A Study of Li-Ion Cells Operated to 4.5 V and at 55°C

Abstract

The long-term cycling behavior of 24 promising electrolyte blends were systematically studied in LaPO4-coated Li(Ni0.4Mn0.4Co0.2)O2/graphite pouch type Li-ion cells tested to 4.5 V at 55°C. Capacity fade during cycling, charge-transfer resistance (Rct) before and after cycling as well as gas evolution during formation and also during cycling were examined and compared head-to-head. Of all the electrolytes tested, triallyl phosphate containing electrolytes including 2% vinylene carbonate + 2% triallyl phosphate in 1 M LiPF6 sulfolane:ethyl methyl carbonate and 2% prop-1-ene sultone + 2% triallyl phosphate in 1M LiPF6 ethylene carbonate:ethyl methyl carbonate electrolytes showed the best capacity retention, the least impedance growth and manageable amounts of gas evolution during long-term cycling. Pyridine boron trifluoride -based additives also showed excellent cycling performance but cells with those additives had higher gas evolution during cycling. Cells containing fluorinated electrolytes had similar cycling performance to cells containing 2% prop-1-ene sultone in ethylene carbonate:ethyl methyl carbonate electrolyte but with much more gas evolution and higher impedance after long-term cycling. The results suggest that electrolytes containing the additives triallyl phosphate or pyridine boron trifluoride may be promising for high voltage Li-ion cells at elevated temperature.