Molten Metal Battery

Figure 1: In this liquid metal battery, the negative electrode (top) is a low-density metal called here Metal A; the positive electrode (bottom) is a higher-density metal called Metal B; and the electrolyte between them is a molten salt. During discharge (shown here), Metal A loses electrons (e-), becoming ions (A+) that travel through the electrolyte to the bottom electrode. The electrons pass through an external circuit, powering an electric load on the way. At the bottom electrode, the Metal A ions and electrons rejoin and then alloy with the Metal B electrode. Photo: MIT

Novel rechargeable battery developed at MIT could one day play a critical role in the massive expansion of solar generation needed to mitigate climate change by midcentury. Designed to store energy on the electric grid, the high-capacity battery consists of molten metals that naturally separate to form two electrodes in layers on either side of the molten salt electrolyte between them. Tests with cells made of low-cost, Earth-abundant materials confirm that the liquid battery operates efficiently without losing significant capacity or mechanically degrading — common problems in today’s batteries with solid electrodes. The MIT researchers have already demonstrated a simple, low-cost process for manufacturing prototypes of their battery, and future plans call for field tests on small-scale power grids that include intermittent generating sources such as solar and wind.

The ability to store large amounts of electricity and deliver it later when it’s needed will be critical if intermittent renewable energy sources such as solar and wind are to be deployed at scales that help curtail climate change in the coming decades. Such large-scale storage would also make today’s power grid more resilient and efficient, allowing operators to deliver quick supplies during outages and to meet temporary demand peaks without maintaining extra generating capacity that’s expensive and rarely used.

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.

A battery for the power grid had to be able to operate reliably for years. It could be large and stationary, but — most important — it had to be inexpensive. “The classic academic approach of inventing the coolest chemistry and then trying to reduce costs in the manufacturing stage wouldn’t work,” says Sadoway. “In the energy sector, you’re competing against hydrocarbons, and they’re deeply entrenched and heavily subsidized and tenacious.” Making a dramatic shift in power production would require a different way of thinking about storage.

Sadoway therefore 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?”

Subsequent investigation led to the liquid metal battery. Like a conventional battery, this one has top and bottom electrodes with an electrolyte between them (see Figure 1 in the slideshow above). 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. But in Sadoway’s battery, all three are liquid. 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.

Choice of materials

For Sadoway and then-graduate student David Bradwell MEng ’06, PhD ’11, the challenge was to choose the best materials for the new battery, particularly for its electrodes. 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.

Read the full Article at MIT


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