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

Abstract The silicon (Si) or silicon monoxide (SiO x )‐graphite (Gr)/nickel‐rich LiNi x Mn y Co z O 2 (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 SiO x ‐Gr/NMC811 full cell system. We find that PFPN in combination with an increased lithium hexafluorophosphate (LiPF 6 ) 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 °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 SiO x ‐Gr/NMC811 pouch cells with a relatively high SiO x content of 20 %. Our work provides a promising avenue for developing safe and high‐performance electrolytes for lithium‐ion batteries with silicon‐based anodes.