Understanding Thermodynamic and Kinetic Contributions in Expanding the Stability Window of Aqueous Electrolytes

Abstract

Aqueous electrolytes come with an intrinsic narrow electrochemical stability window (1.23 V). Expanding this window represents significant benefits in both fundamental science and practical battery applications. Recent breakthroughs made via super-concentration have resulted in >3.0 V windows, but fundamental understanding of the related mechanism is still absent. In the present work, we examined the widened window (2.55 V) of a super-concentrated (unsaturated) aqueous solution of LiNO 3 through both theoretical and spectral analyses and discovered that a local structure of intimate Li + -water interaction arises at super-concentration, generating (Li + (H 2 O) 2 ) n polymer-like chains to replace the ubiquitous hydrogen bonding between water molecules. Such structure is mainly responsible for the expanded electrochemical stability window. Further theoretical and experimental analyses quantitatively differentiate the contributions to this window, identifying the kinetic factor (desolvation) as the main contributor. Such molecular-level and quantitative understanding will further assist in tailor designing more effective approaches to stabilizing water electrochemically.

Water in Rechargeable Multivalent‐Ion Batteries: An Electrochemical Pandora's Box

“…However, Zheng and co‐workers later showed that a mild temperature elevation from 25 to 35 °C immensely raises the solubility of LiNO 3 in water from 12 (as per Yamada et al.) to 22 m ,. They further showed that cheap LiNO 3 salts could be used to cycle aqueous batteries on a par with, or even better than, expensive LiTFSI‐based ionic liquids, due to a polymer‐like aggregation of Li−O−Li−… atoms, which begins at a salt‐to‐water molar ratio of 1:2.5, affording an electrolyte stability window of 2.55 V (Figure ).…”

High‐Performance and Ultra‐Stable Aqueous Supercapacitors Based on a Green and Low‐Cost Water‐In‐Salt Electrolyte

“…What's more, the toxicity and flammability of organic electrolytes have given rise to environment‐related concerns . Based on the above, aqueous electrolytes receive intensive attention due to a combination their being intrinsically safer, highly conductive, low cost and eco‐friendly . Nevertheless, the water decomposition voltage renders their operation only up to 1.23 V in theory, which is the consequence of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), near the negative and positive electrode respectively .…”

“…Moreover, the salt, main component in the highly concentrated electrolyte, inevitably increases the cost of the electrolyte, which represents a significant challenge in accomplishing the commercialization of concentrated electrolytes . Taking into account that, it is significance to explore a novel low cost water‐based electrolyte matching above prerequisites, such as some low concentrations of perchlorate, sulfate, chloride and nitrate, have been widely used for aqueous metal‐ion batteries and supercapacitors . The concentration of sodium nitrate solution with the lowest cost can reach 12 m (mol kg −1 ) among these salts possessing high solubility, which is accordance with the definition of WIS (the weight of the salt outnumbers that of the solvent) .…”

“…[10,28] Based on the above, aqueous electrolytes receive intensive attention due to a combination their being intrinsically safer, highly conductive, low cost and eco-friendly. [23,[29][30] Nevertheless, the water decomposition voltage renders their operation only up to 1.23 V in theory, which is the consequence of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), near the negative and positive electrode respectively. [15,31] Therefore, how to eliminate this ultimate limitation by stabilizing water thermodynamically has been a scientific challenge in both basic research and practical application.…”

Electrolytes for Dual‐Carbon Batteries

“…The high oxidative stability of solvents in HCE is more due to kinetic factors. Beside the passivating SEI film originating from anion oxidation, the polymer‐like chains of −solvent−cation−solvent−, induced by the electrostatic interaction between the lone pairs of solvents and the cations, are mainly attributed to the expanded oxidation stability ,. Overall, the HCEs normally exhibit higher redox stability than the conventional electrolytes, indicating that they are more suitable for DCBs.…”