When used as an electrode in nickel/metal hydride (Ni/MHx) batteries -- the most popular rechargeables -- the alloy has a high capacity for storing charge, a long-lasting ability to be charged and recharged, and good resistance to corrosion. Furthermore, the alloy contains no cobalt, an expensive metal found in many Ni/MHx batteries, and no cadmium, a toxic metal found in nickel-cadmium rechargeables. Composed of lanthanum, nickel, and tin, "this new alloy is inexpensive and relatively environmentally benign," said Reilly, the team leader.
The alloy is based on a classic formula used for Ni/MHx batteries, which consists of a cube-like lattice with lanthanum atoms on the corners and nickel on the inside. The electrode works by storing up hydrogen atoms (from the electrolyte) in the spaces between the atoms during charging, and releasing them into the electrolyte during discharge.
But the added hydrogen atoms have an adverse effect: They cause the crystal lattice to expand, and then contract as the battery discharges. "This expansion and contraction is repeated in each charge/discharge cycle of the battery," said Reilly, "which pulverizes the alloy into small particles that are more susceptible to corrosion. That’s why batteries don’t recharge an infinite number of times. Eventually corrosion takes over."
Through trial and error, scientists originally found that using a mixture of metals, including cobalt, in place of nickel helps the electrode resist this tendency to break apart and corrode. But even small amounts of cobalt can drive up the cost of batteries considerably. For batteries of the sizes needed in electric vehicles, the cost can be prohibitive. So scientists have been trying to understand the role cobalt plays -- and find ways to replace it.
That's what the Brookhaven scientists were doing when they were investigating several relatively simple, cobalt-free alloys. They found a combination of lanthanum, nickel, and tin with a very high storage capacity that didn't decay over many charge/discharge cycles. This surprised the scientists, because normally, combinations of these atoms in the classic ratio of one lanthanum atom to five of the other atoms decayed rather quickly.
As it turns out, there was a mistake. Accidentally, the scientists had added a bit more of the nickel/tin combo, so that the ratio of atoms was no longer 1 lanthanum to 5 nickel or tin, but 1 to 5.157. That small difference in the ratio of the ingredients made a big difference in performance. The superior performance was then confirmed in a further series of experiments, carried out by Adzic and Johnson, with other 1 lanthanum/ 5+ nickel/tin combinations.
Vogt then studied the lattice structure at Brookhaven’s National Synchrotron Light Source to figure out where the extra atoms went. By beaming samples of the material with high-intensity x-rays and looking at how the beam scattered, he determined that "dumbbells" made of two nickel atoms were replacing some of the lanthanum atoms on the cube corners.
These nickel dumbbells make the structure more compact, said Vogt, which decreases somewhat its ability to store hydrogen, and therefore its initial charge capacity relative to the classic 1-5 formula. But it also decreases the alloy's tendency to corrode, therefore increasing its life-span. The result is that the long-term energy-storage capacity of this new alloy exceeds that of cobalt alloys used in commercial batteries.
This work was funded by the U.S. Department of Energy, which supports basic research in a variety of scientific fields.
The U.S. Department of Energy's Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies. Brookhaven also builds and operates major facilities available to university, industrial, and government scientists. The Laboratory is managed by Brookhaven Science Associates, a limited liability company founded by Stony Brook University and Battelle, a nonprofit applied science and technology organization.
Note to local editors: James Reilly lives in Bellport, New York.