Low-cost and Non-flammable Eutectic Electrolytes for Advanced Zn-I2 Batteries.

As a burgeoning electrolyte system, eutectic electrolytes based on ZnCl2/Zn(CF3SO3)2/Zn(TFSI)2 have been widely proposed in advanced Zn-I2 batteries; however, safety and cost concerns significantly limit their applications. Here, we report new-type ZnSO4-based eutectic electrolytes that are both safe and cost-effective. Their universality is evident in various solvents of polyhydric alcohols, in which multiple -OH groups not only involve in Zn2+ solvation but also interact with water, resulting in the high stability of electrolytes. Taking propylene glycol-based hydrate eutectic electrolyte (HEE) as an example, it features significant advantages in non-flammability and low price that is < 1/200 cost of Zn(CF3SO3)2/Zn(TFSI)2-based eutectic electrolytes. Moreover, its effectiveness in confining the shuttle effects of I2 cathode and side reactions of Zn anodes is evidenced, resulting in Zn-I2 coin cells with high reversibility at 1 C and 91.4% capacity remaining under 20 C. After scaling up to the pouch cell with a record mass loading of 33.3 mg cm-2, super-high-capacity retention of 96.7% is achieved after 500 cycles, which exceeds other aqueous counterparts. This work significantly broadens the eutectic electrolyte family for advanced Zn battery design.