The study was conducted as part of Martins’ postdoctoral research supervised by Torresi and part of the Thematic Project “Optimization of the physicochemical properties of nanostructured materials for applications in molecular recognition, catalysis and energy conversion/storage,” for which Torresi is principal investigator. Both projects are supported by FAPESP.
“The term ‘water-in-salt electrolytes’ refers to solutions constituting a very high concentration of salt in a very small amount of water. The amount of water is just sufficient to dissolve the ions to promote solvation. The system contains no free water, unlike conventional solutions,” Torresi told Agência FAPESP.
This is possible only if the salt molecule to be dissolved comprises a large anion and a small cation, Torresi explained. An example is LiTFSI, i.e., lithium bis(trifluoromethane sulfonyl)imide (CF3SO2NLiSO2CF3), whereas NaCI, i.e., sodium chloride or table salt, is of no use, as it has an anion and cation of similar sizes.
“Because there’s no free water in this ultraconcentrated solution, electrolytic splitting of water into hydrogen and oxygen becomes far more difficult, so the electrochemical stability of the solution is very high despite the system containing water,” he said.
In summary, this innovative technological proposal based on a high concentration of salt in water offers significant advantages over conventional technology using salt dissolved in organic compounds. Nevertheless, the technological use of water-in-salt electrolytes also presents challenges.
“The first is that the solution contains little water and is highly hygroscopic: it tends to absorb moisture from the air, and this changes its water content. The second is that ultraconcentrated aqueous solutions are highly corrosive,” Torresi said.
The propensity to absorb ambient moisture is shared with organic solvents and is one of the reasons why conventional batteries have to be shielded, but corrosiveness is a major disadvantage: the organic solvents currently used in lithium batteries do not attack the electrodes, the only metallic components, to a significant extent.
However, according to Torresi, this drawback should not be overestimated. “Corrosion was a major issue for decades. Now, we know how to refine current collectors, and with a few adaptations, it won’t be hard to surmount the problem of corrosion in a future aqueous battery,” he said.
The article “Water-in-salt electrolytes for high voltage aqueous electrochemical energy storage devices” can be retrieved from www.sciencedirect.com/science/article/abs/pii/S245191032030013.
About São Paulo Research Foundation (FAPESP)
The São Paulo Research Foundation (FAPESP) is a public institution with the mission of supporting scientific research in all fields of knowledge by awarding scholarships, fellowships and grants to investigators linked with higher education and research institutions in the State of São Paulo, Brazil. FAPESP is aware that the very best research can only be done by working with the best researchers internationally. Therefore, it has established partnerships with funding agencies, higher education, private companies, and research organizations in other countries known for the quality of their research and has been encouraging scientists funded by its grants to further develop their international collaboration. You can learn more about FAPESP at www.fapesp.br/en and visit FAPESP news agency at www.agencia.fapesp.br/en to keep updated with the latest scientific breakthroughs FAPESP helps achieve through its many programs, awards and research centers. You may also subscribe to FAPESP news agency at http://agencia.fapesp.br/subscribe.
Current Opinion in Electrochemistry