Solar and wind power are great, but what happens when the sun sets and the wind dies down? For years, scientists have been trying to create cheap batteries able to store massive amounts of this green energy, which can be fed into power grids when demand is high. One early contender has had to operate at high temperatures, which cause the battery casings to corrode. Now, researchers have come up with a new design that runs at lower temperatures, potentially giving a new generation of batteries the jolt they need to make it to market.
Conventional solid-state batteries, such as lithium-ion cells, are able to store lots of power. But their electrodes, which collect and release electricity, are complex to produce, and thus expensive. One alternative for lowering the cost has been to create the electrodes from liquid metals. These are separated from one another by a liquid electrolyte that helps ferry charged ions through the battery as it charges and discharges. The metals and electrolytes in such batteries are selected to have different densities so they naturally partition into three layers that remain separated.
An early version created by Donald Sadoway, a materials scientist at the Massachusetts Institute of Technology in Cambridge, and colleagues consisted of a top electrode made from liquid magnesium, a bottom electrode of antimony, and a molten salt electrolyte in between. The problem was that keeping all of these materials liquid required heating the battery to nearly 700°C, which caused other battery components to corrode.
Sadoway’s group has explored replacing the magnesium with lithium, which liquefies at just 180°C. But that solves only half of the problem, because antimony must be heated to at least 630°C to liquefy. The team considered adding other metals to the antimony to create alloys that would liquefy at lower temperatures. But earlier work suggested that such alloys would likely generate less electric voltage, which sharply reduces the amount of power a battery can store.
Nevertheless, Sadoway and his colleagues forged ahead testing out different antimony-based alloys. And in a paper published online today in Nature, they report that when they added different amounts of lead to their antimony they got a pleasant surprise. When lead makes up as much as 75% of the antimony alloy, the combination liquefies at just 327ºC and retains the high voltage produced by antimony alone. “You’re getting antimonylike behavior far below its melting point,” Sadoway says.
Further study revealed that this occurs because as the battery is discharged, lithium atoms in the top electrode give up electrons and migrate through the electrolyte where they bind exclusively with antimony atoms, an arrangement that helps the battery maintain its high voltage. An added benefit, Sadoway notes, is that lead is also much cheaper than antimony, so adding large amounts of lead will likely reduce the battery’s overall cost.
“This looks like an important step in the right direction,” says George Crabtree, who directs the Joint Center for Energy Storage Research at Argonne National Laboratory in Lemont, Illinois. He notes that the technology still has a ways to go to reduce efficiency losses that come from operating a battery at high temperatures. But if these losses can be reduced, he says, the batteries have a good shot at making it to market. One key advantage they have, Crabtree notes, is that because the electrodes are liquids and not solids, they are not prone to breaking down during repeated cycles of charging and discharging, a condition that causes the storage capacity of conventional batteries to fade over time. That would enable liquid metal batteries to operate for many years without being replaced, thereby reducing the long-term cost of using them to help stabilize power grids around the world.