Lithium batteries are currently the belle of the battery ball. They have a lot going for them, including high energy storage for their weight and the ability to charge and recharge many times before losing much capacity.

But we’re all familiar with the drawbacks, too. Lithium-ion batteries pose a fire risk, and the lithium and cobalt used in them aren’t the most abundant elements, which makes things more expensive.

Plenty of other possible battery chemistries could compete with lithium, but getting them to live up to their theoretical potential is difficult. Zinc, for example, performs admirably in your non-rechargeable alkaline batteries, and it could theoretically make a safer and cheaper rechargeable one—with a water-based electrolyte rather than a flammable organic one. This hasn’t happened, though, and the reason becomes apparent if you throw the batteries under a microscope.

Think zinc

Zinc anodes are typically made by binding together particles of zinc powder. Over cycles of charging and recharging, these develop a zinc oxide coating, which is less-conductive and effectively walls off some of the zinc. Branching needles of zinc can also start to grow as a result of this uneven charge distribution. These will eventually pierce through the thin barrier that separates anode from cathode and fatally short-circuit the battery.

A group at the US Naval Research Laboratory has been working on a different way to put together the zinc anode in order to avoid this problem. (The work was funded by a Department of Energy ARPA-E grant—a program the Trump administration has threatened and recently frozen.) In a paper published recently in Science, the team describes how the structure of the zinc anode is more like a sponge, in that the zinc is continuously connected throughout but peppered with tiny pockets of open space. In some ways, it's like a photo-negative of the zinc powder granules.

Add in some trace amounts of bismuth and indium to help control chemical reactions, and you have a new version of an old anode. Zinc oxide can still temporarily form during the battery’s charge cycle, but it lines the open pockets rather than blocking the connections between zinc particles.

The researchers published on this anode structure a couple of years ago, but they have since rounded out the rest of the components needed for a functioning battery. In the new study, they pair the zinc anode with a nickel cathode to run recharging tests. In a couple of standard performance evaluations—drawing the battery down to about 60 percent charge, for example—the battery completed between 100 and 150 cycles before dropping to half its charge capacity.

These results are better than they sound, however, because adding some more electrolyte into the (imperfectly sealed) lab prototypes brought the charge capacity right back. That suggests the problem was electrolyte loss, not a drop in electrode performance. And when the zinc anode went under a scanning electron microscope for inspection, it looked great. The anode wasn’t clogged up with zinc oxide, and there were no pointy zinc needles growing.

Jack of all trades

To demonstrate one possible application, the researchers ran the battery like it was the kind of lead-acid battery used in “microhybrid” vehicles. These batteries allow vehicles to stop a car’s engine at a red light and restart the engine when the light turns green. A constant, low-level charge runs things like the stereo, with occasional strong jolts to start the engine; recharging takes place in between stops.

A 12-volt zinc battery weighing one-fifth as much as a lead-acid battery was run for months through more than 50,000 cycles of activity like that. Again, it needed some electrolyte, but the zinc looked to be in good shape under the microscope.

The researchers ran some numbers to show that zinc batteries could hold more juice in less space (and weight) than lead-acid batteries in electric bikes, as well. Compared to the lithium-ion battery in a full electric vehicle like the Nissan Leaf, they calculate a zinc battery could shave a third of the weight and even more space. Part of that space was because they assumed they can lose the complex fire-prevention system needed for lithium.

(Two of the authors on the paper are associated with a start-up company called EnZinc that has licensed the design and intends to start selling rechargeable zinc batteries—soon. EnZinc CEO Michael Burz told Ars that the company plans to have a battery ready for production by the end of 2019.)

Debra Rolison of the US Naval Research Laboratory team explained that she and her colleagues think their design is ready to leave the lab. She said:

We would want to help that next phase demonstration in a metal casing, a more standard battery package pushing the 500 [cycle lifetime] at lithium-ion-competitive energy content. But we’re definitely ready for the engineers to step in.

The engineering required to scale up a lab prototype to something like an electric vehicle is not trivial, though. The bigger the battery gets, the more challenging it becomes to ensure that the current stays evenly distributed through the anode to prevent problems. And although the heat from zinc batteries would be much easier to work with than the heat from lithium-ion batteries, you still need to design a system to manage and dissipate it.

Even so, there is real reason for optimism, as Princeton’s Dan Steingart (who was not involved in this study, but also works on zinc batteries) told Ars. He went on:

The reason why this work is important in that it shows that you can stabilize a highly porous structure of zinc. And there will be older members in the field of zinc research that will point to work in the ‘80s, ‘90s, even ‘70s, and say this is just derivative of that. And yeah, all zinc work is derivative of that which came before, but this actually works in a way that those did not.

With lithium-ion batteries showing so much promise, zinc research went quiet in the late 1990s and early 2000s. But the door has recently opened again. “I think the most important thing about this paper is that it makes it clear that zinc anodes should be taken seriously and studied and scaled with far stronger effort than [they have] in the past,” Steingart said.

Science, 2017. DOI: 10.1126/science.aak9991 (About DOIs).