<p>Although lignin is the second most abundant forest biomass polymer, it has been largely neglected in hydrogel electrolytes due to its insolubility and inflexibility. In this study, a double-crosslinked hydrogel was prepared using aspartic acid-modified lignin and sodium alginate, significantly improving the mechanical properties. The hydrogel exhibited an exceptional strain of 3008% and a tensile strength of 0.03 MPa, demonstrating its remarkable mechanical properties. In addition, high ionic conductivity (11.7 mS·cm⁻¹) was obtained due to the abundant presence of hydrophilic groups in the hydrogel. The hydrogel-assembled supercapacitor manifested an impressive specific capacitance of 39.46 F·g⁻¹. Notably, the supercapacitor showed a wide potential window of 0–1.5 V and achieved a maximum energy density of 5.48 Wh·kg⁻¹ at the power density of 499.9 W·kg⁻¹. The capacitance retention remained at 115% after 10000 charge-discharge cycles. Finally, the coulombic efficiency was almost 100% during the cycles. Upon reaching a bending angle of 90°, the specific capacitance retention remained impressively high at 94%. These results suggest that the supercapacitor cans maintain normal electrochemical performance under extremely harsh conditions.</p>