Green initiator-free hydrogel electrolytes with lignin-assisted ionic networks for high-performance flexible supercapacitors at subzero temperatures

Abstract

Supercapacitors are widely used in flexible and wearable electronic devices due to their high safety and excellent cycling stability. However, conventional hydrogel electrolytes suffer from a severe decline in flexibility, adhesion, and ion-transport capability under extreme environments, drastically compromising the electrochemical and mechanical performance of supercapacitors. Herein, we develop an anti-freezing hydrogel electrolyte without an initiator, which integrates excellent stretchability (∼1280%), strong adhesion (∼2.4 MPa on wood), and broad environmental adaptability. Addition of sodium lignosulfonate resulted in a high ionic conductivity (36.45 mS cm⁻¹). These merits originate from not only abundant dynamic interactions (hydrogen bonding and electrostatic coupling) but also efficient ion pathways enabled by Zn²⁺ coordination with charged functional groups. Meanwhile, strong O–H⋯F coordination between Zn(BF₄)₂ and H₂O effectively depresses ice crystallization, enabling the electrolyte to remain flexible and maintain 0.86 mS cm⁻¹ conductivity at −40 °C. Additionally, a supercapacitor assembled with BLAD has an operating voltage window of 0–2 V and exhibits high energy-storage efficiency across −30 to 50 °C. At 25 °C and 0.7 A g⁻¹ (0–2 V), the supercapacitor delivers a high specific capacitance of 161.8 F g⁻¹ and an energy density of 89.9 Wh kg⁻¹. It maintains excellent electrochemical performance, achieving a specific capacitance of 87.3 F g⁻¹ and an energy density of 48.5 Wh kg⁻¹ at −20 °C and 0.7 A g⁻¹ (0–2 V). The supercapacitor demonstrates excellent rate capability and cycling stability at low temperatures, with a capacitance retention of over 70% after 2500 cycles (0–1 V) and a coulombic efficiency of 99.7%. Notably, a single device can reliably power a digital timer under both ambient and sub-zero conditions, highlighting its robust practicality for all-weather wearable applications.