A Comparative Study of a Family of Sulfate Electrolyte Additives

The effects of cyclic sulfate additives including ethylene sulfate (or 1,3,2-Dioxathiolane-2,2-dioxide (DTD)), trimethylene sulfate (or 1,3,2-Dioxathiane 2,2-dioxide (TMS)) and propylene sulfate (or 4-methyl-1,3,2-dioxathiolane-2,2-dioxide (PLS)) on Li(Ni1/3Mn1/3Co1/3)O2/graphite pouch cells were studied. These additives have the same SO4 unit bonded to a different hydrocarbon ring so that the impact of changes to the ring on the electrochemical behavior can be studied. The additives were studied in pouch cells singly and in combination with 2% vinylene carbonate (VC) using high precision coulometry, AC impedance, long term storage and using a gas evolution apparatus. DTD and TMS show significant promise as electrolyte additives while PLS appears less useful. When added alone, DTD and TMS decrease cell impedance compared to control cells, improve coulombic efficiency and reduce voltage drop during storage. However, both lead to significant gas generation, comparable to control cells, during formation. DTD in combination with 2% VC gives high coulombic efficiency (CE), stable impedance during cycling and manageable amounts of gas during formation. TMS in combination with VC produces virtually no gas during formation at 40°C. In addition, TMS in combination with VC shows high CE and impedance that is reduced during early cycling. PLS in combination with VC yields cells with relatively poor CE and impedance growth during cycling.