| abstract |
The objective of the present invention is to provide a technique for capturing, in a secondary battery, transition metal ions such as cobalt ions eluted from a positive electrode active material into an electrolyte solution. This porous membrane for a lithium ion secondary battery contains non-conductive particles and a particulate polymer, wherein said particulate polymer includes an aliphatic conjugated diene monomer unit in a proportion exceeding 85 mass%, and the cobalt concentration of a film, which has a thickness of 500 µm and a diameter of 12 mm and is formed from the particulate polymer, after being immersed in a prescribed cobalt chloride solution at 25 °C for five days is 300 mass ppm or more. |