Ultrathin Cellulose Composite Separator for High‐Energy Density Lithium‐Ion Batteries

Abstract Separators can physically inhibit the Li dendrite from piercing through to prevent the short‐circuit. To enhance the energy density of Li metal batteries, separators should be thinner and stronger (<20 µm, >25 MPa). In this work, a ≈10 µm cellulose composite separator by roll‐to‐roll technique is fabricated. Within the films, the Cu(OH) 2 nanoclusters are uniformly confined in the cellulose matrix without any agglomeration, enhancing the mechanical strength to ≈100 MPa. The coordinated copper ions help open the ion transporting channel, enabling the films to exhibit an ion conductivity of ≈2.14 mS cm −1 , among the highest reported values of separators. Compared with the polypropylene separators, the cellulose composite films display excellent cycling stability and effective Li dendrite inhibition with Li anodes. The symmetric cell Li||Li can stable cycle for more than ≈1800 h at 0.5 mA cm −2 with the plating/stripping time of ≈1 h. The cell LiFePO 4 ||Li shows an excellent rate performance of 5C (1C = 170 mA g −1 ). The full cell LiNi 0.8 Co 0.1 Mn 0.1 O 2 ||Li maintains a specific capacity of ≈162 mAh g −1 for ≈420 cycles at 1C (1C = 200 mAh g −1 ). The cellulose composite films enable scalable production with excellent electrolyte‐wettability and non‐flammability, demonstrating the promise in Li metal batteries with high safety and energy densities.