Some Fluorinated Carbonates as Electrolyte Additives for Li(Ni_0.4Mn_0.4Co_0.2)O₂/Graphite Pouch Cells

The effects of four fluorinated carbonates including fluoroethylene carbonate, difluoroethylene carbonate, bis(2,2,2-trifluoroethyl) carbonate and 2,2,3,4,4,4-hexafluorobutyl methyl carbonate as electrolyte additives were studied in Li(Ni 0.4 Mn 0.4 Co 0.2 )O 2 /graphite pouch cells using ultra-high precision coulometry, in situ measurements of gas evolution, gas chromatography, electrochemical impedance spectroscopy, and long-term cycling experiments. The differential capacity vs. voltage curves during formation showed that fluoroethylene carbonate and difluoroethylene carbonate are solid electrolyte interphase (SEI) forming additives, while bis(2,2,2-trifluoroethyl) carbonate and 2,2,3,4,4,4-hexafluorobutyl methyl carbonate do not alter the SEI on the negative electrode during formation. Cells containing difluoroethylene carbonate have the highest coulombic efficiency and lowest charge end point capacity slippage rate at both 4.2 and 4.4 V. However the performance was not as good as that of cells containing a state of the art additive blend. Long-term cycle-hold-rest tests at 55°C showed that at 4.4 V all cells with fluorinated additives that gave promising capacity retention generated unacceptable quantities of gas. Only difluoroethylene carbonate during cycling tests to 4.2 V at 55°C provided promising capacity retention and moderate gas generation. These results suggest that the use of only these fluorinated additives in Li(Ni 0.4 Mn 0.4 Co 0.2 )O 2 /graphite pouch cells with ethylene carbonate-based electrolytes is not competitive to alternative approaches.