Use of Water-In-Salt Concentrated Liquid Electrolytes in Electrochemical Energy Storage: State of the Art and Perspectives

Abstract

Batteries based on organic electrolytes have been raising safety concerns due to some associated 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 narrow 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 chemistries operated with different WIS electrolytes. Finally, we highlight the challenges and future technological perspectives for practical aqueous energy storage systems, including applications in stationary storage/grid, power backup, portable electronics, and automotive sectors.

Solvent-in-Salt Electrolytes for Fluoride Ion Batteries

“…Symmetric Pb|PbF 2 coin cells were cycled at a C/10 rate, with the low-concentration electrolyte showing faster capacity fading (Figure a). This is attributed to the aggravated active material dissolution at lower concentrations (given the constant solubility product, K sp , lower fluoride concentrations will result in higher metal ion dissolution), a problem known to be detrimental in previous FIBs based on conversion of metal fluorides. ,, The WiSE, however, showed improved and more stable capacity retention, with this differentiated cycling performance expected to become more evident with further cycling . For this symmetric cell, increased cycling showed a complete capacity loss for the diluted electrolyte by the 100th cycle (Figure S5).…”

Molecular Modeling of Water-in-Salt Electrolytes: A Comprehensive Analysis of Polarization Effects and Force Field Parameters in Molecular Dynamics Simulations

“…We use a cubic simulation box with an initial side length of 28 Å, chosen large enough that the size effect is negligible (see the SI section A), and periodic boundary conditions in all three directions. 80 salt ion pairs and 480 water molecules are randomly distributed within the simulation box, resulting in a salt concentration of 9.25 m. In the case of a NaOTF aqueous electrolyte, this salt concentration corresponds to the water-in-salt regime. , The energy of the system is minimized via the Polak–Ribiere version of the conjugate gradient (CG) method . For polarizable simulations, Drude particles are added to an energy minimized configuration using the polarizer tool described in ref .…”

Structure–Property Correlations in Aqueous Binary Na+/K+–CH3COO– Highly Concentrated Electrolytes

“…Water-in-salt electrolytes (WISEs) are a new class of ionic solutions, which belong to the wider category of solvent-in-salt (SIS) electrolytes, defined as concentrated solutions, in which the salt/solvent ratio is larger than unity in a weight or molar ratio . These systems have physicochemical properties (electrical conductivity, viscosity, electrochemical stability) that differ considerably from normal ionic solutions (molar salt/solvent ratio <0.2) and make them interesting for a number of electrochemical applications, in particular energy storage . Indeed, although the use of WISEs is recently raising interest in catalysis, electrochromic systems, and surface coatings, most of the work refers to applications in rechargeable batteries or supercapacitors .…”