The research results of Professor Song's team are published in eScience and Angewandte Chemie International Edition.

Aqueous organic flow batteries (AOFBs) have emerged as a frontier in novel energy storage technologies due to their intrinsic safety, decoupling of power and capacity, and ease of modular scalability.

Among various materials, cyclic nitroxyl radicals stand out as highly promising catholyte candidates owing to their high redox potential, excellent reversibility, and green synthetic routes.

However, during charge-discharge cycling, these molecules are prone to side reactions, such as autocatalytic oxidation, disproportionation, molecular aggregation, and ring-opening, leading to irreversible structural degradation and rapid deterioration of electrochemical performance. This constitutes a critical bottleneck that hinders their commercial application.

To address these challenges, Professor Song Jiangxuan's team at Xi'an Jiaotong University's (XJTU) School of Materials Science and Engineering has conducted innovative research on stabilizing core electrolyte molecules, achieving significant breakthroughs.

To solve the issue of denitrogenation ring-opening and active-site deactivation in high-potential five-membered pyrroline nitroxyl radicals during cycling, the team pioneered a host-guest chemistry approach.

They constructed a stabilized molecular armor structure by encapsulating the radicals within the hydrophobic cavity of water-soluble cyclodextrin, orienting the N–O functional groups toward the cavity base.

This spatial configuration significantly inhibits nucleophilic attacks on pyrroline ring hydrogen sites, effectively blocking ring-opening side reactions. Electrochemical tests confirmed the strategy's efficacy: the encapsulated complex exhibited a capacity fade rate of only 0.002 percent per cycle (daily fade rate below 0.233 percent) over 500 cycles at concentrations of 0.05–0.5 mol·L⁻¹, vastly outperforming the unencapsulated state (0.039 percent per cycle, 5.23 percent daily).

Moving beyond traditional linear substituent designs for piperidine nitroxyl radicals, the team developed molecules featuring branched bis-quaternary ammonium substituents. The branched chains induce steric repulsion effects, enhancing electrolyte stability and redox performance.

Dual cationic centers promote intermolecular electrostatic repulsion and steric hindrance, effectively suppressing nucleophilic attacks and adverse reaction pathways. These molecules demonstrated exceptional electrochemical performance at high concentrations.

Paired with bipyridinium salt anolytes, the resulting flow battery achieved 99.992 percent capacity retention per cycle (daily retention >99.85 percent) and a peak power density of 140.3 mW·cm⁻². In situ UV-Vis spectroscopy and theoretical simulations revealed that the branched structure increases the energy barrier between charged states, strengthens charge repulsion, stabilizes molecular conformation, and inhibits side reactions.

Collaborating with Professor Kong Duanyang from Beijing University of Chemical Technology, the team extended the research to a zinc/nitroxyl radical hybrid system, constructing a composite aqueous flow battery with high areal capacity and long cycle life.

These groundbreaking results were published in eScience ("Spatial structure regulation towards armor-clad five-membered pyrroline nitroxides catholyte for long-life aqueous organic redox flow batteries") and Angewandte Chemie International Edition ("Branching-Induced Intermolecular Repulsion Effects Drive Stable and Sustainable Flow Batteries on Condensed Nitroxyl Radicals").