Electrolyte and Interphase Engineering of Aqueous Batteries Beyond “Water‐in‐Salt” Strategy

Abstract

Aqueous batteries are promising alternatives to non‐aqueous lithium‐ion batteries due to their safety, environmental impact, and cost‐effectiveness. However, their energy density is limited by the narrow electrochemical stability window (ESW) of water. “Water‐in‐salts” (WIS) strategy is an effective method to broaden the ESW by reducing the “free water” in the electrolyte, but the drawbacks (high cost, high viscosity, poor low‐temperature performance, etc.) also compromise these inherent superiorities. In this review, electrolyte and interphase engineering of aqueous batteries to overcome the drawbacks of the WIS strategy have been summarized, including the developments of electrolytes, electrode‐electrolyte interphases, and electrodes. Firstly, the main challenges of aqueous batteries and the problems of the WIS strategy are comprehensively introduced. Secondly, the electrochemical functions of various electrolyte components (e.g., additives and solvents) are summarized and compared. Gel electrolytes are also investigated as a special form of electrolyte. Thirdly, the formation and modification of the electrolyte‐induced interphase on the electrode are discussed. Specifically, the modification and contribution of electrode materials toward improving WIS strategy are also introduced. Finally, the challenges of aqueous batteries and the prospects of electrolyte and interphase engineering beyond the WIS strategy are outlined for the practical applications of aqueous batteries.This article is protected by copyright. All rights reserved

Trade-off between H2O-rich and H2O-poor electric double layers enables highly reversible Zn anodes in aqueous Zn-ion batteries

“…Considering that the properties of the Zn/electrolyte interface before cycling are mainly determined by the electrolyte bulk phase properties and EDL structure, relevant characterizations were conducted to explore the relationship between APM and the properties of the Zn/electrolyte interface. 31,32 Fourier transform infrared (FTIR) spectroscopy and Raman spectra were first collected to investigate whether the APM additive affects the Zn 2+ solvation structure, as shown in Fig. 1(b), the bending and stretching modes of the O–H bond of H 2 O experienced no shifting or intensity variation with the increase of APM.…”

Low-Cost “Water-in-Deep Eutectic Solvent” Electrolyte for High-Performance Zinc Ion Hybrid Supercapacitors

“…Under these supersaturated salt conditions, the ions disrupt the wellstructured hydrogen bonding network of water and bound water within their solvation sheaths, 26 and as a result, WIS electrolytes exhibit special characteristics that are fundamentally different from normal water, exhibiting a wider ESW (>1.5 V). 25,27 However, this approach generally fails to take advantage of the cost reduction embodied in the improved electrolyte. For example, the extremely high concentration of 30 M ZnCl 2 WIS leads to the use of just 1 L of electrolyte in actual production, 28,29 which contains more than 4 kg of ZnCl 2 and costs about $250, while this price can be purchased for about 5 mol of lithium hexafluorophosphate, the latter being the main component of current ternary lithium ion batteries.…”

“…However, there are some problems in the development of high-voltage zinc ion hybrid supercapacitors (ZIHSs), and when operating ZIHSs with conventional aqueous electrolytes, parasitic side reactions, including dendrite growth, hydrogen precipitation reaction (HER), corrosion, and passivation, inevitably occur, which can greatly affect the reversibility of ZIHSs, leading to continuous electrolyte depletion and limiting the potential applicability of ZIHSs. , Recently, several strategies have been shown to improve the cycle life and reversibility of ZIHSs, including the application of coatings and interfaces on zinc anodes, changes in the structure of zinc, and new electrolyte designs . Of these, electrolyte design is considered to be the simplest, most cost-effective, and feasible solution, and recently reported electrolyte design strategies for ZIHSs include adding additives, adjusting pH, using gels, deep eutectic solvents, mixing, and concentrating aqueous electrolytes. , The development of highly concentrated “water-in-salt” (WIS) electrolytes has been proven to be an effective way to overcome the narrow voltage window limitations of aqueous electrolytes and to increase the energy density of different water-based battery systems. − This strategy is based on extremely high concentrations of salt dissolved in water. Under these supersaturated salt conditions, the ions disrupt the well-structured hydrogen bonding network of water and bound water within their solvation sheaths, and as a result, WIS electrolytes exhibit special characteristics that are fundamentally different from normal water, exhibiting a wider ESW (>1.5 V). , However, this approach generally fails to take advantage of the cost reduction embodied in the improved electrolyte.…”

“…Of these, electrolyte design is considered to be the simplest, most cost-effective, and feasible solution, and recently reported electrolyte design strategies for ZIHSs include adding additives, adjusting pH, using gels, deep eutectic solvents, mixing, and concentrating aqueous electrolytes. , The development of highly concentrated “water-in-salt” (WIS) electrolytes has been proven to be an effective way to overcome the narrow voltage window limitations of aqueous electrolytes and to increase the energy density of different water-based battery systems. − This strategy is based on extremely high concentrations of salt dissolved in water. Under these supersaturated salt conditions, the ions disrupt the well-structured hydrogen bonding network of water and bound water within their solvation sheaths, and as a result, WIS electrolytes exhibit special characteristics that are fundamentally different from normal water, exhibiting a wider ESW (>1.5 V). , However, this approach generally fails to take advantage of the cost reduction embodied in the improved electrolyte. For example, the extremely high concentration of 30 M ZnCl 2 WIS leads to the use of just 1 L of electrolyte in actual production, , which contains more than 4 kg of ZnCl 2 and costs about $250, while this price can be purchased for about 5 mol of lithium hexafluorophosphate, the latter being the main component of current ternary lithium ion batteries .…”

Air-Stable and Low-Cost High-Voltage Hydrated Eutectic Electrolyte for High-Performance Aqueous Aluminum-Ion Rechargeable Battery with Wide-Temperature Range

“…16 Unfortunately, the practical commercialization of this technology is still restricted by several technical obstacles in the aspect of aqueous electrolyte (Figure 1a). 17 Up until now, the majority of research studies on AAIBs are based on organic/inorganic cathode development, 18−20 such as phenazine (PZ), 21 macrocyclic calix [4]quinone (C4Q), 22 and Al 2/3 Li 1/3 Mn 2 O 4 (ALMO) cathodes, 23 which boosts the development of the high-performance AAIBs. However, we have noticed that the interface of the anode−electrolyte is unstable in an aqueous environment, and this issue has rarely been explored.…”