Donor-Acceptor Index as a Descriptor for Electrolyte Design in High-Voltage Aqueous Batteries

Introducing organic molecules to anchor water can reduce water activity and expand the electrochemical stability window of aqueous electrolytes. However, there is a lack of reliable guidelines for screening suitable organic modulators. Through examining the effects of various organic molecules on the hydrogen bonding network of water, we found that the water-anchoring capabilities of organic modulators are positively correlated with their acceptor-donor index (IA-D)—the sum of the products of donor and acceptor sites and their respective intensities. Furthermore, lithium metal titration-assisted differential electrochemical mass spectrometry (Li-DEMS) technique was developed to systematically investigate the charge-discharge behavior of three electrodes with different redox potentials (Li4Ti5O12, V2O5, and Zn) in nonaqueous/aqueous hybrid electrolytes containing different organic modulators and salts. The contributions of the hydrogen evolution reaction (HER) and solid-electrolyte interphase (SEI) formation to irreversible capacity were decoupled. For all the electrodes studied, a higher IA-D value correlates with a smaller contribution of HER to irreversible capacities and higher initial and average Coulombic efficiencies. Notably, HER becomes the dominant contributor to irreversible capacity after the initial cycles. For the high-potential electrode (V2O5), electrochemical performance is primarily influenced by IA-D, rather than SEI formation, while for the low-potential electrode (Li4Ti5O12), both IA-D and SEI formation play significant roles in determining performance. These findings not only validate IA-D as a reliable descriptor for assessing the water-anchoring capabilities of organic modulators, but also establish a link between organic modulators, electrolyte solution structure, interphase reactions, and battery performances, providing valuable insights for the development of high-performance aqueous batteries.