In this study, Poly(vinyl alcohol)/cellulose nanofiber (PVA/CNF) hydrogels have been successfully prepared using γ-ray irradiation, annealing, and rehydration processes. In addition, the effects of CNF content and annealing methods on the hydrogel properties, including gel fraction, micromorphology, crystallinity, swelling behavior, and tensile and friction properties, are investigated. Consequently, the results show that at an absorbed dose of 30 kGy, the increase in CNF content increases the gel fraction, tensile strength, and elongation at break of irradiated PVA/CNF hydrogels, but decreases the water absorption. In addition, the cross-linking density of the PVA/CNF hydrogels is significantly increased at an annealing temperature of 80 °C, which leads to the transition of the cross-sectional micromorphology from porous networks to smooth planes. For the PVA/CNF hydrogel with a CNF content of 0.6%, the crystallinity increases from 19.9% to 25.8% after tensile annealing of 30% compared to the original composite hydrogel. The tensile strength is substantially increased from 65.5 kPa to 21.2 MPa, and the modulus of elasticity reaches 4.2 MPa. Furthermore, it shows an extremely low coefficient of friction (0.075), which suggests that it has the potential to be applied as a material for artificial joint cartilage.

As a fundamental component of sodium-ion batteries, separators are considered to isolate two electrodes and simultaneously allow for the transport of ions. Cellulose separators have attracted widespread interest for their remarkable properties. In this study, we prepared composite separators comprising cellulose nanofibers (CNFs) and halloysite nanotubes (HNTs) for sodium-ion batteries. When the content of the HNT was up to 60%, the tensile strength and elongation at break of the composite separator (denoted as C/H-60) were 24.39 MPa and 2.22%, respectively. Importantly, the C/H-60 separator demonstrated a high porosity (69.08%), improved ionic conductivity (1.142 mS/cm), decent thermal stability, and good electrolyte retention (91.3% electrolyte uptake). The assembled sodium-ion battery containing the composite separators had an excellent rate capacity and cycling property. The proposed composite separators are expected to be applied in high-performance sodium-ion batteries.