The race for the next big thing in energy storage suddenly got a lot smaller, slimmer, lighter, stretchier, and twistier, now that researchers have solved some kinks in silver-zinc technology. The new battery is not quite ready to slide into an electric vehicle, but it does pave the way for a new generation of lighter, more efficient electronic gear, and better prospects for recycling, too.
A New Era In Energy Storage Was Born 200 Years Ago
According to our friends over at NASA, the silver-zinc energy storage combo first came on the scene 200 years ago, give or take a few. Technology obstacles still linger, but the allure of high energy density, compact size, and low weight provide ample motivation for researchers to keep powering through the problems.
The silver-zinc combo offers “more energy per ounce than any other battery couple,” NASA enthused in a 2016 technology recap, while noting that early versions of the battery were dogged by significant issues.
“The biggest challenge for silver-zinc batteries was that their electrodes — the cell’s negative and positive electrical conductors — were soluble and deteriorated quickly,” NASA explains. “The answer to this challenge, developed first in the late 1920s by professor Henri André and then advanced by the U.S. military in World War II, was a membrane to separate the two electrodes.”
That same 100-year-old technology is still used in some applications by the Department of Defense, including submarines and torpedoes. However, that applies only to disposable batteries. The electrode problem still persisted in relation to rechargeable energy storage devices, meaning that performance would fall off a cliff after just a few charging cycles.
Finally, A Breakthrough For Silver-Zinc Energy Storage
For NASA’s purposes, the zinc-silver energy storage approach also fell short because performance worsened even more if the battery was thermally sterilized, which it had to be in order to prevent the off-chance of oh, say, a lethal microbe making its way around the solar system.
Fast forward from the 1920s to 1972, when a public-private partnership between NASA and the Douglas Aircraft Company yielded a rechargeable version that could hold up under heat sterilization, for up to 500 charging cycles.
That’s close but no cigar. In the 1970s, the nickel-cadmium energy storage platform commonly used in space applications could last for 10,000 cycles.
If you’re wondering why NASA didn’t just stick with nickel-cadmium, that’s a good question. But did you already forget that thing about more energy per ounce? As described by NASA, the silver-zinc combo was 1/3 the size of its nickel-cadmium counterpart, and it weighed a lot less, too.
No, This Time It’s Really A Breakthrough
The size and weight savings provided enough interest to keep NASA plowing away on the R&D end after the 1970s, but a real breakthrough proved elusive, at least for space applications. The charging cycle numbers improved but the best performance only occurred if the battery was charged at lower levels.
NASA did see some potential for silver-nickel energy storage to push lithium-ion technology aside in commercial areas that require pint-sized energy storage devices, like hearing aids, for example. Here, let’s have NASA explain:
“…lithium-ion batteries are prone to a phenomenon known as thermal runaway, which in rare but disastrous cases causes them to catch fire. This is not a possibility with silver-zinc batteries, which use a water-based chemistry. Lithium-ion batteries also require more packaging and other components that take up a larger percentage of their space the smaller they get, so it’s a less efficient technology for small spaces.”
Finally, in the 1990s a company called ZPower adopted NASA’s chemistry and bumped it up a notch.
“The company has improved all four active components of the battery: the two electrodes, the electrolyte and the separators, earning some 100 new patents. The batteries can now survive up to 1,000 discharge cycles without losing significant capacity,” enthused NASA.
Next-Generation Energy Storage For Me, And Thee
If you are wondering what ever happened to ZPower, guess what, it is still here, and the company has been partnering up with the University of California-Davis on the design of a new flexible, rechargeable silver oxide-zinc battery that has 5 to 10 times more real energy density than the best ones available today.
“The battery also is easier to manufacture; while most flexible batteries need to be manufactured in sterile conditions, under vacuum, this one can be screen printed in normal lab conditions. The device can be used in flexible, stretchable electronics for wearables as well as soft robotics,” explains UC Davis.
By easier to manufacture, they mean the device can be screenprinted as an ink in a normal lab environment. Aside from preparing the inks, the printing takes a matter of seconds, and the battery is dry and ready for use in a matter of minutes. The research team also notes that a high speed, roll-to-roll printing process could do the trick, too.
If you caught that thing about oxide, that’s the key to the improvement in energy density. Other flexible silver-zinc batteries are available on the market today, but they use the Ag2O-Zn combo. The UC-Davis team went with AgO-Zn, which was a bit of a challenge considering that AgO is difficult to work with.
“AgO is traditionally considered unstable. But ZPower’s AgO cathode material relies on a proprietary lead oxide coating to improve AgO’s electrochemical stability and conductivity,” UC-Davis explains. “As an added benefit, the AgO-Zn chemistry is responsible for the battery’s low impedance.”
For all the juicy details, check out the team’s research paper in the journal Joule, available in the December 7 issue.
More And Better Energy Storage For The Sparkling Green Future
So far, the new platform has performed without any significant loss of capacity for 80 charging cycles, while being subjected to twisting and bending.
That again brings up the question of why not stick with tried-and-true lithium-ion energy storage, but consider the size angle and all the pieces fall into place. Lithium-ion energy storage requires safety features and other engineering add-ons that contribute bulk. That’s not particularly a problem for larger batteries, but all those extras become an obstacle to miniaturization.
The research team is already working on improvements that cut costs and charge faster, with an eye on new devices that take advantage of new 5G wireless technology as well as markets in the small soft robotics field.
The flexibility angle opens up the potential for a new generation of miniaturized soft robots. It provides engineers with more design leeway, the idea being to tailor the battery to fit the use, rather than having to figure out how to design a robot around a standard battery.
By small, they mean small. Check out this 2018 article in the journal Nature, which reviews the potential for clinical uses in soft robotics. The author notes that formidable obstacles lie ahead, but the payoff could be huge (break added for readability):
“Untethered small robots have the unique capability to non-invasively access and navigate in difficult-to-reach, narrow areas inside our body and to perform therapeutic and diagnostic operations1. As a result, these robots could eliminate the need for an invasive procedure.
“…Recently, the design of tiny soft-bodied robots has brought the clinical translation of miniature robots within reach. These robots are physically intelligent and do not damage tissues owing to their size and soft nature. Moreover, their shape can be actively or passively programmed, which enables them to adapt to their surroundings and endows them with a large number of degrees of freedom.”
The health care field has been playing whack-a-mole with its carbon footprint, and the COVID-19 outbreak hasn’t helped any. A new generation of low-impact diagnostic tools would help move things along in the right direction.
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Image (screenshot): Flexible silver-zinc battery courtesy of UC-Davis.
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