Hierarchical solvation engineering of water/organic hybrid electrolytes for wide-temperature potassium-ion asymmetric supercapacitors

Electrolytes containing water are generally limited by narrow electrochemical stability windows and poor low-temperature performance due to uncontrolled water activity. Herein, a hierarchical solvation engineering strategy is proposed to regulate the K⁺ solvation environment through the synergistic effects of acetonitrile (AN) and propylene glycol methyl ether (PM). Specifically, AN preferentially participates in the inner-shell coordination of K⁺, while PM mainly redistributes H₂O molecules within the surrounding solvent regions, collectively optimizing the local solvation structure and reducing interfacial water activity. The regulated solvation structure suppresses parasitic interfacial reactions and contributes to improved Mn-based electrode stability during cycling, as reflected by the relatively stabilized interfacial Mn chemical-state evolution from XPS analysis, reduced Mn dissolution from ICP-OES measurements, and enhanced long-term cycling durability. As a result, the optimized HPA-0.2 electrolyte exhibits an outstanding ionic conductivity of 21.2 mS·cm⁻¹ at room temperature and retains 3.4 mS·cm⁻¹ at -40 °C. Consequently, the assembled asymmetric supercapacitor achieves a stable operating voltage of 2.6 V, an energy density of 69.5 Wh·kg⁻¹ at 650 W·kg⁻¹, and 90.6% capacitance retention after 10,000 cycles. Even at -40 °C, the device maintains an energy density of 17.9 Wh·kg⁻¹ at 568 W·kg⁻¹. This work highlights the importance of hierarchical solvation engineering in regulating water/organic hybrid electrolytes and provides new insights into the design of high-voltage and temperature-resilient energy-storage systems.