A Robust All-Cellulose Composite Separator with Regulated Ion Transport for High-Performance Lithium Batteries

The development of high-performance battery separators is critical for advancing lithium-ion technology. Conventional polyolefin separators are limited by poor electrolyte wettability, restricted thermal stability, and environmental concerns associated with their petroleum-based origin. In this study, an eco-friendly and robust all-cellulose composite (ACC) separator is designed to overcome these limitations. The separator is fabricated via a straightforward vacuum filtration of cellulose nanofibers (CNF), followed by ultraviolet-induced cross-linking of methacrylated carboxymethyl cellulose (mCMC). In this approach, mCMC serves as a cross-linking monomer integrated within the CNF matrix, substantially enhancing the mechanical integrity. The resulting separator with 5 wt % mCMC (ACC-5) exhibits an impressive tensile strength of 10.5 ± 1.0 MPa even after electrolyte absorption. Moreover, the carboxylate groups of mCMC facilitate lithium-ion transport, endowing the swollen ACC-5 with a high ionic conductivity of (3.22 ± 0.41) × 10–4 S/cm and an enhanced Li+ transference number of 0.59 at room temperature. The facile fabrication process also produces an asymmetric pore structure, the influence of which on battery performance is systematically investigated. Distribution of relaxation times (DRT) analysis indicates that positioning the small-pore side toward the cathode optimally enhances lithium-ion transfer kinetics at the interface. Consequently, the LiFePO4//ACC-5//Li cell demonstrates superior rate capability (112.2 mAh/g at 7C) and excellent cycling stability (92.5% capacity retention after 100 cycles at 1C). This work introduces a good design strategy for sustainable separators that synergistically integrates mechanical robustness and regulated ion transport for high-performance lithium batteries.