Calcium: expanding the chemical repertoire for grid-scale batteries
Rechargeable liquid-metal batteries, suitable for large-scale power storage, are yet to hit the mainstream market. But with an expanding list of cheap, effective materials such as calcium alloys, they may make their move soon. Belinda Smith reports.
Large-scale power storage is an issue with renewable energy that's yet to be surmounted. Right now, there are no practical ways to store excess electricity produced by turbines and solar panels for a town or city to use on still or overcast days.
Could liquid batteries be the answer?
Massachusetts Institute of Technology materials chemist Donald Sadoway thinks so. Unlike batteries we use in cars and phones, his invention – now a decade old – is made of three layers of liquid active materials that can absorb and store large amounts of electricity.
And in the journal Nature Communications, he and colleagues showed calcium, which is cheap and plentiful, can form the bases for two of the three layers. This is despite its high melting point – around 900 ºC – "which is ridiculous," Sadoway says.
"The lesson here is to explore different chemistries for different markets."
The liquid battery cell's three layers consist of an electrolyte sandwiched between two electrodes. In a conventional battery, the electrodes are solid; not so in Sadoway's battery. Each liquid has a different density, a little like floating different liqueurs in a cocktail.
In 2009, Sadoway's team unveiled a prototype with molten magnesium as the top electrode, antimony as the bottom, and a molten salt such as sodium sulfide as the electrolyte.
As electrons flowed into the battery cell, positive magnesium ions in the electrolyte snagged them to form magnesium metal, which rose to the upper electrode.
Meanwhile, antimony ions in the electrolyte lost electrons, causing them to sink to the lower electrode.
Its melting point is a staggering 900 ºC and easily dissolves in salt electrolyte.
Once all magnesium and antimony ions were used up from the electrolyte, the battery was fully charged – with just a thin layer of electrolyte between two thick electrode layers.
To discharge the battery, the process was reversed. The metal atoms returned to the electrolyte as ions, ready to be recharged.
While the liquid battery has a long life span (unlike the battery on your smartphone), can operate at electrical current 10 times higher than other batteries and is made of cheap materials, it must operate at 700 ºC to keep the metal layers molten.
So Sadoway's team looked at alternative metals. Lead author Takanari Ouchi was attracted to calcium, which is cheap, abundant and has an inherent high voltage as a negative electrode.
But burrowing into its properties, the element doesn't seem suitable as an electrode for liquid batteries.
Its melting point is a staggering 900 ºC and easily dissolves in salt electrolyte – yet a crucial element of a liquid battery is that its layers remain distinct.
So Ouchi, Sadoway and colleagues mixed calcium with magnesium – another cheap material – which has a much lower melting point. The resulting alloy ended up with a lower operation temperature while retaining calcium's high-voltage properties.
They then created a salt electrolyte mix of lithium chloride and calcium chloride. And when they sat the magnesium/calcium alloy on top, it didn't dissolve. This solved another impediment to using calcium – its solubility in salts.
A big bonus with the new design, Sadoway says, is magnesium and calcium are often found in ore together.
Purifying one or the other takes energy. But in a battery design that incorporates the mixed alloy, separation isn't necessary. This could save money by using "lower grades" of each that already contain traces of the other.
So it should be full steam ahead from here, right?
We'll be waiting a little while. Liquid batteries are under development at startup Ambri, which planned to start shipping commercial batteries in 2015 or 2016.
But a roadblock popped up not in the form of calcium alloys, but in the battery seal. It needs to withstand constant temperatures of hundreds of degrees, and didn't hold up as well as expected.
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