Effect of fluorinated additives or co-solvent on performances of graphite//LiMn2O4 cells cycled at high potential

Abstract

Herein, a comprehensive study of electrochemical performances of the combined effect of fluorinated additives; fluoroethylene carbonate (FEC); and tris(2,2,2-trifluoroethyl) phosphite (TTFP) or the 2,2,2-trifluoroethyl methyl carbonate (TFEMC) as co-solvent, on Graphite//LiMn2O4 cells cycled at high potential is reported. On one side, each additive has a specific function, the FEC is dedicated to the negative electrode and the TTFP to the opposite one. The electrolyte mixture with (4% FEC + 1% TTFP) additive has shown the best ability to reduce fading of the LiMn2O4 electrode, especially at high rates. On the other side, by studying the comparative thermal and transport properties of the formulated electrolytes with different proportions of TFEMC, we demonstrate that the difference in charge distribution of EMC and TFEMC molecules induced by the presence of fluorine atoms, modifies the solvation model of the Li+ cation, and changes its behavior at the CEI interface and impact strongly the electrochemical performances. Finally, the EIS investigation of the LMO/electrolyte interfaces in the presence of TFEMC demonstrates that despite a spontaneous chemical reactivity of the TFEMC at the cathode interface over time, the conductive and good quality CEI is formed, which positively impact the cyclability. This study shows that against LMO surface phenomena, the combination in adequate proportions of fluorinated additives or solvent can be a solution not only to avoid the oxidative reactivity of LMO-cathode, but also to prevent its harmful consequences on the Li-metal or graphite-anode by controlling the solvation of lithium-ion.