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

Abstract

The silicon or silicon monoxide (SiO x )-graphite/nickel-rich (≥80% Ni) LiNi x Mn y Co z O2 (x+y+z = 1, NMC) cell chemistry is widely regarded as one of the most promising chemistries for next-generation electric vehicle batteries for due to its higher energy density compared to state-of-the-art graphite/Ni-rich NMC batteries. However, the drastic volume change during lithiation and delithiation of silicon poses a significant challenge to realize its high capacity with a practical cycle life. Besides performance degradation, it is desirable to address safety risks stemming from, e.g., the high flammability of carbonate solvents and oxygen release from highly delithiated Ni-rich NMC materials.To address these problems, one of the most effective methods is to develop advanced electrolytes that combine flame-retardant properties and excellent electrochemical performance. Generally, in consideration of the balanced trade-offs between cost, scalability, safety, and electrochemical compatibility, the use of flame retardants is a very promising approach to realize the next-generation nonflammable electrolytes.However, it is still a huge challenge to optimize the electrolyte composition to stabilize Si-based and Ni-rich lithium-ion batteries.Here, the compatibility of the flame retardant ethoxy(pentafluoro)cyclotriphosphazene (PFPN) as additive for carbonate electrolytes is studied, for the first time, in the SiO x -graphite/NMC811 full cells. [1] A synergistic effect is found between PFPN and lithium hexafluorophosphate (LiPF6) for the flame retardancy, and a modified electrolyte (1.35M LiPF6 in ethylene carbonate (EC):ethyl methyl carbonate (EMC) (3:7 by vol.) with 10 wt.% fluoroethylene carbonate (FEC) and 5 wt.% PFPN) displays a very low self-extinguishing time of only 3 s/g while also maintaining a high ionic conductivity of 8.4 mS/cm at 25 °C. Furthermore, the morphology evolution and interphase composition of both anode and cathode are investigated, gaining important insights into the decomposition mechanism of PFPN to stabilize the electrode-electrolyte interfaces. Meanwhile, the wetting properties of the modified electrolyte are also improved due to the excellent wettability of PFPN. As a result, half and full coin cells with SiO x -graphite anode and NMC811 cathode display better capacity retention during long-term cycling with the modified electrolyte than with 1M LiPF6 in EC:EMC (3:7 by vol.) with 10 wt.% FEC. In addition, the enhanced cycling performance with the modified PFPN-contained electrolyte is also confirmed for 1 Ah SiO x -graphite/NMC811 pouch cells cycled under various pressures. These results open a promising avenue for the development of safe and high-performance electrolytes for silicon-based lithium-ion batteries.