Poly Ionic Liquid Solid Electrolyte Interphase (SEI) Formation in an Improvement of 5V Class Lithium Ion Batteries

Abstract

Electric vehicles (EVs) are poised to dominate the next generation of transportation, but meeting the power requirements of EVs with lithium ion batteries is challenging because electrolytes containing LiPF6 and carbonates do not perform well at high temperatures and voltages. However, lithium benzimidazole salt is a promising electrolyte additive that can stabilize LiPF6 through a Lewis acid–base reaction [1]. The imidazole ring is not eligible for high-voltage applications owing to its resonance structure, but in this research, electron-withdrawing (−CF3) and electron-donating (−CH3) substitutions on imidazole rings were investigated. According to the calculation results, the CF3 substitution facilitates a high electron cloud density on imidazole ring structures to resist the electron releases from bezimidazole in oxidation reactions. In addition, through CF3 substitution, electrons are accepted from the lattice oxygen (O2−) in Lithium-rich (Li-Excess) layer material and O− is converted by electron to a great quantity, which catalyzes the electrolyte reduction and forms a new polyionic liquid solid electrolyte interphase on the cathode’s surface. Furthermore, the cycle performance tested at 60 °C and 4.8 V showed that the CF3 substitution maintains the battery retention effectively and exhibits almost no fading compared with both the blank electrolyte and the CH3 substitution. In addition, the results of this study confirm that bezimidazole salt greatly catalyzed the reduction of EC to a stage II reaction, and therefore less Li2CO3 formation was found. A possible reaction mechanism is shown in Figure 1. Figure 1