Formation of Solid–Electrolyte Interfaces in Aqueous Electrolytes by Altering Cation‐Solvation Shell Structure

Abstract

materials in dilute aqueous electrolytes, respectively, [7] although a common strategy for adjusting the operating window is to change the pH of the electrolyte. However, even in strongly alkaline electrolytes (pH = 14), hydrogen evolution still occurs at 2.213 V, which is too high to accommodate most of the anode materials desired by battery scientists. [8] Additionally, a downshift in O 2 evolution potential will occur simultaneously when the pH is increased, keeping the gap between the anodic and cathodic limits constant at 1.23 V. This gap, known as the electrochemical stability window in aqueous electrolytes under thermodynamic equilibria, is defined by the Pourbaix diagram. The slightly higher voltage gap of 1.8 V that has been realized in existing aqueous battery setups is the result of "kinetic overpotentials" of H 2 or O 2 evolutions, which depends on changing the salt concentration, catalytic activity of the electrodes, and applied currents. [9] As a result, very few suitable electrode materials have been reported for AIBs at present, and their energy densities are still far from those required for practical applications. A particular challenge is that O 2 or H 2 evolution will result in low energy efficiencies, which is of vital importance for this technology. [10] The working potential of lithium-ion battery electrode materials is also far beyond the thermodynamic stability limits of organic electrolytes. [11] Hence, solvent molecules coordinated with Li + to form solvation shells can suffer from decomposition when ions move between electrode surfaces during the charging/discharging processes. These decomposition reactions produce dense, solid products that deposit on electrode surfaces to form a solid-electrolyte interface (SEI), which prevents direct contact between the electrodes and electrolytes. [12] A large number of inorganic salts or precipitates originating from salt reductions may also be present in the SEI at the anode, making it insulating to long-range electron transport but conductive to ions. Therefore, it can prevent sustained electrolyte decomposition while still allowing electrochemical reactions to proceed. The presence of an SEI substantially expands the usable electrochemical stability window of nonaqueous electrolytes. [13] However, protective interfaces are not formed in conventional aqueous electrolytes, because the decomposition products from coordinated water mostly are gases such as H 2 and O 2 , which will escape from the electrolyte on formation. [2] Unfortunately, traditional inorganic lithium salts such as Li 2 SO 4 , LiCl, and LiNO 3 are stable enough to be reduced Aqueous lithium/sodium-ion batteries (AIBs) have received increasing attention because of their intrinsic safety. However, the narrow electrochemical stability window (1.23 V) of the aqueous electrolyte significantly hinders the development of AIBs, especially the choice of electrode materials. Here, an aqueous electrolyte composed of LiClO 4 , urea, and H 2 O, which allows the electrochemical stability windo...

Non-concentrated aqueous electrolytes with organic solvent additives for stable zinc batteries

“…The cation-solvent or cation-anion complexes with a lower LUMO level indicates their easier electron reception from anode than free solvent/anion. 43,47,48 The predicted reduction potentials of free DMC solvent (-2.39 V) and free OTfanion (-2.46 V) are much lower than that of the Zn 2+ /Zn redox couple (Fig. 5b).…”

“…It is well-known that the electron could preferably transfer from cation to solvent (or anion) in the cation-solvent (or cation-anion) unit and induce the solvent/anion decomposition, where the SEI could be formed at the electrode interface. 37,39,48 Herein, the introduction of DMC solvent into the aqueous Zn(OTf) 2 electrolyte leads to a significant change To support the above mechanism, LSV measurements were conducted at 5 mV s -1 using a three-electrode configuration employing Ti foil as working electrode, Zn foils as reference and counter electrodes ( Fig. 5c and Fig.…”

Perspective on High‐Energy Carbon‐Based Supercapacitors

“…[161] Besides, the microstructural variations also affect the solvent properties, such as reaction activity, volatility, and diffusion. [162][163][164][165][166][167] Features of electrolytes such as melting and boiling points, toxicity, flammability, and price should be also considered. [28,149] Low melting and high boiling points allow the electrolyte to operate in a large applicable temperature range.…”

“…[ 161 ] Besides, the microstructural variations also affect the solvent properties, such as reaction activity, volatility, and diffusion. [ 162–167 ]…”

Solid electrolyte interphase in water-in-salt electrolytes

“…After all, the SEI is not a simple combination of each ingredient; the morphology and distribution of diverse chemical components might be as important as the components themselves. So far, the main methods used to enhance the SEI include increasing the salt concentration and introducing additional components in the SEI via adding organic co-solvents, for example, DMC, AN, and urea [33,35,46,47]. In consideration of the electrolyte additives, which have been extensively investigated in the organic system, additives for WiSEs can also be developed to manipulate the SEI in the aqueous system.…”