Thermal management is critical to improving battery performance and suppressing thermal runaway. Besides developing external cooling technologies, it is important to understand and control thermal transport inside batteries. In this paper, heat transfer inside batteries is first analyzed and the thermal conductivity of each component is measured. The results show that low thermal conductivity of the separator is one major barrier for heat transfer in Li-ion batteries. To improve thermal conductivity of the separator, a hierarchical nano/micro-Al2O3/polymer separator is prepared with thermal conductivity of ~1Wm−1 K−1, representing an enhancement of 5× compared to commercial polyethylene-based separators. Modeling has been performed to understand mechanism behind the enhancement of thermal conductivity, which suggests that addition of nanoparticles significantly reduces thickness of polymer coating on micron-sized Al2O3 particles and thus increase the thermal conductivity of the composite separator. This Al2O3-based separator also has similar ionic conductivity with commercial polymer separators. Such composite separator may have potential applications in developing batteries with better performance and safety.