In this study, novel temperature stable electrolyte mixtures are investigated with respect to their use in Li-ion cells. Sulfolane, 3-methylsulfolane, and the ionic liquid N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis(trifluoromethanesulfonyl) azanide (DMMA-TFSA) are chosen as main components in the electrolyte mixtures. The electrolytes are studied in cell tests with LiNi1/3Mn1/3Co1/3O2 (NMC) as cathode and graphite as anode material. The performance of the cells is tested at room temperature and at elevated temperatures (40 °C) which demonstrates the significant improvement of the cell performance by raising the temperature slightly. Furthermore, the additives vinylene carbonate and hexamethyldisilazane are used to improve the electrolyte properties with respect to cell performance and long term stability.