Not only could rechargeable zinc-based batteries possibly store as much energy as lithium-ion batteries, they could also be safer, cheaper, smaller and lighter, new research finds. The results suggest zinc batteries could find use in mild hybrids (microhybrids), electric vehicles, electric bicycles, and eventually, perhaps smartphones and power grid storage.
The researchers are now aggressively testing these batteries and exploring scaling up this technology. “We feel we can have a battery ready for the market by the end of 2019,” says Michael Burz, CEO of energy technology firm EnZinc in San Anselmo, Calif., which helped engineer the new batteries.
When it comes to electric vehicles, the new batteries will “be 30 to 50 percent cheaper than comparable lithium-ion systems,” Burz says.
Lithium-ion batteries have become notorious for safety incidents resulting from overheating, at times bursting into flames and even exploding. The U.S. Navy was researching alternative technologies because “there's a Navy and a broader military concern with the safety of lithium-ion batteries—on soldiers, on sailors, on platforms,” says Debra Rolison, head of the advanced electrochemical materials section at the U.S. Naval Research Laboratory in Washington, D.C., and one of the researchers involved in the zinc breakthrough.
Zinc-based batteries do not pose the same fire risk linked with lithium-ion batteries, and can in principle match or surpass them in terms of specific energy (energy per unit mass), as well as energy density (energy per unit volume). Moreover, zinc is cheap and widely available. All these features help explain why zinc-based batteries “are the go-to global battery for single-use applications,” Rolison says.
However, zinc-based batteries “are not considered rechargeable in practice due to their tendency to grow conductive whiskers—dendrites—inside the battery, which can grow long enough to cause short circuits,” Rolison says. As such, zinc-based batteries typically die after several cycles of discharging and recharging, she explains.
Now Rolison, Burz and their colleagues have developed a zinc-based battery whose internal structure can suppress dendrite formation. The zinc anode has a porous, sponge-like architecture that helps charge move uniformly across the entire structure when the battery discharges and recharges. “Electric currents are more uniformly distributed within the sponge, making it physically difficult to form dendrites,” says Joseph Parker, a research chemist at the U.S. Naval Research Laboratory.
Zinc sponge before (left) and after 100 cycles testing.Image: U.S. Naval Research Laboratory
The zinc anodes are currently made by drying zinc emulsions in cylindrical molds overnight and baking the resulting disks in a furnace for a few hours. “It's like we're baking muffins,” Burz says.
The researchers paired this zinc anode with a nickel cathode. Their experiments revealed the battery could withstand more than 50,000 brief cycles of discharging and recharging, similar to how lead-acid batteries are used in a start-stop manner in microhybrid vehicles. In addition, such a battery could deliver the same amount of energy in a smaller mass and volume:
| Replacing a mild hybrid’s battery | Zinc | Lead Acid |
|---|---|---|
| Energy (watt-hours) | 1720 | 1720 |
| Mass (kilograms) | 21.7 | 45 |
| Specific energy (watt-hours per kilogram) | 79.2 | 38.2 |
| Energy density (watt-hours per liter) | 164 | 126 |
Their experiments also suggested that a rechargeable zinc battery could meet the 24 kilowatt-hour demands of the Nissan Leaf in a smaller, lighter package:
| Replacing a Nissan Leaf’s battery | Zinc | Lithium-ion |
|---|---|---|
| Energy (watt-hours) | 24,000 | 24,000 |
| Mass (kilograms) | 220 | 339 |
| Specific energy (watt-hours per kilogram) | 109 | 71 |
| Energy density (watt-hours per liter) | 216 | 96 |
Similar findings were seen for electric bicycles:
| Replacing an electric bike’s battery | Zinc | Lead Acid |
|---|---|---|
| Energy (watt-hours) | 504 | 504 |
| Mass (kilograms) | 5.9 | 12.2 |
| Specific energy (watt-hours per kilogram) | 91.8 | 44.3 |
| Energy density (watt-hours per liter) | 225 | 140 |
All in all, this work “offers the energy of lithium-ion batteries but at the cost of lead-acid batteries, and it's also safer, recyclable, and uses an abundant material,” Burz says.
“Zinc is the fourth-most mined material on the planet—over 14 million tons of it are mined per year,” Burz says. Burz says that when he talked with the International Zinc Association about how much zinc was needed if these new batteries did find use in vehicles, “they laughed and said, 'We spill more zinc than you need.'”
The researchers note the zinc anodes can take practically any shape desired. “We should be able to use these batteries for anything from micro-devices to consumer electronics to vehicles to the grid,” Rolison says.
EnZinc has a partially exclusive license for this research when it comes to zinc-nickel battery use in wheeled vehicles. Licenses for other applications and other cathodes are still open, says Steven Marquis at the U.S. Naval Research Laboratory's technology transfer office.
The Navy researchers are also exploring cathodes other than nickel to use with the zinc anodes. “Silver-zinc shows power not seen with lithium-ion,” Rolison says. “It could be of extreme interest to the Navy for submersible applications.”
The scientists detailed their findings this week in the journal Science.
