To improve the thermal shrinkage and ionic conductivity of the separator for lithium-ion batteries, adding carboxylic titanium dioxide nanofiber materials into the matrix is proposed as an effective strategy. In this regard, a poly(vinylidene fluoride-hexafluoro propylene)/dibutyl phthalate/carboxylic titanium dioxide (PVDF-HFP/DBP/C-TiO) composite separator is prepared with the phase inversion method. When the content of TiO nanofibers reaches 5%, the electrochemical performance of the battery and ion conductivity of the separator are optimal. The PVDF-HFP/DBP/C-TiO (5%) composite separator shows about 55.5% of porosity and 277.9% of electrolyte uptake. The PVDF-HFP/DBP/C-TiO (5%) composite separator has a superior ionic conductivity of 1.26 × 10 S cm and lower interface impedance at room temperature, which brings about better cycle and rate performance. In addition, the cell assembled with a PVDF-HFP/DBP/C-TiO separator can be charged or discharged normally and has an outstanding discharge capacity of about 150 mAh g at 110 °C. The battery assembled with the PVDF-HFP/DBP/C-TiO composite separator exhibits excellent electrochemical performance under high and room temperature environments.