Abstract: Cellulose/Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) composite separators were fabricated via an environmentally benign and scalable papermaking process. Systematic modulation of the cellulose-to-LATP mass ratio (1 ∶ 1~1 ∶ 8) established composition-structure-performance correlations. The separator with a 1 ∶ 4 mass ratio exhibited optimal performance: LiFePO₄/Li half-cells showed a capacity decay rate of 0.034% per cycle over 1 000 cycles at 0.5C, while high-voltage LiNi_0.5Mn_1.5O₄/Li batteries (>4.7 V) achieved 78.4% capacity retention after 1 000 cycles at 2C. This approach constructs a high-performance organic-inorganic composite separator system and provides a strategy for regulating ion transport properties through inorganic electrolyte content, offering new design perspectives for next-generation high-safety, high-energy-density lithium metal battery separators.