In Search of the Ideal Grid Battery
Researchers at Stanford make an electrode that can be recharged 40,000 times without losing much capacity.
Energy utilities are increasingly looking for batteries that can help stabilize the grid. By quickly storing and delivering charge, batteries could accommodate fluctuations in supply and demand, and help to incorporate variable sources of power such as wind and solar. However, currently available battery technologies are either too expensive or don’t last for enough charge cycles to be practical.
Researchers at Stanford University have now demonstrated a high-efficiency new nanomaterial battery electrode that lasts for 40,000 charge cycles without significantly losing its charge-holding capacity. The work was led by Yi Cui, a materials science and engineering professor at Stanford University. Cui says the electrode is a first step toward a new type of low-cost battery suitable for storing large amounts of electricity within the power grid.
Cui’s new battery chemistry uses inexpensive, abundant materials. It relies on the same principle employed in lithium-ion batteries—moving sodium or potassium ions between electrodes during charging and discharging—but does it much more cheaply. “For grid storage, the battery can be huge, and using sodium and potassium is very attractive because they are so abundant and cheap,” Cui says. These batteries will use water-based electrolytes that are cheaper and easier to use than organic solvent-based electrolytes used in lithium-ion batteries.
The new electrodes, demonstrated in a paper posted online today in the journal Nature Communications, are also based on commonly available materials. The researchers start with the pigment Prussian Blue, an iron and cyanide compound. They replace half the iron with copper, and make crystalline nanoparticles of the resulting compound, which they coat on a cloth-like carbon substrate. Then they immerse this electrode in a potassium nitrate electrolyte solution.
The electrodes maintain 83 percent of their charge capacity after 40,000 cycles—in comparison, lead-acid batteries last a few hundred cycles, while lithium-ion batteries typically last for 1,000. The electrodes also show 99 percent energy efficiency. “You want the voltage you put in during charging and the voltage you take out during discharge to be same,” Cui says. “Compared to any other battery material, this is absolutely the best.”
Jay Whitacre, a professor of materials science and engineering at Carnegie Mellon University and founder of the sodium-ion battery startup Aquion Energy in Pittsburgh, says that the electrodes show good cycle life, but notes that their charge capacity is relatively low: 60 milliampere-hours per gram of material, compared to 100 for Aquion’s manganese oxide cathode. Besides, he says, “it’s based on copper, which is actually pretty expensive these days.”
However, the most important metric for large-scale grid storage is price per unit of energy per cycle, says Donald Sadoway, a materials science and engineering professor at MIT. In that respect, the new material, with its tens of thousands of cycles, could have an edge over other batteries. “In the end, it comes down to the cost,” he says. “If they can deliver this performance at a cost that’s substantially lower than sodium-sulfur, they’ve got a winner here.”
Other than cost and cycle life, “round-trip energy efficiency is also very important for grid energy storage so that you’re not wasting energy during recharging,” says Christopher Johnson, a battery researcher at Argonne National Laboratory. While the cost of the new electrode isn’t known, its efficiency and cycle life “are impressive,” he says. The researchers still need to demonstrate a full battery cell with two electrodes, though, which could change the numbers, Johnson adds.
The electrode made so far acts as a cathode. Cui says his team is tuning the chemistry of the material to make an anode and is working on making prototype batteries.