"This novel approach provides a number of benefits. Because the components are liquid, the transfer of electrical charges and chemical constituents within each component and from one to another is ultrafast, permitting the rapid flow of large currents into and out of the battery. When the battery discharges, the top layer of molten metal gets thinner and the bottom one gets thicker. When it charges, the thicknesses reverse. There are no stresses involved, notes Sadoway. “The entire system is very pliable and just takes the shape of the container.” While solid electrodes are prone to cracking and other forms of mechanical failure over time, liquid electrodes do not degrade with use.

Indeed, every time the battery is charged, ions from the top metal that have been deposited into the bottom layer are returned to the top layer, purifying the electrolyte in the process. All three components are reconstituted. In addition, because the components naturally self-segregate, there’s no need for membranes or separators, which are subject to wear. The liquid battery should perform many charges and discharges without losing capacity or requiring maintenance or service. And the self-segregating nature of the liquid components could facilitate simpler, less-expensive manufacturing compared to conventional batteries."1

Sadoway and then-graduate student David Bradwell MEng ’06, PhD ’11 investigated methods exist for predicting how solid metals will behave under defined conditions. But those methods “were of no value to us because we wanted to model the liquid state,” says Sadoway — and nobody else was working in this area. So he had to draw on what he calls “informed intuition,” based on his experience working in electrometallurgy and teaching a large freshman chemistry class.

To keep costs down, the need to use electrode materials that were earth-abundant, inexpensive, and long-lived. To achieve high voltage, they had to pair a strong electron donor with a strong electron acceptor. The top electrode (the electron donor) had to be low density, and the bottom electrode (the electron acceptor) high density. “Mercifully,” says Sadoway, “the way the periodic table is laid out, the strong electropositive [donor] metals are low density, and the strong electronegative [acceptor] metals are high density”. And finally, all the materials had to be liquid at practical temperatures.

The first combination, Sadoway and Bradwell chose magnesium for the top electrode, antimony for the bottom electrode, and a salt mixture containing magnesium chloride for the electrolyte. They then built prototypes of their cell — and they worked. The three liquid components self-segregated, and the battery performed as they had predicted. Spurred by their success, in 2010 they, along with Luis Ortiz SB ’96, PhD ’00, also a former member of Sadoway’s research group, founded a company — called initially the Liquid Metal Battery Corporation and later Ambri — to continue developing and scaling up the novel technology.

Not there yet

As always there are problems with new approaches. To keep the components melted, the battery had to operate at 700 degrees Celsius (1,292 degrees Farenheit). Running that hot consumed some of the electrical output of the battery and increased the rate at which secondary components, such as the cell wall, would corrode and degrade. The search for alternate materials continue.

Early results from the magnesium and antimony cell chemistry had clearly demonstrated the viability of the liquid metal battery concept; as a result, the on-campus research effort received more than $11 million from funders including Total and the U.S. Department of Energy’s ARPA–E program.

"Within months, the team began to churn out new chemistry options based on various materials with lower melting points. For example, in place of the antimony, they used lead, tin, bismuth, and alloys of similar metals; and in place of the magnesium, they used sodium, lithium, and alloys of magnesium with such metals as calcium. The researchers soon realized that they were not just searching for a new battery chemistry. Instead, they had discovered a new battery “platform” from which a multitude of potentially commercially viable cell technologies with a range of attributes could spawn.

New cell chemistries began to show significant reductions in operating temperature. Cells of sodium and bismuth operated at 560 degrees Celsius. Lithium and bismuth cells operated at 550 C. And a battery with a negative electrode of lithium and a positive electrode of an antimony-lead alloy operated at 450 C.

While working with the last combination, the researchers stumbled on an unexpected electrochemical phenomenon: They found that they could maintain the high cell voltage of their original pure antimony electrode with the new antimony-lead version — even when they made the composition as much as 80 percent lead in order to lower the melting temperature by hundreds of degrees.

“To our pleasant surprise, adding more lead to the antimony didn’t decrease the voltage, and now we understand why,” Sadoway says. “When lithium enters into an alloy of antimony and lead, the lithium preferentially reacts with the antimony because it’s a tighter bond. So when the lithium [from the top electrode] enters the bottom electrode, it ignores the lead and bonds with the antimony.”

That unexpected finding reminded them how little was known in this new field of research — and also suggested new cell chemistries to explore. For example, they recently assembled a proof-of-concept cell using a positive electrode of a lead-bismuth alloy, a negative electrode of sodium metal, and a novel electrolyte of a mixed hydroxide-halide. The cell operated at just 270 C — more than 400 C lower than the initial magnesium-antimony battery while maintaining the same novel cell design of three naturally separating liquid layers."

The role of the new technology

But for grid-scale storage, both capa­bilities are important — and the liquid metal battery can potentially do both. It can store a lot of energy (say, enough to last through a blackout) and deliver that energy quickly (for example, to meet demand instantly when a cloud passes in front of the sun). Unlike the lithium-ion battery, it should have a long lifetime; and unlike the lead-acid battery, it will not be degraded when being completely discharged. And while it now appears more expensive than pumped hydropower, the battery has no limitation on where it can be used. With pumped hydro, water is pumped uphill to a reservoir and then released through a turbine to generate power when it’s needed. Installations therefore require both a hillside and a source of water. The liquid metal battery can be installed essentially anywhere. No need for a hill or water.

Bringing it to market

Ambri has now designed and built a manufacturing plant for the liquid metal battery in Marlborough, Massachusetts. As expected, manufacturing is straightforward: Just add the electrode metals plus the electrolyte salt to a steel container and heat the can to the specified operating temperature. The materials melt into neat liquid layers to form the electrodes and electrolyte. The cell manufacturing process has been developed and implemented and will undergo continuous improvement.

According to Bradwell, Ambri scientists and engineers have built more than 2,500 liquid metal battery cells and have achieved thousands of charge-discharge cycles with negligible reduction in the amount of energy stored. Those demonstrations confirm Sadoway and Bradwell’s initial thesis that an all-liquid battery would be poised to achieve better performance than solid-state alternatives and would be able to operate for decades.

Ambri researchers are now tackling one final engineering challenge: developing a low-cost, practical seal that will stop air from leaking into each individual cell, thus enabling years of high-temperature operation. Once the needed seals are developed and tested, battery production will begin.

Impacts

Due to the materials they are using one question that has not been addressed is how dangerous is the waste when these batteries are discharged? Are we putting our water and land supply in danger as noted with lead in the past via paint? Although the technology is impressive, one must also look at the full picture from creation to replacement. The question is what to do with the replaced battery and the impacts have not been studied or considered at this point and that is dangerous and can lead to major problems in the future.

Also, they in the reference article noted only renewable that would not work in areas where there is little to no sunlight or consisted and at the required speed to recharge these batteries. Think Alaska for sun and many locations of this nation for wind. This will have to be considered as well.

I hope you found this article interesting as I did about this amazing technology.

Reference: https://news.mit.edu/2016/battery-molten-metals-0112