Recycling-regenerating salt enables the economic viability of aqueous high-concentration electrolytes

The rapid increase in energy demand has stimulated the development of energy storage devices with high energy density. A variety of rechargeable batteries shine in different energy storage fields due to their unique characteristics. However, traditional electrolytes have disadvantages such as low safety, poor cycle stability, unfriendly environments, and high cost. Although aqueous electrolyte can solve some of these problems, its narrow electrochemical window severely limits its development and application. Due to its unique solvation structure, high-concentration electrolyte exhibits excellent performance in rechargeable batteries. The stable electrode–electrolyte interface, wide electrochemical window, high thermal stability, low volatility, and good flame retardancy provide a new direction for the development of next-generation batteries. In Chapter 10 we outline the application and development of high-concentration electrolytes and local high-concentration electrolytes in different rechargeable batteries, and look forward to the challenges and prospects of organic and aqueous high-concentration electrolytes.

Among several energy storage technologies, supercapacitors have achieved considerable attention because of their rapid charge-discharge rates, high power density, and excellent cycling stability, but low energy density and fast self-discharge are some of serious disadvantages for the supercapacitors. Electrodes with combination of battery and pseudocapacitance behaviors offer the advantages of both supercapacitors and the advanced batteries, it shows higher energy density and the same time maintains the extended cycle life and fast charge capability. In this study, a new type of tetragonal LixMn2O4 (LMO) electrode exhibiting combined pseudocapacitance and battery behaviors in aqueous electrolytes has been synthesized by electrochemical cation-exchange conversion from ZnMn2O4 (ZMO) [1]. It is shown that the Zn+ ion in ZMO can be replaced by the Li+ ion, resulting in the formation of LMO in aqueous Li2SO4 electrolytes by cyclic voltammetry. The resulting LMO electrode is in tetragonal structure but yet exhibits redox potentials essentially the same as those of the cubic LMO (Fig. a), delivering a redox capacity over 100 mAh/g under low current rate or a specific capacitance of nearly 300 F/g when behaving as a pseudocapacitor at higher current rates. Despite its tetragonal structure, the electrode shows outstanding cycle stability, exhibiting an interesting self-healing structure transformation process to the cubic structure and retaining 100% capacity after 40, 000 cycles. Using the high-concentration LiTFSI electrolyte enables the expansion of operating potential window to 1.55 V. The material synthesis process and charge-transfer mechanism are characterized in detail. [1] M. Abdollahifar, S.S. Huang, Y.H. Lin, Y.C. Lin, B.Y. Shih, H.S. Sheu, Y.F. Liao, N.L. Wu, J. Power Sources 378, 90, 2018. Figure 1

The inherent safety and potential cost-efficiency of rechargeable batteries based on aqueous electrolytes, led to their development as a large-scale energy storage option for grid applications enabling wide spread integration of renewables. In this field cost and safety are more important than energy density, which is why water based systems are a promising candidate. In recent reports, highly-concentrated electrolytes have been employed to extend the narrow electrochemical stability window of water (thermodynamically ~1.23 V) and stability windows beyond 3 V have been reported for lithium based systems [1]. Using 35 molal aqueous sodium bis(fluorosulfonyl)imide (NaFSI) solutions, we recently translated the concept to sodium-ion batteries and reported a stability window of 2.6 V on stainless steel current collectors [2]. However, all so called water-in-salt electrolytes reported to date suffer from crystallization at room temperature, which leads to premature cell failure. We developed a ternary aqueous sodium-ion electrolyte based on NaFSI which is kinetically robust against crystallization and allows long term cycling at temperatures down to at least -10 °C. Addressing the raw materials supply chain as important challenge for scaling, we developed a 2 V class aqueous sodium-ion battery employing only abundant raw materials, namely NaTi2(PO4)3 [3] and Na3(VOPO4)2F [4] on the anode and cathode side, respectively. With 64 Wh kg-1, based on the active masses of both electrodes, this battery displays an energy density that is almost twice as high as that of previously reported aqueous sodium-ion batteries [5]. The cell displays excellent cycling stability at 30 °C with capacity retention of 85% after 100 cycles at C/5 and 77% after 500 cycles at 1C. At -10 °C, we obtain capacity retention of 74% after 500 cycles at C/5.

Superconcentrated aqueous electrolytes have recently emerged as a new class of electrolytes, called water-in-salt electrolytes. They are distinguished, in both weight and volume, by a quantity of salt greater than water. Currently, these electrolytes are attracting major interest, particularly for application in aqueous rechargeable batteries. These electrolytes have only a small amount of free water due to an ultrahigh salt concentration. Consequently, the electrochemical stability window of water is wider than the predicted thermodynamic value of 1.23V. Hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) have been shown to be shifted to more negative and positive potentials, respectively. The decrease in free water population is recognized as being involved in the increase in the electrochemical stability window of water. Here, we study the quantitative contribution of the decrease in the free water molecule concentration to the permittivity of the solution and of the activity of water to the OER and HER overpotentials when the salt concentration increases. We compare our model with that of Kornyshev and get three types of electrolyte structures: diluted, gradient of water contents, and aggregation. The theoretical calculation of the redox potentials of the OER and HER is compared with the experimentally determined electrochemical properties of aqueous LiTFSI electrolytes.

