A Yarn-Like Seawater Battery

After soaking in salty water, these rechargeable strands lit up LEDs (top image) and powered a timer (bottom image). (Image: Adapted from ACS Applied Materials & Interfaces 2024, DOI: 10.1021/acsami.4c16439)

Most batteries are rigid and incompatible with water. But people work and play in oceans and estuaries, and they could benefit from flexible and saltwater-safe power sources. Now, researchers report in ACS Applied Materials and Interfaces a yarn-like battery prototype that works when immersed in seawater. They knotted the rechargeable strands into a fishing net that lit up LEDs and wove a fabric that powered a timer.

Flexible, yarn-like batteries can be knit or woven into various shapes. These stringy energy sources are lightweight and are often designed to be waterproof. But rather than avoid battery exposure to water entirely, scientists have proposed using salty water as a critical battery component — the electrolyte — a liquid that conducts electricity through ions. Yan Qiao, Zhisong Lu, and colleagues previously developed a water-friendly battery made with carbon fiber and cotton yarn. This prior work used sweat from the body as the electrolyte for exercise monitors. Like sweat, seawater, which contains sodium, chloride and sulfate ions, can serve as an electrolyte. So, Qiao, Lu and a new team wanted to develop a marine version of a rechargeable battery that could be used to power lights on fishing nets, life jackets, or mooring lines for buoys.

To create electrodes for the seawater batteries, the group treated carbon fiber bundles with electrically conductive coatings: nickel hexacyanoferrate for the positive electrode (cathode) and polyamide for the negative electrode (anode). Then researchers twisted two bundles together to form yarn-like cathode and anode strings. To prepare a battery, the researchers wrapped the cathode string in a layer of fiberglass, laid it along the anode, and encased both strands in a nonwoven, permeable fabric. The fabric protects the electrodes while also allowing in seawater to contact the electrodes. In tests using saltwater, the battery continued to store an electrical charge after being bent 4000 times. Then, when evaluated in seawater, it retained most of the initial charging efficiency and storage capacity over 200 charge and discharge cycles.

Finally, as a proof-of-concept, the group knotted battery strands together into a fishing net and wove a rectangular piece of fabric. The net was then soaked in seawater to absorb the electrolyte and was charged. The net battery lit up a 10-LED panel. Similarly, the fabric submerged in a sodium sulfate solution powered a timer for more than an hour. The researchers say their yarn-like battery has potential as an energy source in marine applications.

Here is an exclusive Tech Briefs interview, edited for length and clarity, with Lu.

Tech Briefs: What was the biggest technical challenge you faced while developing this seawater battery?

Lu: The most significant technical challenge we encountered would be how to efficiently immobilize active materials within yarn-like electrodes. The goal was to maximize the use of the internal spaces between the carbon fibers to boost performance. Traditionally, active materials are immobilized on yarns using a "twist-before-coating" method, where fibers are twisted first and then coated with active materials. But in this study, we developed a new approach called the "twist-after-coating" technique. We first coat the well-aligned carbon fibers with active materials and then twist them to form the yarn-like electrodes. This method ensures that the active materials are evenly distributed both inside the yarn and on its surface, thereby making full use of each carbon fiber. As a result, we reduced the charge transfer resistance and achieved a higher specific capacity.

Tech Briefs: Can you please explain in simple terms how it works?

Lu: Seawater acts as a natural electrolyte because it contains positively charged ions like sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), and magnesium (Mg²⁺). When the battery charges, these ions are embedded into the yarn-based negative electrode, storing energy. During discharge, the ions are released from the negative electrode, move through the seawater, and embed themselves into the positive electrode. The back-and-forth movement of the ions between the electrodes enables the battery to work.

Tech Briefs: How does this battery differ from the water-friendly battery made with carbon fiber and cotton yarn that your team previously developed?

Lu: In our earlier work, we created a water-friendly battery called a "yarn-based sweat-activated battery." That design used zinc wire as the positive electrode, carbon fiber as the negative electrode, and human sweat as the electrolyte instead of seawater. Another key difference is that the sweat-activated battery was a single-use, non-rechargeable device, whereas this newly developed seawater battery is fully rechargeable.

Tech Briefs: Do you have plans for further research/work/etc.?

Lu: Future research will focus on two main areas: (1) optimizing the active materials and device structure to improve the performance of these yarn-like seawater batteries, and (2) developing simpler, scalable methods for mass production to meet the demands of marine and deep-sea applications.

Tech Briefs: Do you have any updates you can share?

Lu: In our latest work, we’ve improved the design by adjusting the intertwined structure of the positive and negative electrodes, which resulted in a better negative-to-positive capacity ratio. Compared to earlier versions, the new yarn-like seawater batteries show a 10.2 percent increase in specific capacity.

Tech Briefs: Is there anything else you’d like to add that I didn’t touch upon?

Lu: One important point is that traditional seawater batteries often use non-aqueous electrolytes and sodium anodes, which can be harmful to the environment and limit their widespread use. Our yarn-like seawater battery avoids these issues entirely — it doesn’t rely on hazardous electrolytes or active metal anodes, making it a safe and sustainable power source that works seamlessly in seawater. Plus, its open structure eliminates the need for sealing layers and solid ion-conducting membranes, which significantly cuts down on costs.