A Nonionic Bola-Form Surfactant Additive with Inverted U-Shaped Adsorption for Highly Reversible Zn Metal Batteries

The development of aqueous Zn batteries is severely impeded by side reaction and dendrite issues. Herein, stevioside, a bio-derived bola-amphiphilic surfactant, is employed as an additive to improve the above issues. The stevioside features a central hydrophobic steviol backbone linked to hydrophilic sugar groups at both ends by β-1,2-glycosidic and ether bonds. The polar sugar groups facilitate preferential adsorption on the Zn surface, orienting the hydrophobic steviol aglycone outward in an inverted U-shape to create a hydrophobic layer, thereby decreasing the desolvation energy and preventing H₂O-related side reactions. Furthermore, the stevioside can reduce the surface energy of Zn, which promotes smaller Zn nucleation and preferential (002) crystal facets. Consequently, the stevioside renders the symmetrical Zn battery with a stable lifespan at 10 mA h cm^-2 with a depth of discharge (DOD) for 78%, and the Zn/I₂ battery with a capacity retention of 80% after 9,000 cycles at a high-loading of 15 mg_iodine cm^-2.