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

As the need for sustainable battery chemistry grows, non-metallic ammonium ion (NH₄ ⁺ ) 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 NH₄ ⁺ and solvents are elucidated and design principles for NH₄ ⁺ weakly solvated electrolytes are proposed. Given that hydrogen bond interactions dominate the solvation of NH₄ ⁺ 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 NH₄ -ion batteries exhibit superior reversible rate capability (energy density of 65 Wh kg^-1 _{total active mass} 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 NH₄ ⁺ -based energy storage systems.