Donnan-Engineered Inner Helmholtz Plane Enabling Ultra-Stable Aqueous Bismuth Electrode

The electrochemical instability of the solid-liquid interface remains a critical bottleneck in rechargeable aqueous metal batteries (RAMBs), where traditional strategies fail to resolve the inherent conflict between electrochemical and parasitic reactions at the inner Helmholtz plane (IHP). Herein, inspiring from the ion-sieving principles of the Donnan effect, classical electrostatics is integrated with interfacial engineering by creating a phosphate anion (PO₄ ^3-)-adsorbed IHP on a bismuth (Bi) electrode. The immobilized PO₄ ^3- establishes a sustained Donnan potential, driving three key functions: i) ion and reaction sieving through charge-selective ion partitioning, enriching Na⁺ while excluding OH⁻ to enable selective (de)alloying over corrosion reaction; ii) electron confinement through Donnan potential to suppress parasitic electron leakage; and iii) dynamic stabilization of the IHP through strong anion chemisorption, bridging the classical Donnan model with electrochemistry. As a result, the Bi electrode demonstrates a superior cycling stability (200 mAh g^-1 retention after 3000 cycles at 2 A g^-1) and ultrahigh-rate performance (134 mAh g^-1 at 16 A g^-1). By extending the Donnan effect in electrochemistry, the research creates a universal interfacial paradigm based on charge-selective ion partitioning and electron confinement. This breakthrough provides a transformative strategy to decouple desired electrochemical reactions from parasitic side reactions, paving the way for advanced RAMBs.