Hierarchical design of cellulose-based sandwich separators for advanced lithium-ion batteries

The development of sustainable and high-performance separators is crucial for advancing next-generation lithium-ion batteries (LIBs). To address the growing demand for eco-friendly alternatives to conventional polyolefin-based separators, a fully cellulose-based LIB separator with a well-defined sandwich-like architecture was fabricated via layer-by-layer vacuum filtration in this study. Carboxyl groups (-COOH) were grafted onto oxidized cellulose (OC), while regenerated cellulose microspheres (RCM) were lithiated to form RCM-Li bearing lithium-functional groups. The synergistic interaction between -COOH and Li resulted in the improvement of electrolyte compatibility, interfacial stability, and supplying additional mobile Li⁺. The optimized separator with 10 wt% RCM-Li exhibited remarkable electrochemical performance, which showed a high electrolyte uptake of 496.32%, an ionic conductivity of 1.95 mS·cm^-1, and a lithium-ion transference number of 0.65. Additionally, the separator demonstrated excellent cycling stability and effectively suppressed lithium dendrite formation as confirmed by SEM and molecular dynamics simulations. This work provides a green and scalable strategy for designing functionalized cellulose-based separators and offers mechanistic insights into the role of bio-derived functional groups in enhancing ion transport and interfacial stability in LIBs.