Zinc-based metal halide electrolytes for all-solid-state zinc-metal batteries

The advancement of electrolyte materials is fundamental to the advancement of batteries. While zinc-metal batteries offer compelling advantages in safety, cost, and environmental sustainability, their performance is constrained by conventional aqueous electrolytes. All-solid-state zinc-ion electrolytes, which can inherently address the issues associated with aqueous electrolytes, are promising but yet currently in their nascent stages. Here, we show a composition engineering strategy to design metal halide-based zinc-ion electrolytes, and two types of zinc-based hybrid metal halides, C₄N₂H₁₂ZnBr₄ and C₄N₂H₁₂ZnCl₄, are obtained. Both solid-state electrolytes exhibit good electronically insulating properties and low zinc-migration energy barriers. C₄N₂H₁₂ZnBr₄ demonstrates a high ionic conductivity of 2.9×10^-4 S cm^-1 at 25 °C and a wide electrochemical window of ~3.74 V, making it suitable for all-solid-state zinc-metal batteries. The C₄N₂H₁₂ZnBr₄ electrolyte also establishes a tight contact with the zinc electrode, thereby minimizing interface resistance for enhanced electrochemical performance. As a result, an all-solid-state Zn | |I₂ cell using C₄N₂H₁₂ZnBr₄ electrolyte achieves a high capacity retention of 234.5 mAh g^-1 after 200 cycles at 0.2 mA cm^-2, with a low capacity degradation rate of 0.056% per cycle. This work provides a design approach for metal halide-based solid electrolytes in zinc-metal batteries.