Electrochemical and thermal studies were performed to investigate: a) a fluorinated electrolyte (FE) containing fluoroethylene carbonate (FEC) as the co-solvent, and b) a non-fluorinated electrolyte (NFE) containing ethylene carbonate (EC) as the co-solvent, for lithium ion cells. Electrochemical impedance spectroscopy (EIS) measurements were performed on both electrolytes to estimate ionic conductivity. Cyclic voltammetry was employed to probe the effect of co-solvent fluorination on electrolyte oxidative stability. Cycling, rate capability, and EIS tests were performed on lithium ion coin cells (of 2032 size) to investigate the effect of the FEC co-solvent on the electrochemical performance of (LiNi0.8Co0.15Al0.015O2) at the cathode. Differential scanning calorimetry (DSC) studies were performed to quantify the exothermic heat associated with delithiation of the (LiNi0.8Co0.15Al0.015O2) cathode in the presence of electrolytes containing FEC and EC as the co-solvent respectively. A remarkable improvement in the thermal stability of (Li0.36Ni0.8Co0.15Al0.015O2) was observed when FEC was used as the co-solvent. Besides the upward shift of the onset temperature by 24 degrees C, the exothermic heat was also reduced by 15%. Cells prepared with FE showed good electrochemical performance at low rates and moderate performance at high rates when compared to cells prepared with NFE. (C) 2014 Elsevier Ltd. All rights reserved.