Battery separator and manufacturing method thereof, and lithium ion secondary battery and manufacturing method thereof

A battery separator having excellent oxidation resistance and a lithium ion secondary battery having excellent oxidation resistance and little deterioration in battery capacity even after being kept at a high temperature and in a charged state for a long time are provided. A battery separator 13 of the present invention includes a porous film 12 as a base material and a crosslinked polymer layer 11 supported on the porous film 12. The crosslinked polymer layer 11 reacts a reactive polymer having a functional group in the molecule with a polyfunctional compound having reactivity with the functional group to crosslink at least a part of the reactive polymer. The resulting crosslinked polymer and inorganic particles are included and are non-porous. The lithium ion secondary battery of the present invention includes a positive electrode 14, a negative electrode 15, a battery separator 13 of the present invention disposed between the positive electrode 14 and the negative electrode 15, and a nonaqueous electrolytic solution. The battery separator 13 is disposed such that the porous film 12 is on the negative electrode 15 side and the crosslinked polymer layer 11 is on the positive electrode side 14. [Selection] Figure 1