Since 5 V lithium ion batteries have attracted more and more attentions and are deemed to be an important tendency in the future, the matched design of the separators has also become a necessary and significant work. In this work, the lithium ionic conducting glass ceramic Li1.5Al0.5Ge1.5(PO4)3-polypropylene (PP) based inorganic–organic composite separator (LAGP-PP) is prepared. Compared with the pristine PP separator, the LAGP-PP separator owns enhanced thermal stability and wettability. Meanwhile, the LAGP-PP separator shows higher ion conductivity than the traditional Al2O3 coated PP separator due to the more facile lithium ion diffusion channels in the coating layer. The superior C-rate capacity and cyclability in the LiNi0.5Mn1.5O4 based 5 V lithium ion batteries indicate that the LAGP-PP separator is a good alternative for the traditional inert inorganic ceramic coated polyolefin separators and is a kind of promising candidate separator for the high voltage lithium ion batteries.