Nanocellulose has attracted great attention as one of the most promising separator materials owing to the advantage of excellent electrolyte wettability and high thermal stability. It also has excellent natural abundance, biocompatibility and renewability. However, the dispersion of nanocellulose is inadequate, leading to self-agglomeration and other issues. This not only hinders the formation of a uniform porous separator but also diminishes the transport channels for Li+. To overcome this challenge, the rare earth ions Ce3+ were added. The complexation between the rare earth ions and the hydroxyl oxygen atoms of cellulose weakened the original hydrogen bond interactions among cellulose nanofibers, thereby enhancing the dispersion of CNF. In this work, a cerium-modified nanocellulose (0.050% Ce-CNF) separator with excellent thermal stability and high wettability using rare earth metal ions and cellulose nanofibers (CNF) as raw materials was prepared. Compared with conventional nanocellulose, the 0.05% Ce-CNF exhibited better processability for separator manufacturing and the separator obtained in this way possessed high stability in the electrolyte. The contact angle of 0.050% Ce-CNF separator with the electrolyte was smaller (18.7°) compared to that of polypropylene (PP) at 42.3°, and the ionic conductivity was higher. At the same time, due to the high porosity of 67.4% (compared to only 41.8% for the PP separator), batteries using CNF separators showed significantly improved rate performance compared to those using PP separators. In addition, the preparation process of Ce-CNF modified composite separator was environmentally friendly. Therefore, cerium-modified nanocellulose separator is expected to be a strong candidate for the next generation of high-safety and high-performance battery separators.