Electrolytes with flame retardant pentafluoro(phenoxy)cyclotriphosphazene for nickel-rich layered oxide/graphite cells

Abstract

Ni-rich NMC cathode materials with a Ni content of over 80% are currently regarded as a promising pathway to further boost the energy density of rechargeable lithium-ion batteries. Meanwhile, cell safety is an important aspect when commercializing high-capacity batteries. Here, the compatibility of the flame retardant pentafluoro(phenoxy)cyclotriphosphazene (FPPN) when added to an ethylene carbonate/ethyl methyl carbonate/lithium hexafluorophosphate (LiPF₆ in EC:EMC) electrolyte is studied for Ni-rich NMC/graphite full cells. Addition of 15 wt.% FPPN to the baseline electrolyte renders it non-flammable as defined by a self-extinguishing time of zero. When compared to the baseline electrolyte, both coin and pouch cells containing the electrolyte with FPPN show promising electrochemical performance. In addition, fluorinated carbonates are combined with FPPN to reduce the amount of FPPN additive. Excellent flame retardancy is found when combining 35 vol.% methyl 2,2,2-trifluoroethyl carbonate with just 5 wt.% FPPN. The results of this study confirm that FPPN is a potent flame retardant for carbonate electrolytes and indicate good compatibility with Ni-rich NMC cathode materials.