The Effects of a Ternary Electrolyte Additive System on the Electrode/Electrolyte Interfaces in High Voltage Li-Ion Cells

The effects of a ternary electrolyte additive system on Li[Ni0.42Mn0.42Co0.16]O2 (NMC442)/graphite pouch cells balanced for 4.7 V was thoroughly investigated. The additive system consisted of 2% prop-1-ene-1,3-sultone (PES) + 1% methylene methane disulfonate + 1% tris-(trimethylsilyl)-phosphite (TTSPi) which is denoted "PES211". "PES211" significantly improved the capacity retention, impedance and gas evolution of NMC442/graphite pouch cells cycled at constant current up to 4.4 V or 4.5 V. X-ray photoelectron spectroscopy results suggest more stable and passivating SEI films at both graphite and NMC442 surfaces due to the preferential electrochemical reaction of PES and MMDS as well as the preferential chemical reaction of TTSPi at the graphite surface. "PES211" modifies the reactivity of the LiPF6 salt at the graphite surface and hinders its degradation at high potential. However, during extended exposure to 4.5 V, less passivating SEI films were observed leading to extensive electrolyte degradation, capacity loss, impedance increase and gas evolution even for NMC442/graphite pouch cells containing "PES211".