Water-in-Salt Electrolyte Stabilizes Pyrazine Radical: Suppression of Its Aggregation by Interaction between Pyrazine and Li(H2O) n +

Abstract

Stabilizing radical intermediates of redox-active organic molecules in aqueous media is crucial for advancing applications in energy storage, catalysis, and electrosynthesis. This study investigates the stabilization of protonated radical intermediates of pyrazine derivatives in water-in-salt electrolytes (WISEs) with 7-8 m LiTFSI. Strong interactions between pyrazine derivatives and Li+-coordinated water (Li(H2O) n +) in WISEs prevent molecular aggregation and protect radical intermediates from disproportionation and oxygen-induced degradation. Voltammetric results show that higher concentrations of LiTFSI enhance both the stability and redox reversibility of dimethylpyrazine (DMP) radical intermediates, with protonation identified as a key stabilizing factor. Notably, these stabilizing effects were absent in solutions containing concentrated LiCl or LiNO3. Fourier-transform infrared (FTIR) spectroscopy and molecular dynamics (MD) simulations confirmed reduced DMP aggregation in LiTFSI-based electrolytes, driven by interactions with Li(H2O) n +, while no similar solvation structure modification occurred with LiNO3. The protonated radical intermediates in LiTFSI-based WISEs exhibited greater resistance to oxygen-induced degradation compared to conventional acidic solutions. Additionally, substitution of methyl or ethyl groups on the pyrazine ring destabilized the corresponding radical intermediates in LiTFSI-based WISEs, primarily due to the alkyl inductive effect, as evidenced by electrochemical and UV-visible absorption spectroscopy. Charge-discharge tests in an H-cell further demonstrated significantly improved Coulombic efficiency of pyrazine redox reactions in LiTFSI-based WISEs compared to acidic Salt-in-Water electrolytes, underscoring the importance of radical intermediate stabilization.