Manganese dissolution from positive electrodes seriously reduces the useful life of Li-ion batteries, especially with positive electrode materials having spinel phases. We show herein that Mn ion trapping separators containing inexpensive mass-produced materials may dramatically increase the life of Li-ion batteries. LiMn2O4-graphite cells containing these materials and a LiPF6 based electrolyte solution display excellent capacity retention during cycling at both room and elevated temperatures, over baseline cells with plain separators. After 30 days of cycling at 55 °C and C/5 rate, LiMn2O4-graphite cells containing three different Mn-trapping materials with nitrogen functionalities retain between 75% and 125% more of the initial capacity than the baseline cells. Mn amounts in graphite negative electrodes from cells with the functional separators are 13–21 times lower than in baseline cells. LiMn2O4 lattice shrinkage in cells with functionalized separators is negligible compared to baseline cells, indicating major reductions in the loss of electrochemically active Li+ ions and increased stability of the LiMn2O4 crystal lattice.