It survived punctures, razor cuts, and a full month of abuse — without a drop spilled.
At first glance, it looks like a flimsy patch of silicone, the kind you might peel off a sticker. But inside, this sliver of softness hides a breakthrough in energy storage. Unlike most lithium-ion batteries, which are rigid, flammable, and sealed tight in metal shells, this one can stretch to more than 13 times its original length, heal itself after being sliced in two, and keep powering an LED light even as it’s twisted, folded, and stabbed. And it does all of this in open air — without leaking, exploding, or stopping.
Built to Bend, Designed to Survive
Lithium-ion batteries make the modern world go round. But they’re also brittle and potentially dangerous. Their organic liquid electrolytes, which shuttle charged lithium ions between electrodes, are flammable and toxic. To avoid leaks and water damage, manufacturers encase them in rigid, impermeable metal.
That’s fine for phones and cars. But for the coming wave of wearable devices, soft robots, and electronic skin, flexibility is a necessity and rigid casings are a liability.
Scientists have tried using water-based hydrogels as safer, stretchier electrolytes. The problem is that water breaks down at low voltages, limiting the power such batteries can safely deliver. Earlier attempts to stretch this limit relied on fluorinated lithium salts — themselves expensive, environmentally persistent, and still toxic. And that’s obviously not ideal for devices that go on your skin or inside your body.
This time, scientists at the University of California, Berkeley tried a different path.
Instead of overloading the gel with salt or relying on fluorinated compounds, the team engineered what they call a “water-scarce zwitterionic hydrogel” (WZH). Inside it, lithium ions are stabilized not by free water molecules, but by the carefully tuned chemistry of the gel’s backbone: a mix of quaternary ammonium and sulfonic acid groups that attract and trap lithium ions while binding water tightly enough to keep it from misbehaving.
Long story short, this is a special hydrogel that operates at a voltage window of up to 3.11 volts — enough to match many commercial lithium-ion batteries — without needing a rigid, hermetically sealed case. Its water content is just 19%, making it low enough to prevent unwanted reactions, yet high enough to keep ions flowing freely.
This material is, in effect, a new kind of electrolyte.
Tougher than it looks
To test it, the researchers built full lithium-ion batteries with flexible, wavy electrodes and their WZH hydrogel. Then they put them through the ringer. They twisted, bent, and stretched them up to 50% of their original length. They stabbed them repeatedly with needles and sliced them in half. In one test, the battery lit an LED even as it was being punctured five times in a row.
After each injury, the battery was allowed to rest — or in some cases, briefly heated to 70°C. Within minutes, it stitched itself back together. Ten cycles of cutting and healing resulted in less than a 10% change in resistance.
And it kept working. Over 500 charge cycles, the battery discharged 95 percent of the energy it received. That’s comparable to many commercial smartphone batteries, which are also designed for around 500 cycles.
This resilience comes from the gel’s molecular makeup. Its backbone contains not just ion-trapping groups, but also hydrogen bond donors and acceptors that snap back together after damage. These dynamic bonds let the material reassemble itself, much like living tissue might.
Still, there’s room for improvement. After 500 cycles, the prototype retained only 60 percent of its original capacity, less than the industry standard of 80 percent. And its energy density is only about one-tenth that of commercial lithium-ion cells.
But the researchers argue that this lower density isn’t a dealbreaker.
“Your smartwatch is powered by a battery, but the band for this watch today performs only the mechanical function,” said Liwei Lin, the study’s senior author and a professor of mechanical engineering. “If you can replace the band with our battery, you have more area, more volume to work with. Instead of needing a recharge once a day, it could perhaps work for, like, a week.”
From the lab to your skin
Wearables, which now depend on awkwardly placed batteries, could become fully flexible. Soft robots, designed to move like living creatures, could carry their own power in muscle-like structures. Medical implants could become less invasive, more adaptable, and safer.
And then there’s the peace of mind.
“Current-day batteries require a rigid package because the electrolyte they use is explosive,” Lin said. “One of the things we wanted to make was a battery that would be safe to operate without this rigid package.”
The team has big plans. They’re exploring ways to increase the battery’s capacity using 3D porous electrodes, and new cathode materials that can better exploit the gel’s high voltage window. They’re also looking into how the technology might work in other battery chemistries, including zinc or sulfur-based designs.
For now, the WZH battery is a striking proof of concept. It shows that a soft, safe, stretchable lithium-ion battery isn’t just possible — it can survive real-world abuse and keep working.
The findings were reported in the journal Science Advances.
