The safety of lithium ion batteries is correlated closely to the mechanical property and thermal stability of separators. The polymer/ceramic particle composite separators are widely investigated for quasi-solid gel separators of lithium ion batteries, but the nonuniform dispersion of ceramic particles limits their performance. In this study, ball-milled Al-doped Li6.75La3Zr1.75Ta0.25O12 suspension is incorporated into poly (vinylidene fluoride-co-hexafluoropropylene) gel to prepare a highly uniform quasi-solid composite separator. The results show that adding only 4.29 wt% Li6.75La3Zr1.75Ta0.25O12 into PVDF-HFP increases ductility by three times and the mechanical strength by 38% as compared with pure PVDF-HFP separator. Batteries with the composite separator show significantly improved cyclic stability and rate performance at both room temperature and temperatures up to 80 °C. In particular, the composite separator promotes homogenous Li+ flux through the separator and suppresses Li dendrite growth in Li-metal based batteries.