Batteries used in hearing aids could be key to the future of renewable energy
Lithium-ion batteries—giant versions of those found in electric vehicles—are the current front-runners for storing renewable energy, but their components can be expensive. Zinc batteries are easier on the wallet and the planet—and lab experiments are now pointing to ways around their primary drawback: They can’t be recharged over and over for decades.
The need for grid-scale battery storage is growing as increasing amounts of solar, wind, and other renewable energy come online. This year, President Joe Biden committed to making the U.S. electricity grid carbon free by 2035. To even out dips in supply, much of that renewable power will have to be stored for hours or days, and then fed back into the grid. In California alone, the public utilities commission envisions deploying more than 8800 megawatts of rechargeable batteries by 2026, and last week, California Governor Gavin Newsom proposed $350 million in state funding to develop long-duration energy storage technologies. “That trend will not go down. It will only continue to grow,” says Mark Baggio, vice president for business development at Zinc8 Energy Solutions, a zinc battery producer.
For power storage, “Lithium-ion is the 800-pound gorilla,” says Michael Burz, CEO of EnZinc, a zinc battery startup. But lithium, a relatively rare metal that’s only mined in a handful of countries, is too scarce and expensive to back up the world’s utility grids. (It’s also in demand from automakers for electric vehicles.) Lithium-ion batteries also typically use a flammable liquid electrolyte. That means megawatt-scale batteries must have pricey cooling and fire-suppression technology. “We need an alternative to lithium,” says Debra Rolison, who heads advanced electrochemical materials research at the Naval Research Laboratory (NRL).
Enter zinc, a silvery, nontoxic, cheap, abundant metal. Nonrechargeable zinc batteries have been on the market for decades. More recently, some zinc rechargeables have also been commercialized, but they tend to have limited energy storage capacity. Another technology—zinc flow cell batteries—is also making strides. But it requires more complex valves, pumps, and tanks to operate. So, researchers are now working to improve another variety, zinc-air cells.
A better battery
Zinc is cheaper than many battery metals and could store more energy.
In these batteries, a water-based electrolyte spiked with potassium hydroxide or another alkaline material separates a zinc anode and a cathode made of other conductive materials, often porous carbon. During discharge, oxygen from the air reacts with water at the cathode to form hydroxide ions, which migrate to the anode, where they react with zinc to eventually produce zinc oxide. The reaction releases electrons that flow from anode to cathode through an external circuit. Recharging the batteries means reversing the flow of current, causing zinc metal to re-form on the anode.
But zinc batteries don’t like to run in reverse. Irregularities across the anode’s surface cause the electric field to intensify at certain spots, which causes zinc to deposit there, further enhancing the electric field. As the cycle repeats, tiny spikes called dendrites grow, eventually perforating and shorting out the battery. Equally troublesome, water in the electrolyte can react at the anode, splitting into oxygen and hydrogen gas, which can burst the cells apart.
Researchers have begun to deal with these downsides, churning out nearly 1000 papers per year. In 2017, for example, Rolison and colleagues reported in Science that they reengineered the anode as a 3D network of zinc metal pocked with tiny voids. The electrode’s vast surface area reduced the local electric field, which prevented the buildup of dendrites and reduced the likelihood of splitting water molecules. NRL licensed the technology to EnZinc.
This month, Wang and his colleagues reported in Nature Nanotechnology that when they added a fluorine-containing salt to their electrolyte, it reacted with zinc to form a solid zinc fluoride barrier around the anode. Ions could still wriggle through during charging and discharging. But the barrier prevented dendrites from growing and repelled water molecules, blocking them from reaching the anode.
“It’s a great development,” says Wei Wang, who directs the Energy Storage Materials Initiative at the Pacific Northwest National Laboratory. Still, Chunsheng Wang notes his device is somewhat slow to discharge. To improve that, his team wants to add catalysts at the cathode to speed up the reaction between oxygen and water.
The same strategy features in work by researchers led by Jung-Ho Lee from Hanyang University. In Nature Energy on 12 April, they reported creating a fibrous and conductive cathode from a mix of copper, phosphorus, and sulfur that also served as a catalyst, dramatically speeding up oxygen’s reaction with water. That and other advances produced batteries that could be charged and discharged quickly and had high capacity, 460 watt-hours per kilogram (compared with about 75 Wh/kg for standard zinc cells with manganese oxide cathodes and 120 Wh/kg for scaled-up lithium-ion systems). The batteries were stable for thousands of cycles of charge and discharge. The result “looks like another important step,” Chunsheng Wang says.
Such advances are injecting new hope that rechargeable zinc-air batteries will one day be able to take on lithium. Because of the low cost of their materials, grid-scale zinc-air batteries could cost $100 per kilowatt-hour, less than half the cost of today’s cheapest lithium-ion versions. “There is a lot of promise here,” Burz says. But researchers still need to scale up their production from small button cells and cellphone-size pouches to shipping container–size systems, all while maintaining their performance, a process that will likely take years. Burz also notes electric utilities and other companies looking to buy cheap large-scale batteries want to see years of steady operation first. “These customers need to see that it works in the real environment,” he says.