Fluorinated hyperbranched cyclotriphosphazene additive for high-performance LiNi0.4Mn1.6O4 cathodes in lithium ion batteries

Abstract

One of an international problem is global warming which is rapidly going on increasing average surface temperature of the Earth. To reduce greenhouse gases which have increased to the Earth temperature, there have been numerous efforts to replace fossil fuels with eco-friendly batteries such as lithium ion cells, fuel cells, solar cells, and redox flow cells. Among them, the lithium ion cells is promising candidate for new energy sources due to relatively high gravimetric, volumetric energy density, and cost-effectiveness. To increase the energy density for lithium ion battery, LiNi0.4Mn1.6O4 has been considered due to its high operating potential of 4.7V vs. Li/Li+ and its reasonably high theoretical capacity of 147 mAh g-1. However, the LiNi0.4Mn1.6O4 shows poor cycling performances in full cell at elevated temperatures and has defects regarding dissolution of transition metal ion such as Ni, Mn, and decomposition of the electrolyte at high voltage, causing to inherent structure degradation of the LiNi0.4Mn1.6O4 cathode. In addition there exist obstacles about safety of the lithium ion battery. Despite of elaborate precautions, unpredictable accidents such as battery explosion happened due to exothermic reactions between highly volatile electrolyte and electrode materials at elevated temperatures. In this study, fluorinated hyperbranched cyclotriphosphazene, hexakis(2,2,2-trifluoroethoxy)cyclotriphosphazene (HFEPN), was employed as a functional additive to simultaneously enhance electrochemical performances and thermal stability of the LiNi0.4Mn1.6O4 cathode in lithium ion batteries. Along with improvement of the thermal stability and self-extinguishing time, it enhanced the electrochemical performances of Li/ LiNi0.4Mn1.6O4 half cells, Li/graphite half cells, and graphite/LiNi0.4Mn1.6O4 full cells by forming thermally stable and robust HFEPN-derived SEI layer on the cathode and anode. To understand the role of HFEPN as a flame-retardant and stabilization of the delithiated LiNi0.4Mn1.6O4 cathode with the electrolyte at elevated temperatures, the thermal stability was evaluated by differential scanning calorimetry (DSC). To investigate surface chemistry between the LiNi0.4Mn1.6O4 cathode and the electrolyte with or without HFEPN, the X-ray Photoelectron Spectroscopy (XPS) measurements for cycled cathode were performed. In addition, the dissolution of transition metal ions such as Ni and Mn were measured by inductively coupled plasma (ICP). The action of HFEPN for HF removal, which is expected to suppress the formation of resistive LiF component on the LiNi0.4Mn1.6O4 cathode surface was examined by nuclear magnetic resonance (NMR). These results suggested that the electrolyte with HFEPN can simultaneously enhance thermal stability and electrochemical performances of LiNi0.4Mn1.6O4 cathode. In full cells composed of the LiNi0.4Mn1.6O4 cathode and graphite anode with practical loading level, the HFEPN additive led to significant enhancement of discharge capacity and Coulombic efficiency of LiNi0.4Mn1.6O4 cathode during cycling.