Electrolyte design for aqueous Zn batteries

Context & scaleZn-based batteries with aqueous electrolytes are garnering great interest as the most promising next-generation batteries due to their intrinsic high safety, low cost, and environmental friendliness. However, the short cycle life of the Zn battery, originating from the low reversibility of the Zn metal electrode, is far from satisfactory. To achieve high reversibility of Zn metal electrode, tackling the water decomposition reaction and the inhomogeneous deposition/dissolution reaction is crucial. We present design principles for aqueous co-solvent electrolytes with high Zn reversibility and suggest a new parameter for accurately selecting beneficial organic molecules for Zn-based batteries. Electrolytes prepared using this principle effectively overcome the challenges in aqueous systems and demonstrate record-high cycling stability. This work provides new insight into aqueous electrolyte systems.Highlights•Thorough electrolyte design principles are suggested for aqueous batteries•An effective descriptor for selecting organic co-solvent is discovered•Designed electrolyte provides high reversibility and cycling stability of Zn batteriesSummaryPolarity scales are often used as descriptors for selecting organic molecules for aqueous Zn battery (AZB) electrolytes. However, failure to accurately predict the solvation of Zn2+ raises questions about their applicability for designing high-performance AZB electrolytes. Here, Dimroth and Richardt's Et(30) polarity scale is introduced as an effective guideline for screening organic molecules. A clear volcanic correlation is demonstrated between Et(30) and Zn Coulombic efficiency (CE). This challenges the common consensus in the aqueous electrolyte design formula, which typically uses highly polar organic molecules to improve Zn CE, and indicates that the roles of organic molecules beyond altering the Zn2+ solvation structure are critical for obtaining high AZB performances. Based on the Et(30) scale, the designed electrolyte achieves a high average Zn CE (99.8%), an exceptionally long cycle life (5,500 h), and a high specific energy (110 Wh kg−1). Et(30) polarity scale offers general frameworks for selecting organic molecules in aqueous electrolytes.Graphical abstract