Decoding the Entropy-Performance Relationship in Aqueous Electrolytes for Lithium-Ion Batteries

Developing aqueous low-temperature electrolytes aligns with the societal demand for lithium batteries in extreme climates and environments. However, the main challenges include high thermodynamic freezing points, slow ion diffusion, and instability at the interface under low temperatures, resulting in low energy density and poor cycle performance. Here, the role of mixing entropy ΔS_mix, hydrogen bonding, and electrostatic interactions in achieving an optimal electrolyte composition is explored. By systematically varying the ethyl acetate (EA)/H₂O ratio, a critical “mixing entropy optimal point” at a molar ratio of 3.91, where the electrolyte exhibits the best balance between molecular disorder and interfacial stability is identified. At this optimal point, EA molecules with polar ester group (-COO-) effectively break the hydrogen-bond network of water, enhancing the ΔS_mix and lowering the freezing point to −106.95 °C. Furthermore, the stable interfacial chemistry derived from entropy-driven solvation structure effectively suppress hydrogen evolution and expand the electrochemical window to 6.2 V. Full aqueous Li-ion batteries with LiMn₂O₄-Li₄Ti₅O₁₂ full cell delivered an initial discharge specific capacity of 135.1 mAh g⁻¹ for 1000 cycles under rapid 10 C rate. The results provide a promising thermodynamic foundation for designing high-performance aqueous electrolytes, with implications for next-generation low-temperature aqueous lithium-ion batteries.