A sandwich-structured sustainable separator based on pristine bacterial cellulose nanofiber network for lithium dendrite early-warning and enhanced battery safety.

The uncontrollable growth of lithium dendrites triggers battery short circuits, thermal runaway, and even explosions. In this work, a novel safe sandwich-structured bacterial nanocellulose (BNC) separator (SBCB) was designed. In order to facilitate scaling up and future industrialization, a conventional industrial static fermentation method was employed for its preparation. The intermediate layer features a conductive network constructed by the pristine three-dimensional (3D) nanoscale network structure of BNC interconnected with carboxylated multi-walled carbon nanotubes (MWCNTs-COOH). When lithium dendrites penetrate the conductive intermediate layer of SBCB, an electrical signal warning is triggered, enhancing battery safety. Moreover, leveraging the advantages of BNC, the separator exhibits no observable macroscopic dimensional shrinkage at 200 °C, with its natural nano-network pores enabling high electrolyte uptake (536.1%). The internal hydrogen bonds endow it with exceptional tensile strength (193.39 MPa) and puncture resistance (5.24 N). The uniformly interconnected pore structure facilitates efficient lithium-ion transport (tLi+ = 0.59), enabling stable lithium plating/stripping behavior for approximately 4500 h in LiLi cells. In LiFePO4-Li cells, it achieves 200 cycles at 0.2C with 92% capacity retention. These results indicated that the SBCB separator would provide a new strategy for addressing the lithium dendrite issue and offer a simple, bio-based, and potentially sustainable technological pathway.