Adsorption-Driven Interfacial Engineering of Aluminum Anodes via Saponin for Enhanced Performance in Aluminum-Air Batteries

Abstract Alkaline aluminum-air batteries (AABs) are promising high-energy-density power sources, but their practical application is hindered by severe anodic self-corrosion and unstable Al/electrolyte interfaces in concentrated alkaline media. Herein, saponin, a low-cost and environmentally benign amphiphilic biosurfactant, is introduced into 4 M NaOH to regulate the Al anode interface through adsorption-driven interfacial engineering. Experimental characterization and molecular simulations indicate that oxygen-containing groups in saponin interact with the Al surface through Al–O interactions, forming a saponin-derived adsorption layer. This interfacial layer limits the access of H2O and OH– to active Al sites, suppresses parasitic hydrogen evolution, and promotes more uniform anodic dissolution. At the optimal concentration of 15 mM, saponin achieves a corrosion inhibition efficiency of 58.9% and increases Al utilization from 47.8% to 79.4%. In full-cell tests using identical air cathodes, the specific capacity increases from 1424.9 to 2364.7 mAh g–1, while the energy density increases from 1652.9 to 2812.0 Wh kg–1. These results demonstrate that adsorption-driven interfacial regulation using amphiphilic molecules is an effective strategy for mitigating Al corrosion and enhancing the discharge performance of alkaline AABs.