Optimizing the Electrolyte Additive Content in Practical Li-Ion Cells with Si-Dominant Anodes

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Due to their high capacity, Silicon (Si) anodes are very promising for the next generation of lithium-ion batteries. However, Si materials suffer from a high volume expansion, which cause cracking and accumulated growth of the solid electrolyte interphase (SEI), resulting in low coulombic efficiencies and lifetime of the Si anodes. Applying additives in the electrolyte is a facile and straightforward approach to improve the composition of the SEI and reduce cracking. For Si-containing anodes, fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are considered the most effective electrolyte additives. The salt lithium difluorophosphate (LiDFP) is known to improve the interphases of cathode materials and graphite when applied as additive, but has been poorly investigated for Si anodes. In our study, we combine VC, FEC, and a variation of LiDFP concentrations to investigate the effect on Si-dominant anodes in detail. We conducted detailed electrochemical testing in half coin cells and Si/NCM pouch cells, including long-term cycling, rate capability tests, potential monitoring of the anode and electrochemical impedance spectroscopy. For the latter two, we applied a novel perforated reference electrode concept, which allows for broad and precise measurement in pouch cells, while avoiding a blocking effect of the lithium ions. Furthermore, post-mortem analysis was conducted to gain detailed insights into the SEI formation and microstructure influenced by the different additive concentrations.