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• CNFs reinforced pure cellulose paper (CCP) was prepared by facile filtration process. • The ionic conductivity of CCP with 20 wt.% CNFs was 25 % higher compared to polypropylene (PP) separator. • The tensile of CCP with 20 wt.% CNFs was 227 % higher compared to cellulose paper (CP) separator. • CCP with 20 wt.% CNFs has better working stability than control. • The LIB assembled with CCP separator displays better cycle performance than control. Separator is a vital component of lithium-ion batteries (LIBs) due to its important roles in the safety and electrochemical performance of the batteries. Herein, we reported a cellulose nanofibrils (CNFs) reinforced pure cellulose paper (CCP) as a LIBs separator fabricated by a facile filtration process. The nanosized CNFs played crucial roles as a tuner to optimize the pore size of the as-prepared CCP, and also as a reinforcer to improve the mechanical strength of the resultant CCP. Results showed that the tensile strength of the CCP with 20 wt.% CNFs was 227 % higher compared to the commercial cellulose separator. In addition, the lithium cobalt oxide/lithium metal battery assembled with CCP separator displayed better cycle performance and working stability (capacity retention ratio of 91 % after 100 cycles) compared to the batteries with cellulose separator (52 %) and polypropylene separator (84 %) owing to the multiple synergies between CCP separator and electrolytes. [ABSTRACT FROM AUTHOR]