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Environmental-friendly nanocellulose films with excellent mechanical strength have attracted extensive attention in the gel polymer electrolytes (GPEs) for lithium-ion batteries. However, their dense structure causes poor electrolyte uptake capacity. In this research, carboxymethylated nanocellulose (CMNC) with a strong swelling behavior and anionic character was chosen as raw materials to not only improve the electrolyte absorption ability and lithium ion transfer number of the resulting GPEs but also avoid short circuits by their dense morphology. Further crosslinking by epichlorohydrin and solvent casting, CCMNC films were obtained, which exhibited excellent mechanical strength (36.46 to 67.45 MPa), thermal stability (up to 250 °C) and flexibility. After electrolyte uptaking, the optimal GCCMNC GPE at an ECH content of 5 wt% displayed an electrolyte uptake ratio, ionic conductivity, lithium ion transfer number, and electrochemical stability window of 312 wt%, 3.93 mS cm−1, 0.82, and 4.65 V, respectively. Particularly, the initial discharge capacity of our assembled Li/GCCMNC-5/NCM523 battery was 151.4 mAh g−1. After 50 cycling, the cell still maintained a capacity of 74 % and a coulombic efficiency of 98 %. The good mechanical strength, thermal stability and electrochemical properties of our designed GCCMNC GPE indicated its promising application in the lithium-ion batteries.