Electrolyte Engineering Toward High‐Voltage Aqueous Energy Storage Devices

Abstract

Aqueous electrochemical energy storage (EES) devices are highly safe, environmentally benign, and inexpensive, but their operating voltage and energy density must be increased if they are to efficiently power multifunctional electronics, new‐energy cars as well as to be used in smart grids. This Minireview summarizes the key breakthroughs and progress in expanding the electrochemical stability window (ESW) of aqueous EES devices over the past five years. After briefly introducing the electrode engineering ways to widen ESW, we focus on four ground‐breaking electrolyte engineering strategies and classify them into two kinds from the perspective of salts and exotic solutes/solvents. The widening degree toward ESW of these emerging electrolytes is compared and the universal fundamental mechanism relating to the interactions between limited water molecules and high‐concentration salts (or large amounts of exotic solutes/solvents) is elucidated. Key challenges and perspectives for high‐ESW electrolytes as well as recent advances in low‐cost and other metal ion (sodium, potassium, zinc, etc.)‐based electrolytes for expanding ESW are also outlined.

Liquid Structure of a Water-in-Salt Electrolyte with a Remarkably Asymmetric Anion

“…Upon increasing the salt content, conductivity reaches a maximum and, due to increased viscosity, progressively decreases when the salt content reaches concentrations of the order of a few molal (mol salt /kg solvent ). Accordingly, the highly concentrated regime that characterizes WiS systems has barely been explored in the past and only recently, more systematic studies are being developed in this new regime. − Recent reviews have addressed the nature of the structural, dynamic, and electrochemical properties of these systems. ,,,,− Much of the structural investigations have been focused on the first WiS system, namely, LiTFSI-H 2 O, mostly due to LiTFSI high solubility in water (>20 m at 25 °C) and stability against hydrolysis . The phase diagram of this binary system has been characterized, showing the existence of a eutectic LiTFSI/H 2 O = 1:1, with a melting point at ca.…”

Recent advances in materials and device technologies for aqueous hybrid supercapacitors

“…As expected, the Na + IC-WISE could expand the electrochemical window to 3.3 V, with the suppression of transition metal dissolution from the battery-type electrode during cycling. Moreover, using super sucrose electrolyte and molecular crowding electrolyte without the high-concentration salt can also expand the electrochemical stability window of aqueous devices by reducing the amount of free water and destroying the tetrahedral structure (the hydrogen bonds) of free water molecules [49].…”

“…Nevertheless, SCs should also deliver high power density to differentiate them from batteries, and fast ion transport in electrolytes and efficient charge transfer at the electrode/ electrolyte interface must be ensured. In this regard, special attention should be paid in future to increasing the ionic conductivity of high-voltage aqueous electrolytes, without drastically increasing the viscosity and avoiding the inevitable salt precipitation at low temperatures [49].…”

“…Thus, according to the equation E = 1/2CV 2 (where C and V are the specific capacitance and operating voltage of the device, respectively), the energy density of AHSCs is much lower than that of conventional SCs. Fortunately, recent advances have indicated that the V can be significantly widened via designing advanced electrolyte, electrode, and robust electrode/electrode interface [49]. To efficiently power flexible and wearable electronics, afford miniaturized energy delivery or harvest with high power capabilities for Internet of Things, significant research efforts have also been devoted to developing thin-film flexible AHSCs using quasi-solid-state/gel electrolytes and flexible/ bendable current collectors [50][51][52].…”

Surface and Interface Engineering of Nanoarrays toward Advanced Electrodes and Electrochemical Energy Storage Devices

“…In addition, challenges such as electrode volume expansion and dissolution, dendrites on the metal anode surface, interfacial side reactions with water or dissolved oxygen, and detrimental passivation layers remain and restrict the overall performance of many aqueous EESDs. [32][33][34] It is evident that all these challenges are essentially related to the surface and interface issues in the devices.…”