Multifunctional Additive Ethoxy(pentafluoro)cyclotriphosphazene Enables Safe Carbonate Electrolyte for SiO_x-Graphite/NMC811 Batteries

The silicon (Si) or silicon monoxide (SiOx)-graphite (Gr)/nickel-rich LiNixMnyCozO2 (NMC, x+y+z=1, with x >= 80 %) cell chemistry is currently regarded as a promising candidate to further improve the energy density of rechargeable lithium-ion batteries, but is confronted with safety and cycling stability issues. Here, the flame retardant ethoxy(pentafluoro)cyclotriphosphazene (PFPN) is studied as electrolyte additive in the SiOx-Gr/NMC811 full cell system. We find that PFPN in combination with an increased lithium hexafluorophosphate (LiPF6) concentration renders carbonate-based electrolytes non-flammable based on a very low self-extinguishing time of 3.1 s g(-1) while the electrolyte maintains a high ionic conductivity of 8.4 mS cm(-1) at 25 degrees C. Importantly, PFPN in combination with fluoroethylene carbonate (FEC) also stabilizes the solid-electrolyte interphase of Si-based anodes beyond the level achieved only with FEC. Furthermore, PFPN improves the wetting property of the electrolyte, rendering it a multifunctional additive. As a result, excellent capacity retention of 87 % after 200 cycles at 1 C was achieved for SiOx-Gr/NMC811 pouch cells with a relatively high SiOx content of 20 %. Our work provides a promising avenue for developing safe and high-performance electrolytes for lithium-ion batteries with silicon-based anodes.