Eco-friendly polyvinyl alcohol/cellulose nanofiber-Li⁺ composite separator for high-performance lithium-ion batteries

A novel eco-friendly polyvinyl alcohol/cellulose nanofiber-Li⁺ (PVA/CNF-Li) composite separator was prepared for lithium-ion batteries. In this membrane by a non-solvent induced phase separation (NIPS) wet-process, CNF-Li originating from wood pulp was successfully prepared and characterized by FT-IR and TEM. The composite separators showed excellent porosity of over 60%, and better ionic conductivity (∼1.1 mS cm^-1) as well as remarkable electrolyte uptake approaching 2.3. CNF-Li combined the excellent properties of both nanofibers and ion-conductive polymers such as CMC-Li. The introduction of CNF-Li in the separator increased the thermal dimensional stability and mechanical performance. Simultaneously, CNF-Li as a constituent in the membrane increased the contents of Li⁺, opening a way for Li⁺ transportation to improve batteries' Li⁺ diffusion efficiency and specific capacity. The battery with a 2 wt% CNF-Li separator retained 93% of the initial reversible capacity after 50 cycles, which was much higher than that of the commercial polypropylene (PP) separator with a value of 80%. The PVA/CNF-Li composite membrane produced via a relatively low cost and eco-friendly method can serve as a potential alternative of commercial PP separators applied in high-performance lithium-ion batteries.