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Perspective Use of Water-in-Salt Concentrated Liquid Electrolytes in Electrochemical Energy Storage: State of the Art and Perspectives Shahid Khalid 1, Nicolò Pianta 1, Piercarlo Mustarelli 1,2,* and Riccardo Ruffo 1,2 Citation: Khalid, S.; Pianta, N.; Mustarelli, P.; Ruffo, R. Use of Water-in-Salt Concentrated Liquid Electrolytes in Electrochemical Energy Storage: State of the Art and Perspectives. Batteries 2023, 9, 47. https://doi.org/10.3390/ batteries9010047 Academic Editor: Hans-Georg Steinrück Received: 2 December 2022 Revised: 31 December 2022 Accepted: 4 January 2023 Published: 7 January 2023 Copyright: © 2023 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (https://cre- ativecommons.org/licenses/by/4.0/). 1 Department of Materials Science, University of Milano Bicocca, 20126 Milan, Italy 2 National Reference Center for Electrochemical Energy Storage (GISEL), Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali (INSTM), 50121 Firenze, Italy * Correspondence: piercarlo.mustarelli@unimib.com Abstract: Batteries based on organic electrolytes have been raising safety concerns due to some as- sociated fire/explosion accidents caused by the unusual combination of highly flammable organic electrolytes and high energy electrodes. Nonflammable aqueous batteries are a good alternative to the current energy storage systems. However, what makes aqueous batteries safe and viable turns out to be their main weakness, since water molecules are prone to decomposition because of a nar- row electrochemical stability window (ESW). In this perspective we introduce aqueous batteries and then discuss the state-of-the-art of water-in-salt (WIS) electrolytes for aqueous energy storage systems. The main strategies to improve ESW are reviewed, including: (i) the use of fluorinated salts to make a solid electrolyte interphase (SEI); (ii) the use of cost-effective and highly soluble salts to reduce water activity through super concentration; and (iii) the use of hybrid electrolytes combining the advantages of both aqueous and non-aqueous phases. Then, we discuss different battery chem- istries operated with different WIS electrolytes. Finally, we highlight the challenges and future tech- nological perspectives for practical aqueous energy storage systems, including applications in sta- tionary storage/grid, power backup, portable electronics, and automotive sectors. Keywords: metal-ion batteries; supercaps; water-in-salt; electrolyte; aqueous battery 1. Introduction Driven by a variety of needs such as the grid-scale deployment of intermitted renew- able energy sources together with the electrification of transportation, the development of efficient electrochemical energy storage systems (EESS) has long been at the forefront of energy technologies. Much progress has been accomplished thus far; for instance, the 2019 Nobel Prize in Chemistry honored the development of Li-ion batteries (LIBs) [1–3]. The electrolyte, which is an essential component of electrochemical systems, is often the limiting factor directly affecting the performance of EESS. Supported liquid electrolytes are still used in EESS because solid electrolytes are yet unable to compete in terms of cost, electrochemical, thermal, and interfacial properties. Therefore, the study and improve- ment of liquid electrolytes is remaining an active research area [4,5]. Currently, non-aque- ous electrolytes based on organic solvents, ionic liquids, or deep eutectic solvents are pri- marily used to attain high operating voltage in EESS due to their broad electrochemical stability windows (ESW). The extended ESW of organic solvent-based electrolytes over water is mainly due to: (i) the absence of acidic protons, (ii) to the formation of an ionically conductive passivation layer formed by the decomposition of part of the electrolyte, as in the case of carbonate-based solutions in LIBs. This passivation layer was named solid elec- trolyte interphase (SEI) by Peled in 1979 after its discovery in 1970 by Dey et al. by soaking a Li metal anode in propylene carbonate (PC) based electrolyte [6–9]. SEI forms on the surface of most anodes and some cathodes, where it is generally referred to as the cathode electrolyte interphase, CEI. The SEI is a thin solid film that allows the conduction of ions Batteries 2023, 9, 47. https://doi.org/10.3390/batteries9010047 www.mdpi.com/journal/batteriesPDF Image | Water-in-Salt Electrolytes Electrochemical Energy Storage
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