POROUS MEMBRANE FOR LITHIUM ION SECONDARY BATTERY, AND LITHIUM ION SECONDARY BATTERY

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Provided is a technique for capturing transition metal ions, such as cobalt ions, in a secondary battery that elute into an electrolysis solution from a positive electrode active material. A porous membrane for a lithium ion secondary battery contains non-conductive particles and a particulate polymer. The particulate polymer includes an aliphatic conjugated diene monomer unit in a proportion of greater than 85 mass%. The cobalt concentration in a film of 500 µm in thickness and 12 mm in diameter, formed from the particulate polymer, after the film is immersed in a specific cobalt chloride solution for 5 days at 25°C is at least 300 mass ppm.