Trade-off between reversibility and fast Zn2+ kinetics: Toward ultra-stable low-temperature aqueous zinc-ion batteries

Abstract

Despite their environmental friendliness, security and high volumetric energy density of zinc anodes, aqueous Zinc-ion batteries (AZIBs) still face poor reversibility of Zn anodes, especially under high current density, originating from various parasitic reactions induced by high activity of water. The hydrated deep eutectic electrolyte (HDEE) effectively suppresses parasitic reactions, but the electrochemical performance still needs to be optimized. Here, our research emphasized the importance of balancing enhanced reversibility and fast Zn²⁺ transfer kinetics. A new green and low-cost HDEE (Zn(ClO₄)₂·6H₂O/Glycerol) is developed, and then an optimized solvation structure [Zn(H₂O)_2.0(Gl)_1.3(ClO₄)_2.7]²⁺ can be formed by adding glycerol (Gl), which not only maintains a high Zn²⁺ diffusion coefficient (1.2 × 10⁻⁷ cm² s⁻¹), but also disrupts the bulk water network via strong H-bonding with ClO₄⁻ and water, significantly lowering the freezing point (−65 °C) and inhibiting the parasitic reactions/cathode dissolution. Furthermore, the evolution of the HDEEs solvation chemistry and its impact on the electrode/electrolyte interfacial stabilities can be understood through precise adjustments of the molar ratios of Zn(ClO₄)₂·6H₂O and Gl, molecular dynamics and COMSOL simulation. The Zn//Zn with the HDEE (Zn|HDEE|Zn cells) can cycle for ∼5000 h without short-circuiting at 1 mA cm⁻², which is roughly 12.5 times more stable than ordinary aqueous electrolyte, indicating effective suppression of parasitic reactions. The Zn//NH₄V₄O₁₀ with HDEE (Zn|HDEE|NH₄V₄O₁₀ cells) can stably cycle 3500 cycles with 120 mAh g⁻¹ at 10 A g⁻¹ at room temperature and 1000 cycles with 95 mAh g⁻¹ at 5 A g⁻¹ at a low temperature of -20 °C. This study provides a path toward the development of HDEE electrolyte and a thorough comprehension of the influence of Zn²⁺ solvation structure on reversibility.