Low-temperature transition glycerol-based electrolyte toward sustainable zinc-ion batteries

Aqueous zinc-ion batteries (ZIBs) are promising candidates for low-cost stationary energy storage because they rely on safer and more sustainable components than conventional lithium-ion batteries (LIBs). The use of aqueous electrolytes mitigates the flammability and toxicity issues associated with the organic electrolytes of LIBs, while zinc is more abundant and geopolitically accessible than lithium, making ZIBs attractive for large-scale applications. However, their practical deployment is still hindered by electrolyte-related challenges, particularly corrosion at the Zn anode due to hydrogen evolution reactions (HERs) that interfere with Zn plating/stripping (-0.76 V vs SHE), and proton co-intercalation into oxide cathodes, which causes structural degradation and accelerated capacity fading. Low-temperature transition mixtures, including deep eutectic solvents or merely eutectic mixtures, have recently been proposed as alternative electrolytes for ZIBs. These systems offer enhanced electrochemical and thermal stability, low flammability and volatility, ease of preparation, and often good environmental compatibility. In hydrated LTTMs, water actively participates in the hydrogen-bond network, improving transport properties without significantly compromising electrochemical stability. Building on previous work on supercapacitors, we developed sustainable and cost-effective LTTMs based on ZnSO4·7H2O and glycerol, both inexpensive and widely available (glycerol being a biodiesel by-product). Through a comprehensive investigation, we correlated thermal behavior (i.e., glass transition), transport properties, and electrochemical stability window. The 1:10 molar ratio showed efficient and uniform Zn plating/stripping on carbon cloth, as demonstrated through SEM/EDX analysis. As proof of concept, full-cell tests using a Zn anode and zinc hexacyanoferrate cathode demonstrated good power capability and long-term cycling stability, supporting the feasibility of these electrolytes for affordable stationary energy storage.