Improved performance of lithium-ion cells with the use of fluorinated carbonate-based electrolytes

There has been increasing interest in developing lithium-ion electrolytes that possess enhanced safety characteristics, while still able to provide the desired stability and performance. Toward this end, our efforts have been focused on the development of lithium-ion electrolytes which contain partially and fully fluorinated carbonate solvents. The advantage of using such solvents is that they possess the requisite stability demonstrated by the hydrocarbon-based carbonates, while also possessing more desirable physical properties imparted by the presence of the fluorine substituents, such as lower melting points, increased stability toward oxidation, and favorable SEI film forming characteristics on carbon. Specifically, we have demonstrated the beneficial effect of electrolytes which contain the following fluorinated catbonate-based solvents: methyl-2,2,2-trifluoroethyl carbonate (MTFEC), ethyl-2,2,2-trifluoroethyl carbonate (ETFEC), propyl-2,2,2trifluoroethyl carbonate (PTFEC), methyl-2,2,2,2',2',2'-hexafluoro-i-propyl carbonate (MHFPC), ethyl-2,2,2,2',2',2'-hexafluoro-i-propyI carbonate (EHFPC), and di-2,2,2-trifluoroethyl carbonate (DTFEC). These solvents have been incorporated into multi-component ternary and quaternary carbonate-based electrolytes and evaluated in lithium-carbon and carbon-LiNi0.8Co0.2O2 cells (equipped with lithium reference electrodes). In addition to determining the charge/discharge behavior of these cells, a number of electrochemical techniques were employed (i.e. Tafel polarization measurements, linear polarization measurements, and electrochemical impedance spectroscopy (EIS)) to further characterize the performance of these electrolytes, including the SEI formation characteristics and lithium intercalation/deintercalation kinetics. (C) 2003 Elsevier Science B.V. All rights reserved.