Soap-inspired electrolyte for designing longer-lasting batteries

A new study published in Nature Materials has found that a promising electrolyte for designing longer lasting lithium batteries has complex nanostructures that act like micelle structures do in soaped water. When someone washes their hands with soap, the soap forms structures called micelles that trap and remove grease, dirt and germs when flushed with water. The soap does this because it acts as a bridge between the water and what is being cleaned away, by binding them and wrapping them into those micelle structures.

Researchers from Brown University and Idaho National Laboratory observed that a similar process occurs in a new type of electrolyte called a localised high-concentration electrolyte, which could be promising for the design of longer-lasting lithium batteries. This new understanding of how this process works might be the missing piece to enhancing this emerging technology.

Yue Qi, a professor at Brown’s School of Engineering, said the researchers want to improve and increase the energy density for batteries, by enhancing how much energy they store per cycle and how many cycles the battery lasts. “To do this, materials inside of traditional batteries need to be replaced to make long-life batteries that store more energy a reality — think batteries that can power a phone for a week or more, or electric vehicles that go for 500 miles,” Qi said.

Scientists are working to transition to batteries made from lithium metal because they have a higher energy storage capacity than lithium-ion batteries. However, traditional electrolytes for lithium-ion batteries, which are made of low-concentration salt dissolved in a liquid solvent, don’t effectively enable an electrical charge to pass between a battery’s two terminals, sparking the electrochemical reaction needed to convert stored chemical energy to electric energy.

Scientists at Idaho National Laboratory developed localised high-concentration electrolytes to address this challenge. The electrolytes are made by mixing a high concentration of salt in a solvent with another liquid called a diluent, which makes the electrolyte flow better so that the power of the battery can be maintained. This new type of electrolyte has shown promising results, but how it works and why has never been fully understood, which limits how effective it can be and how it can be better developed.

Bin Li, a senior scientist at Oak Ridge National Laboratory, said the new research findings provide a unified theory as to why this electrolyte works better. “Here we see that the role of the soap or surfactant is played by the solvent that binds both the diluent and the salt, wrapping itself around the higher concentration salt in the centre of the micelle,” Li said.

By understanding this, the researchers were able to break down the ratios and concentrations needed to bring about the optimal reactions for the batteries. This should help engineer this electrolyte, as it allows researchers to find the proper balance for all three ingredients. In fact, the work not only provides better guidelines for making localised high-concentration electrolytes that function, but for making ones that work even more effectively. The researchers put this theory into action and found that it holds up and helps to extend the life of lithium metal batteries. The researchers look forward to discovering what designs for localised high-concentration electrolytes come from their work, but progress must first overcome the electrolyte design bottleneck for high-density batteries.

“The concept of the micelle may be new for the electrolyte, but it’s actually very common for our daily life. Now we have a theory, and we have guidelines to get interactions we want from the salt, the solvent and the diluent in the electrolyte, and what concentration they have to be at and how you mix them,” Qi said.

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