Why a Rechargeable Zinc Battery Is Such a Big Deal

Lithium-ion battery cells on the production line of the Eliiy Power Co. plant in Kawasaki City, Japan Photograph by Tomohiro Ohsumi/Bloomberg

Picture a Nike FuelBand that’s just a small ring on your index finger, or a cell phone that’s as slim and pliable as a credit card. Such thin, tiny or just downright unusual shapes could be created if there were batteries slim, flexible and also powerful enough to run the gadgets. The batteries, it turns out, are the main barrier to modern electronics design.

But in a small, brightly lit lab in an office park behind the Oakland Airport in Alameda, Calif., a young startup called Imprint Energy, is using research created at the University of California, Berkeley to develop just such a battery that could free gadget makers from the constraints of the standard lithium ion battery. Well, that’s the plan, anyway.

Using zinc instead of lithium, along with screen-printing technology, Imprint Energy is already churning out low volumes of its ultra-thin, energy-dense, flexible, low-cost, rechargeable batteries for pilot customers.

It’s hard to make standard lithium ion batteries thin and flexible, explains Imprint Energy Chief Executive Officer Devin MacKenzie in an interview in the startup’s lab. There’s a “lot of packaging” required to seal off the highly reactive lithium in the battery from the environment, says MacKenzie. If you’ve ever seen YouTube videos of lithium batteries that catch fire in the air or water, you know why those barriers are needed.

But this architecture also makes lithium ion batteries rigid and potentially bulky. Even the slimmest laptops such as the Macbook Air, or tablets like the iPad, face design limitations posed by the size and weight of the batteries. The Nike FuelBand uses a curved (called conformal in battery terms) lithium polymer battery, but if you look closely at the shape of the band, the battery is the only part of the bracelet that isn’t pliable.

Imprint Energy’s battery tackles the problem of rigidity and bulkiness by throwing out the lithium. The company, which has a staff of 8, was founded in 2010 by Berkeley PhD students Christine Ho and Brooks Kincaid, who recently raised seed funding from Dow Chemical and CIA fund In-Q-Tel.

The company uses zinc for the anode part of the battery, combining it with a solid polymer electrolyte and a cathode made of a metal oxide. A battery is made up of an anode on one side and a cathode on the other, with an electrolyte in between. In Imprint’s case, zinc ions travel from the anode to the cathode through the electrolyte, creating a chemical reaction that allows electrons to be harvested along the way.

MacKenzie tells me that while zinc has been used for years in batteries, it’s been difficult to make zinc batteries rechargeable. That’s because when zinc is combined with a liquid electrolyte, it creates something called dendrites—tiny fibers that grow and get in the way of the charging reaction. Imprint Energy solved this hurdle by using an electrolyte made of a solid polymer combined with the zinc.

Using zinc means Imprint’s batteries can have far less “packaging” because zinc isn’t highly reactive with the environment. In other words, the batteries can be made much more thinly. They can also be made as tiny as a few hundred microns thick (the width of a couple of human hairs). Batteries that small could power tiny digital smart labels, like freshness-detector stickers on food.

Zinc also makes Imprint’s batteries safer and less toxic than lithium-based batteries are. The team at Imprint can work on zinc batteries in the open air. And zinc batteries are a safer option for creating devices that sit on—or even in—the body. Imagine a lithium battery powering a heart device inside a person’s chest cavity—and the battery leaks lithium into the person’s body. Yikes.

An additional innovation Imprint Energy has developed is that it’s printing out batteries using standard screen-printing technology. Most batteries are made by coating the materials onto foils that are then assembled into cells.

In Imprint Energy’s Alameda lab, CEO MacKenzie shows me one of two on-site battery-printing machines and a variety of screens that look sort of like t-shirt silk-screening screens. The battery materials are printed like ink onto the screens in whatever shapes a client requires. Customers will pay a premium for batteries created to the custom shapes of their devices.

The company can churn out 100 cells a day on the machines in its lab. That’s tiny in a world of giant Asian battery makers, but it’s large enough to get samples out to potential customers. Down the road—potentially in two to three years—Imprint will shape up manufacturing to a large commercial scale. It probably won’t build its own factories, but will work closely with manufacturing partners, or license its technology.

While it’s still early days for Imprint Energy, the team’s end goal is the wearable electronics market, both for consumers (as with Nike’s FuelBand and the FitBit line) as well as the health sector (such as implanted monitors). The wearables industry could reap the greatest benefits from the batteries’ novel and thin shapes, as well as from using safer, less-toxic materials.

Co-founder Kincaid is a wearables buff. He wears a Nike FuelBand on his wrist during the interview and says he’s eagerly awaiting the arrival of his Misfit Shine. For the wearables industry, Imprint Energy’s zinc poly batteries could enable an entirely new type of device that’s more hidden, streamlined, even more functional. Given that wearable electronics is an emerging sector—and one that could become a lot more mainstream over the next few years—disruptive design could ultimately completely change the wearable industry.

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