Lithium difluorophosphate as an additive to improve the low temperature performance of LiNi0.5Co0.2Mn0.3O2/graphite cells

Abstract

Lithium difluorophosphate (LiPO2F2) was used as an electrolyte additive to promote the low temperature performance of LiNi0.5Co0.2Mn0.3O2/graphite cells. The impact of LiPO2F2 on the solid electrolyte interface (SEI) film-formation on electrodes was demonstrated by various electrochemical methods and microscopy techniques, such as transmission electron microscopy (TEM), scanning electron microscope (SEM) as well as X-ray photoelectron spectroscopy (XPS), in the pouch cells and half cells. The results showed that the cells containing 1% LiPO2F2 performed 71.9% (−20°C) and 57.93% (−30°C) of initial capacity, while the cells without LiPO2F2 discharged only 49.41% and 9.6% of initial capacity under the same condition. In addition, the enhancement of cyclic performance at 0°C was attributed to a conductive and stable SEI film formed on the graphite by the sacrifice of LiPO2F2, which led to a low impedance and richer content of LiF and Li2CO3 in SEI components, as depicted in XPS.