Phase inversion is a powerful alternative process for preparing ultra-thin separators for various secondary batteries. Unfortunately, separators prepared from phase inversion generally suffer from uneven pore size and pore size distribution, which frequently results in poor battery performance. Here, a straightforward route is demonstrated to solve the drawbacks of phase-inversion-based separators for Li-ion batteries by means of directly incorporating 2D clay sheets in the skeleton of poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) with multiscale pore generation from a simple one-step solution coating method. Additionally generated pores by the inclusion of 2D nanosheets in PVdF-HFP skeletons, combined with the multiscale pores (several micrometers + sub-micrometers) originally generated by means of the controlled phase inversion, can generate additional ionic transport pathways, leading to Li-ion battery performances better than those of commercialized polyethylene separators. Moreover, the addition of extremely low contents of 2D clay sheets in PVdF-HFP separators allows thermally stable polymer separators to be realized.