Chinese researchers have greatly improved the performance of sodium-ion batteries by methylating hydrogel electrolytes, increasing their salt absorption and stability. This advance not only improves the efficiency of these environmentally friendly batteries, but also opens up new possibilities for the use of hydrogels in a variety of technologies.
Flexible water batteries commonly used in portable electronic devices typically include a hydrogel electrolyte composed of water and salt. A Chinese research team has made significant progress in improving the salt stability of hydrogels in sodium-ion batteries. They methylated the structural polymer of the hydrogel, which prevented salting out, thereby improving the battery's capacity and cycling performance.
Their research results were recently published in the journal AngewandteChemie.
Sodium-ion batteries are a promising alternative to lithium-ion batteries because they contain cheaper and more environmentally friendly materials than lithium-ion batteries. However, new batteries require the development of many new components, all of which must be adapted to sodium ions. One of the most basic components is the electrolyte, which often takes the form of a hydrogel in thin, flexible batteries. These flexible water-containing materials absorb dissolved sodium salts and conduct ions.
Although hydrogels are suitable, an unresolved issue is the occurrence of phase separation and salting out at the high salt concentrations required for a broad electrochemical stability window. Cui Guanglei and colleagues from the Qingdao Institute of Energy, Chinese Academy of Sciences, have now successfully improved sodium-ion battery hydrogels so that they can absorb more salt stably and safely.
To achieve this, they used a technique also used in nature to regulate the water-salt binding of large biological macromolecules: methylation. In proteins, methylation causes "capping" of amine and amide groups, thereby reducing the accessibility of water molecules to cross-links and salt ion solubilization within the protein structure.
Since the polyamide polymers used in hydrogels also contain amide groups, their extensive cross-linking by water molecules can lead to salting out and thus electrolyte decomposition. With this in mind, the team compared hydrogels made from ordinary polyamides with those made from polyamides containing methylated amide groups. The latter absorbs significantly more salt than the original variant. The hydrogel electrolyte remains transparent and stable even when salt concentrations reach record high levels.
Higher salt content means that the electrochemically available voltage range of the battery can be broadened. In addition, the team did not observe any signs of electrode disintegration, cycle stability was better, and the assembled battery had a greater capacity than its unmethylated counterpart. In this system, even cheap aluminum foil can be used as a current collector.
The authors suggest that simple polyamide methylation could also be used in other technologies, such as drug development, to make hydrogels more salt-tolerant and thus more stable.