Effect of Fluoroethylene Carbonate on the Electrochemical Battery Performance and Surface Film Formation Mechanism of Amorphous MoO2 Lithium-Ion Secondary Battery Negative Electrodes

Abstract

Lithium-ion secondary batteries (LIBs) are the most promising electrochemical devices for energy storage system based on their superior energy storage performance. In spite of their success, many intensive works have consistently been devoted to improve capacity, energy and power density of LIBs by developing new electrode materials and controlling stable interphase. Among the various electrode material candidates, molybdenum dioxide (MoO2) has been reviewed as one of negative electrode materials due to its attractive properties such as fairly low electrical resistivity, high thermal and chemical stability. However, similar to other electrode materials, solid electrolyte interphase (SEI) is also formed on MoO2 electrode surface after repetitive galvanostatic cycling. SEI formation is responsible for prolonged cycleability since it is an inevitable side reaction that leads to battery performance degradation. Hence well characterization and control of the highly ion conductive, mechanically and electrochemically stable, and thin SEIs are also required for MoO2-based LIBs. In this research, the positive impact of fluoroethylene carbonate (FEC), a typical SEI former additive, on the electrochemical battery performance and SEI formation mechanism of nano-sized amorphous molybdenum dioxide (a-MoO2) was investigated. The various contents of FEC were combined in the electrolyte as an alternative co-solvent to identify different SEI formation mechanism which can explain the electrochemical behavior of a-MoO2 electrodes. The capacity retention was enhanced as a function of FEC concentration up to approximately 7% after 50 cycles. To explain the electrochemical performance of a-MoO2 in FEC-containing system, the surface chemistry was characterized by using electrochemical impedance spectroscopy (EIS), field emission-scanning electron microscope (FE-SEM), Fourier transform-infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). As a result of successive reductive decomposition of FEC on the electrode surface, a highly ion conductive, mechanically and electrochemically stable, and thin SEI was developed. It originated from FEC-reduced products that are rich in polycarbonates and LiF. Based on the findings, we identified the properties of the surface films and developed their formation mechanism with and without FEC for ethylene carbonate (EC)-derived, FEC-derived, and EC-/FEC-co-derived SEI, respectively. The identification of SEI formation mechanism proposed herein might provide a good idea in understanding the effect of FEC as an effectual alternative co-solvent for modifying the surface chemistry of typical LIB negative electrodes. To the best of our knowledge, this is the first report on identifying surface chemistry mechanism of a-MoO2 negative electrode with FEC.