When scientists at the University of Surrey decided to leave water inside a battery material, their colleagues thought they were making a mistake. Water and batteries do not usually mix. But the researchers were wrong about that assumption in the best possible way.
The team, based in Guildford, United Kingdom, discovered that keeping water in a sodium-based battery material did not harm it. Instead, it made the material work nearly twice as well. The finding could open the door to cheaper, more sustainable batteries and maybe even help clean salty water.
Sodium-ion batteries are made from sodium, an element found in seawater, table salt, and minerals underground. Lithium-ion batteries, which power most phones, laptops, and electric cars, rely on lithium, which is scarcer and more expensive to extract. Sodium is far more common and easier to find around the world, making it an attractive alternative for storing large amounts of energy.
The problem has been performance. Sodium batteries have historically struggled to store as much charge as lithium ones, charge as quickly, or last as long. Researchers have spent years trying to close that gap.
Dr. Daniel Commandeur and his team at the University of Surrey School of Chemistry and Chemical Engineering were studying a material called nanostructured sodium vanadate hydrate, or NVOH for short. The word hydrate means water molecules are actually built into the material's structure. Normally, researchers would heat the material to drive out this water because moisture is considered bad for batteries. But Commandeur's team decided to test the opposite.
They left the water in. The results stunned them. In laboratory tests, the water-containing version stored almost twice as much charge as typical sodium-ion materials. It charged faster and kept working reliably for more than 400 complete charge cycles, which is one charge and one discharge. That placed it among the strongest cathode materials ever reported for this type of battery.
But the surprise did not stop there. The researchers then placed the material in salt water, an environment that usually causes problems for battery components. Not only did it continue working, but it also began pulling salt out of the water. The sodium vanadate hydrate drew sodium particles from the water while a graphite electrode removed chloride, the two parts that make up ordinary table salt. The researchers call this process electrochemical desalination.
Commandeur said the discovery was completely unexpected. "We decided to challenge that assumption," he explained, "and the outcome was far better than we anticipated."
The implications stretch far beyond the laboratory. If sodium-ion batteries could eventually run safely on seawater, they might one day help provide clean energy and fresh water at the same time. "That means we might be able to design systems that use seawater as a completely safe, free and abundant electrolyte," Commandeur said, "while also producing fresh water as part of the process."
The study was published in the Journal of Materials Chemistry A.