Eco-friendly cellulose nanofiber paper-derived separator membranes featuring tunable nanoporous network channels for lithium-ion batteries

Eco-friendly cellulose nanofibers (CNFs), a core constituent of cellulose, have garnered increasing attention as a promising sustainable building block source for advanced materials in various application fields. In the present study, we successfully fabricate a cellulose nanofiber paper from a CNF suspension and explore its potential application to a separator membrane for lithium-ion batteries. In contrast to macro/microscopic cellulose fibers that have been commonly used for typical papers, the CNFs are characterized by the nanometer-scale diameter/length up to several micrometers and highly crystalline domains, contributing to excellent mechanical/thermal properties and nanoporous structure evolution. A salient feature of the cellulose nanofiber paper-derived separator membrane (referred to as "CNP separator") is an electrolyte-philic, nanoscale labyrinth structure established between closely piled CNFs. The unusual porous structure is fine-tuned by varying the composition ratio of the solvent mixture (= isopropyl alcohol (IPA)–water) in the CNF suspension, wherein IPA is introduced as a CNF-disassembling agent while water promotes dense packing of CNFs. Based on a solid understanding of separator characteristics, electrochemical performances of cells assembled with the CNP separators are investigated. Notably, the CNP separator manufactured with IPA–water = 95/5 (vol/vol%) exhibits highly interconnected nanoporous network channels and satisfactory mechanical properties, which play a significant role in improving separator properties and cell performance. This study underlines that the porous structure-tuned cellulose nanofiber papers provide a promising new route for the fabrication of advanced separator membranes, which will also serve as a key component to boost the development of next-generation paper batteries.