Mn cation trapping by polymeric aza-15-crown-5 ethers is an effective means for mitigating the consequences of Mn dissolution in Li-ion batteries. Mn cations trapping was investigated in lithium manganese oxide (LMO) spinel–graphite (GR) cells containing 1 M LiPF6 in ethylene carbonate (EC):diethyl carbonate (DEC) 1:2 v/v. A commercial polyolefin separator membrane coated with poly[divinylbenzene-(vinylbenzyl-aza-15-crown-5)-vinylbenzylchloride)] effected a 39% reduction in capacity loss rate during cycling at 50 °C with 100% depth of discharge (DOD) at C/5 rate. Simultaneously, a 50–60% reduction in the Mn deposited at the negative electrode, and a 6× to 10× increase in the Mn on the coated separator were observed for cells with coated separators, over baseline cells with plain separators. X-ray absorption near-edge spectroscopy (XANES) yielded average oxidation states near +3 for Mn cations in graphite electrodes and separators from cycled cells, suggesting that Mn metal or in oxidation state +2 can only be minor fractions of the Mn existing outside the positive electrode. We discuss the implications of these results for choosing an optimal chelating agent. We also show that the cation chelating polymer reported here is compatible with existing manufacturing processes for Li-ion battery separators.