Professor Ying Shirley Meng and her team at UC San Diego have developed a working prototype of a flexible silver oxide-zinc battery that has at least ten times the areal energy density of a typical lithium-ion battery. It can be manufactured with standard screenprinting technology in a normal room environment.
Tech Briefs: How did you get the idea for this project?
Meng: I’ve had an independent group since 2008 working on lithium-ion batteries. Some people using batteries started asking whether we could make flexible, stretchable, high-energy batteries. There has always been a demand for applications where lithium-ion would be difficult to use. And there have always been problems having to do with lithium-ion chemistry.
My colleague, Professor Joseph Wong, is a printable electronics person and we started thinking that the two disciplines could work together to figure out how to make batteries of any shape and be stretchable and flexible. The rise of IoT particularly fueled our desire to develop such a technology.
Tech Briefs: I'm curious why there is such a big demand for flexible, bendable stretchable batteries. What are some of the applications?
Meng: In recent years, there have been quite a few applications. One example is Fitbit — there are a lot of health monitoring devices. Many sensors, biosensors, are already implemented in clothing or on mobile devices, but they're mostly in rigid form. They're wearable, but you’re conscious of wearing them. The batteries are often the limiting factor. I think we need a stretchable, bendable battery because people want to wear them, but don’t want to feel bulky batteries.
One good example to think about as a first-generation product is that the wristband on the Apple Watch or Fitbit could be a battery. Also, some people might want to wear a headband with an integrated biosensor. If it also contained a battery, the sensor data could be directly transmitted to an app. A lot of biometric information could be obtained about the status of your health by monitoring your sweat and saliva.
Those are the very exciting research topics we're working on now. Apparel companies are interested; and the defense industry is interested because the military wants their people to be in top condition. As a result, their body signals are constantly monitored, so having power devices that are soft and flexible with high power, and high performance is really important.
Tech Briefs: I would think that the sensor part doesn't need such high power, but I guess transmitting the signal does.
Meng: That's right, most sensors don't need high power, but you need something to read the signals and transmit them to a phone, or Bluetooth device, for example. Of course, you could use a hard-cased coin cell, but the device won’t look very cool.
Tech Briefs: And as you said, it makes it uncomfortable because it’s stiff. What kind of capacity are you talking about in milliamp hours? How long can it run before you need to recharge it?
Meng: Our present battery, which is not rechargeable, has a capacity of about 100 mW-hour per square centimeter. It's a few times higher than lithium battery capacity.
Tech Briefs: How about the life of your battery vs. lithium?
Meng: We do two or three times better than the duration of operation of a typical lithium-ion coin cell.
Tech Briefs: You have said that normally, flexible batteries have to be produced in sterile conditions under vacuum, but yours does not. How are you able to achieve that?
Meng: We stayed away from lithium chemistry. We use zinc-based batteries, so the ions that get transported are zinc ions. It is a benign chemistry that can be produced at ambient conditions. That allows us to use screen-printing rather than a vacuum method. It’s a method we adopted from printable biosensors in order to mass produce our flexible batteries. The cost reduction here is very significant with this type of manufacturing process.
Tech Briefs: Have you looked into scaling up the process of producing the ink?
Meng: We have a major industrial partner who is co-developing this with us. So, for the next stage, they are considering the scaling. We have been talking about 10s of millions in sales, based upon our projections of industry demands. So, we have a very steep learning curve — right now the battery is being hand-crafted by students in our lab — we need to automate the entire process including the printing.
Tech Briefs: In the description I read it talks about a current collector. What is that?
Meng: The current collectors are positive and negative tabs for connecting to the output of the battery. We think we will also be able to print them.
Tech Briefs: You said the economics of producing it will be very good ultimately.
Meng: The only concern we have is the price of silver. But these batteries will be recycled, so we are hoping the silver used will eventually be a closed loop. We can more than 90% recycle all the materials, particularly the silver oxide on the cathode. We’re doing follow-up work on the scalability and the techno-economic analysis of this chemistry.
Tech Briefs: How would the shape and size of your battery compare to lithium-ion?
Meng: Our shape is actually custom made right now. We have done 2 x 2cm, 1 x 5, 2 x 5, and 3 x 3. So, we can make multi-dimensional cells and then can also stack them. In printing, it’s easy to stack layers to build the voltage. In principle with the same area, if I do multi-layer printing, I can achieve the same energy density as coin cells but still have flexibility. Many customers want an elongated shape, you know, like 1 x 5 for a wristband.
These are all in a design spreadsheet. You can plug in the energy you need, as well as print conditions. So, for example, if you are designing a 5 x 5 patch energy source, at a certain energy density, we can custom print it. The printing is easy to adjust.
Tech Briefs: What are the next steps in your research?
Meng: Next, we will release the first-generation non-rechargeable version. The second generation will be rechargeable for 300 cycles. for those, the energy density will come down a little bit, but there are different requirements for rechargeable batteries. We have demonstrated it in a small cell in the lab, but now we need to demonstrate it in the actual printed cells.
I think the major bottleneck is the scaling. From 10s of cells to making thousands — that's a very critical step.
Tech Briefs: Do you have some idea of the timeline for commercializing it?
Meng: We are hopeful that the product will be ready in about two years. As far as the demand is concerned, that is somewhat tied to the growth of the IoT, for example, autonomous vehicles. The car has to talk to everything, right? Pedestrians, trees, buildings, everything needs to be communicated so that you know where everything is. I would have predicted it should have happened already, but you know those things haven’t yet.
The other thing is the IoT for personalized health monitoring. From my scientist perspective, I think this should have already happened. People should be able to use their cell phones to immediately read blood sugar level, for example. For Covid, we should be able to self-monitor. But I do see that in the future most the things: infrastructure, buildings, everything, will be tagged with sensors and batteries.
I think telehealth is a really important area now. We have the coronavirus, so this is one of our major driving forces. I told my students it’s really important because I think the coronavirus will be with us for a long, long, time, so each person has a responsibility for monitoring their own health level. Currently sensors are being developed, including for virus detection, that need very reliable power sources.
Tech Briefs: And then being able to transmit this information directly to your doctor.
Meng: Yes, your doctor and also public health officials need to know if a person is infected. Not to invade your privacy, but you need to do that to keep things under control.
An edited version of this interview appeared in the March 2021 issue of Tech Briefs.