Abstract: To address the bottleneck that traditional lithium-ion battery separators struggle to balance high safety with environmental friendliness, a gel polymer separator was prepared using lignocellulose (LC₁) extracted from biomass Arundo donax via an alkali treatment, and a modified composite separator (LC₁@PVA) was further constructed by compounding with poly(vinyl alcohol) (PVA) for comparison. The two separators were characterized by SEM, XRD, BET, FT-IR, TG-DTG, and electrochemical tests. The results showed that the modified LC₁@PVA composite system exhibited a high electrolyte uptake of 529%, and the average pore size increased to 3.173 nm. The ionic conductivity at room temperature was elevated to 0.069 mS·cm⁻¹, and the electrochemical stability window was widened to 5 V. Furthermore, Li/LiFePO₄ half-cells were assembled for performance evaluation. The LC₁@PVA system delivered a discharge specific capacity of 174 mAh·g⁻¹ at 0.5 C, which was significantly higher than that of the unmodified LC₁ system (133 mAh·g⁻¹). It also demonstrated excellent rate capability and cycling stability at 0.1 C, 0.2 C, and 0.5 C rates. This study provides a new strategy for fabricating high-performance gel polymer separators from low-cost biomass, which is of great significance for promoting the development of green and sustainable energy storage technologies.