Enhancing Ionic Transport in Green Gel Polymer Electrolytes Based on Alginate/Lignin/Na₂SO₄ Systems

Green gel polymer electrolytes (GPEs) based on renewable polymers are promising candidates for sustainable energy storage; however, achieving high ionic conductivity remains a major challenge due to limited salt dissociation and restricted ion mobility. In this work, a biobased GPE system combining sodium alginate, lignin, and sodium sulfate (Na₂SO₄) is proposed to enhance ionic transport through the interplay between salt concentration, polymer-ion interactions, and matrix organization. A systematic compositional study was performed to evaluate the effects of Na₂SO₄ and lignin incorporation on the electrochemical properties of the system. The ionic conductivity increased from 2.09 × 10^-5 to 2.91 × 10^-4 S cm^-1, with the optimized composition (Alg-Lig-Na20.6) achieving among the highest values reported for alginate-based electrolytes. This enhancement is attributed to an optimal balance between charge carrier density and mobility, while excessive salt content led to ion association and reduced transport efficiency. Dielectric and electric modulus analyses revealed reduced relaxation times and enhanced charge carrier dynamics, while transference number measurements confirmed predominantly ionic conduction (t _ion up to 0.98). The electrolytes exhibited good electrochemical stability, with a stability window up to 2.85 V and stable cyclic voltammetry response over multiple cycles. These findings demonstrate that lignin incorporation, combined with controlled salt content, is an effective strategy to tailor ionic transport and electrochemical stability in biobased GPEs.