Effects of the LiPO2F2 additive on unwanted lithium plating in lithium-ion cells

The effect of the additive lithium difluorophosphate (LiPO2F2) on unwanted lithium plating in lithium-ion cells was studied in this work. Li[Ni1/3Mn1/3Co1/3]O2/graphite pouch cells were cycled at high charge rates where unwanted lithium plating was identified as the major aging mechanism. LiPO2F2 used as a single electrolyte additive reduced cell impedance growth during cycling even at high charge rate (i.e. 2C) at 20 °C. Electrochemical impedance spectroscopy (EIS) measurements on symmetric cells showed that LiPO2F2 reduced the growth of both positive and negative electrode impedances during cycling. The effects of LiPO2F2 depend strongly on the presence of other additives. When LiPO2F2 was combined with 2% propene sultone (PES) + 1% ethylene sulfate (DTD) + 1% tris(-trimethyl-silyl)-phosphite (TTSPi) (called PES211), the cell impedance increased which led to unwanted lithium plating. When 1% LiPO2F2 was used in combination with ethylene sulfate (DTD) or fluoroethylene carbonate (FEC) the impedance of the cells grew more slowly with cycling than with DTD or FEC added alone. Normally, but not always, additions of LiPO2F2 reduce cell impedance. The choice of additives to use along with LiPO2F2 must be made wisely to ensure synergistic action.