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[Ni_1/3Mn_1/3Co_1/3]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.