Multifunctional Additive Ethoxy(pentafluoro)cyclotriphosphazene Enables Safe Carbonate Electrolyte for SiO_<i>x</i>‐Graphite/NMC811 Batteries

The silicon (Si) or 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 confronted safety and cycling stability issues. Here, flame retardant ethoxy(pentafluoro)cyclotriphosphazene (PFPN) studied electrolyte additive in SiOx-Gr/NMC811 full system. We find that PFPN combination an increased lithium hexafluorophosphate (LiPF6) concentration renders carbonate-based electrolytes non-flammable based on very low self-extinguishing time 3.1 s g−1 while maintains high ionic conductivity 8.4 mS cm−1 at 25 °C. Importantly, fluoroethylene carbonate (FEC) also stabilizes solid-electrolyte interphase Si-based anodes beyond level achieved only FEC. Furthermore, improves wetting property electrolyte, rendering it multifunctional additive. As result, excellent capacity retention 87 % after 200 cycles 1 C was for pouch cells relatively SiOx content 20 %. Our work provides avenue developing safe high-performance batteries silicon-based anodes.