The interaction of charged LiNi0.5Mn1.5O4 cathodes in contact with standard electrolyte solvents as e.g. ethylene carbonate and ethyl methyl carbonate is investigated. Due to the high electrochemical potential of the charged LiNi0.5Mn1.5O4 cathode electrolyte decomposition occurs, which results in the formation of gases and other compounds. The corresponding decomposition products are successfully detected and analyzed by means of gas chromatography. We propose that decomposition products react on the surface of the graphite anode, which leads to strong capacity fading. Therefore, an electrochemically active separator containing Li4+x Ti5O12 is used in graphite/LiNi0.5Mn1.5O4 cells in order to scavenge and neutralize these decomposition products as well as soluble transition metal ions. This separator most probably prevents the migration of decomposition products to the anode and their subsequent reaction on the anode surface. Pouch cells with graphite anode, LiNi0.5Mn1.5O4 cathode, standard electrolyte and the Li4+x Ti5O12 based separator are cycled successfully for hundreds of cycles without any significant capacity fading. In contrast, similar cells equipped with a conventional separator show strong degradation.