A flexible supercapacitor with high energy density and wide range of temperature tolerance using a high-concentration aqueous gel electrolyte

CO2-regulated, ultrahigh-concentration aqueous electrolytes for high energy and power of supercapacitors

Understanding Li-ion thermodynamic and kinetic behaviors in concentrated electrolyte for the development of aqueous lithium-ion batteries

Lithium‐ion batteries with aqueous electrolytes can be excellent candidates for battery applications at low temperatures. In contrast to a common misconception, aqueous lithium ion batteries can operate at several tens of degrees below the freezing point of water when high concentration electrolyte solutions are utilized. Furthermore, it is reported here that the performance of intercalation cathodes in aqueous electrolytes is quite remarkable and superior to that in common organic electrolytes at very low temperatures down to about −40 °C. Here in the performance of water‐based electrolyte solutions‐based on three low‐cost inorganic salts (LiNO3, Li2SO4, and LiCl) and that of the corresponding aqueous battery systems is studied in order to understand the rate‐limiting step at sub‐zero temperatures. It is found that the charge transfer resistance is the largest impedance contributor at low temperatures. However, layered cathodes in aqueous electrolytes do not exhibit a significant increase in the charge‐transfer resistance, or a reduction in the accessible capacity during charging until the temperature is close to the solution freezing point. This is in sharp contrast to their behavior in organic electrolytes that do not support any performance below −20 °C. This different behavior explains the dramatically superior performance of lithium ion battery cathodes in water‐based electrolytes at lower temperatures.

High energy aqueous sodium-ion capacitor enabled by polyimide electrode and high-concentrated electrolyte

Aqueous batteries represent a significant research area due to their low cost and high safety advantages. However, aqueous electrolytes suffer from high side‐reaction activity, narrow electrochemical windows, and insufficient interface stability and are frozen at low temperatures, thus hampering practical applications. This review focuses on high‐concentration brine‐based aqueous electrolyte optimization strategies to address the above problems. The solvation structure, hydrogen‐bond network, and interfacial components are the key factors that are altered by the appropriate salts, solvent selection, and electrode interaction. A high concentration of brine decreases the free water content, inhibits the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), and widens the electrochemical window. Additional salts and solvents in the electrolyte can further promote the formation of the solid electrolyte interphase (SEI) and the cathode electrolyte interphase (CEI) to reduce deleterious interfacial side reactions. At the same time, the synergistic effects between the cathodes/anodes and the electrolyte expand the electrochemical window, improve the interface stability, and enhance the electrochemical properties of aqueous batteries. In this review, we describe the optimization strategies and mechanisms to provide guidance to future research on high‐concentration electrolytes (HCE) and the challenge of high‐energy and wide‐temperature‐range applications.

Predicting CO2 solubility in aqueous and organic electrolyte solutions with ePC-SAFT advanced

Improving the cycle life of cryptomelane type manganese dioxides in aqueous rechargeable zinc ion batteries: The effect of electrolyte concentration

Ion–ion correlation, solvent excluded volume and pH effects on physicochemical properties of spherical oxide nanoparticles

Supercapacitors with high power density and life cycles are an important family of power supplies among energy storage systems. The operating voltage window of a supercapacitor is determined by both the chemistry of electrode materials and the electrochemical kinetics of electrolytes while the water hydrolysis potential of 1.23 V is the typical limit for capacitors based on aqueous electrolytes. Here, we briefly outline the working mechanism of electrochemical supercapacitors, including electron double layer capacitors (EDLC) and pseudo‐capacitors, and investigate the limitations of their working voltages. The principles and examples of different designs of electrodes and electrolytes for high‐voltage supercapacitors beyond the hydrolysis limit are discussed, such as asymmetric electrodes, high concentration electrolytes, and electrolytes with different pH values. Insights into high voltage capacitor cells and future prospects are provided for the development of electrochemical energy storage systems.

In this work, the layered double hydroxide (LDH) Mg2Al(OH)6 was intercalated with redox active ferrocene carboxylate anions in order to implement charge storage capability to the interlayer spaces of the LDH structure. Two sets of anions, namely mono‐ and dicarboxylic ferrocene, were intercalated to produce two different active materials: MgAl‐FcMono and MgAl‐FcDi. The electrochemical investigation of these two materials was performed in two model electrolytes: 1 M LiTFSI in H2O and Pyr13TFSI. In the aqueous electrolyte, the first charge reaches the full theoretic capacity of ca. 60 and 40 mAh g−1 for both materials. However, significantly less capacity is stored and delivered during subsequent cycles. In‐situ UV/vis experiments identified the loss as a release of charged ferrocene anions from the LDH during oxidation in the charging process, which is more severe for MgAl‐FcMono. It is possible to prevent this release of redox species by the use of the ionic liquid Pyr13TFSI as a high concentrated electrolyte. Subsequently, both materials cycled very steadily with high coulombic efficiency for 150 cycles. This better understanding of the capacity degradation of the LDH‐ferrocene active material is key to improving this new and promising concept of using modified LDHs as active material in energy storage application.