Guanidinium chloride as multifunctional electrolyte additives for highly reversible aqueous zinc batteries

Aqueous zinc-ion battery (ZIB) show promise for energy storage but face challenges in reversibility due to side reactions and dendrite growth, limiting practical application. Herein, we present a highly reversible aqueous ZIB where guanidinium chloride (GdmCl) assists the construction of protective solid-electrolyte interface (SEI), modifying the solvation structure of Zn²⁺, manipulate crystallographic orientation of Zn deposition. The Gdm⁺-based interface not only physically insulate free water in the electrolyte but also remove the reactive coordinated water molecules when hydrated Zn²⁺ pass through the interface. In addition, the Cl^- from additive repel coordinated water molecules which further reduce the number active coordinated water molecules. Therefore, the side reactions were dramatically inhibited. Furthermore, Gdm⁺ facilitate the preferential growth of the Zn (002) crystal planes of Zn by blocking the (101) and (100) planes, inducing planar and dendrite-free deposition. As a result, dendrite-free Zn plating/stripping is achieved with a coulombic efficiency of ∼99.7 % over 300 cycles in Zn//Cu cells, stable charge/discharge performance for 1500 h in Zn//Zn cells, and an impressive cycle retention of 89 % over 1200 cycles in Zn//MnO₂ cells. The additive also endows the electrolyte with antifreeze properties which enabling the cell to be operated at −20 °C, showing vast practical application potential