Thermal and electrochemical stability of organosilicon electrolytes for lithium-ion batteries

Organosilicon (OS) electrolytes that integrate an ethylene glycol oligomer with a trimethylsilane head group are promising substitutes for commercial carbonate-based electrolytes because of their low flammability and their high electrochemical and thermal stability. To explore the factors that control thermal and electrochemical stability of these compounds, we developed a real-time headspace analysis apparatus with a mass spectrometer to detect the evolution of decomposition products during thermal cycling and during electrochemical measurements. Here we present mass spectroscopy, XPS, and SEM results exploring the thermal stability of [2-[2-(2-Methoxyethoxy)ethoxy]ethoxy]trimethylsilane (1NM3) with LiPF6, and its electrochemical stability against graphite anodes and LiCoO2 cathodes. Our results show that 1NM3 + LiPF6 shows no significant decomposition below 100 °C and at potentials below 4.5 V. At higher temperatures and/or potentials, decomposition of LiPF6 induces hydrolysis of 1NM3. Our results show that LiPF6 decomposition is the limiting factor controlling stability of 1NM3 + LiPF6 electrolytes and also provide fundamental insights into the molecular bonds of 1NM3 that are attacked by PF5 and its decomposition products. Full-cell measurements of 1NM3 + LiPF6 + vinyl carbonate show Coulombic efficiencies of >99.6%. These results point the way to new molecular structures that may have even further enhanced electrochemical and thermal stability.