Metal-Free Eutectic Electrolyte with Weak Hydrogen Bonds for High-Rate and Ultra-Stable Ammonium-Ion Batteries

Abstract

As the need for sustainable battery chemistry grows, non-metallic ammonium ion (NH<inf>4</inf>⁺) batteries are receiving considerable attention because of their unique properties, such as low cost, nontoxicity, and environmental sustainability. In this study, the solvation interactions between NH<inf>4</inf>⁺ and solvents are elucidated and design principles for NH<inf>4</inf>⁺ weakly solvated electrolytes are proposed. Given that hydrogen bond interactions dominate the solvation of NH<inf>4</inf>⁺ and solvents, the strength of the solvent's electrostatic potential directly determines the strength of its solvating power. As a proof of concept, succinonitrile with relatively weak electronegativity is selected to construct a metal-free eutectic electrolyte (MEE). As expected, this MEE is able to significantly broaden the electrochemical stability window and reduce the solvent binding energy in the solvation shell, which leads to a lower desolvation energy barrier and a fast charge transfer process. As a result, the as-constructed NH<inf>4</inf>-ion batteries exhibit superior reversible rate capability (energy density of 65 Wh kg^–1<inf>total active mass</inf> at 600 W kg^–1) and unprecedent long-term cycling performance (retention of 90.2% after 1000 cycles at 1.0 A g^–1). The proposed methodology for constructing weakly hydrogen bonded electrolytes will provide guidelines for implementing high-rate and ultra-stable NH<inf>4</inf>⁺-based energy storage systems.