Effective SEI Formation via Phosphazene-Based Electrolyte Additives for Stabilizing Silicon-Based Lithium-Ion Batteries

Silicon, as potential next-generation anode material for high-energy lithium-ion batteries (LIBs), suffers from substantial volume changes during (dis)charging, resulting in continuous breakage and (re-)formation of the solid electrolyte interphase (SEI), as well as from consumption of electrolyte and active lithium, which negatively impacts long-term performance and prevents silicon-rich anodes from practical application. In this work, fluorinated phosphazene compounds are investigated as electrolyte additives concerning their SEI-forming ability for boosting the performance of silicon oxide (SiOx)-based LIB cells. In detail, the electrochemical performance of NCM523 || SiOx/C pouch cells is studied, in combination with analyses regarding gas evolution properties, post-mortem morphological changes of the anode electrode and the SEI, as well as possible electrolyte degradation. Introducing the dual-additive approach in state-of-the-art electrolytes leads to synergistic effects between fluoroethylene carbonate and hexafluorocyclotriphosphazene-derivatives (HFPN), as well as enhanced electrochemical performance. The formation of a more effective SEI and increased electrolyte stabilization improves lifetime and results in an overall lower cell impedance. Furthermore, gas chromatography-mass spectrometry measurements of the aged electrolyte with HFPN-derivatives as an additive compound show suppressed ethylene carbonate and ethyl methyl carbonate decomposition, as well as reduced trans-esterification and oligomerization products in the aged electrolyte.