The Optimal Amount of Lithium Difluorophosphate as an Additive for Si-Dominant Anodes in an Application-Oriented Setup

Fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are considered the most effective electrolyte additives for improving the solid electrolyte interphase (SEI) of Si-containing anodes while lithium difluorophosphate (LiDFP) is known to improve the interphases of cathode materials and graphite. Here, we combine VC, FEC, and different amounts of LiDFP in a highly-concentrated electrolyte to investigate the effect on Si-dominant anodes in detail. Cycle life tests, electrochemical impedance spectroscopy and rate tests with anode potential monitoring were conducted in Si/NCM pouch cells. The results reveal that adding LiDFP to the electrolyte improves all performance criteria of the full cells, with a concentration of 1 wt% being the optimal value for most cases. Post-mortem analyses using scanning electron microscopy and x-ray photoelectron spectroscopy showed that a more beneficial SEI film was formed for higher LiDFP concentrations, which led to less decomposition of electrolyte components and a better-maintained anode microstructure. Lithium difluorophosphate (LiPO2F2, LiDFP) was applied as a co-additive in application-oriented Si/NCM pouch cells in four different concentrations and detailed electrochemical and post-mortem analyses were performed. 1 % of LiDFP exhibited the best performance for most criteria. However, 2 % of LiDFP was better at some conditions, especially when applying low C rates.image